Schemes of LED indicators of overcurrent. Fuse blown indicator

There is a need to track the presence of current flowing in the circuit in two states: either there or not. Example: you are charging a battery with a built-in charge controller, connected to a power source, but how to control the process? You can, of course, include an ammeter in the circuit, you say, and you will be right. But you won't do it all the time. It’s easier to build a charge flow indicator into the power supply once, which will show whether current is flowing into the battery or not.
Another example. Let's say there is some kind of incandescent lamp in the car that you do not see and do not know if it is on or burned out. In the circuit to this lamp, you can also turn on the current indicator and control the flow. If the lamp burns out, it will be immediately visible.
Or is there a sensor with a filament. Tapa gas or oxygen sensor. And you need to know for sure that the filament is not broken and everything is working properly. This is where the indicator will come to the rescue, the scheme of which I will give below.
There can be a lot of applications, of course the main idea is the same - control of the presence of current.

Current indicator circuit

The scheme is very simple. The resistor with an asterisk is selected depending on the controlled current, it can be from 0.4 to 10 ohms. To charge a lithium-ion battery, I took 4.7 ohms. A current flows through this resistor (if it flows), according to Ohm's law, a voltage is released on it, which opens the transistor. As a result, the LED lights up, indicating that charging is in progress. As soon as the battery is charged, the internal controller will turn off the battery, the current in the circuit will disappear. The transistor will turn off and the LED will turn off, indicating that charging is complete.
Diode VD1 limits the voltage to 0.6 V. You can take any, for a current of 1 A. Again, it all depends on your load. But you can’t take a Schottky diode, since it has a too small drop - the transistor simply may not open from 0.4 V. Through such a circuit, you can even charge car batteries, the main thing is to choose a diode with a current higher than the desired charging current.

In this example, the LED turns on during the passage of current, but if you want to show when there is no current? In this case, there is a scheme with the reverse logic of work.

Everything is the same, only an inverting key is added on one transistor of the same brand. By the way, a transistor of any of the same structure. Domestic analogues are suitable - KT315, KT3102.
In parallel with the resistor with the LED, you can turn on the buzzer, and when there is no current when controlling, say, a light bulb, there will be an audible signal. What will be very convenient, and do not attach to the output of the LED is not the control panel.
In general, there can be many ideas on where to use this indicator.

Exceeding the output current in power supplies indicates an increase in power consumption in the load device. Sometimes the consumed current in the load (due to a malfunction of the connections or the load device itself) can increase up to the value of the short circuit current (short circuit), which will inevitably lead to an accident (if the power source is not equipped with an overload protection unit).

The consequences of an overload may turn out to be more significant and irreparable if you use a power source without a protection unit (as radio amateurs often do today by making simple sources and buying inexpensive adapters) - power consumption will increase, the mains transformer will fail, individual elements may ignite and an unpleasant smell.

In order to notice in time the output of the power source in the "non-standard" mode, simple overload indicators are installed. Simple - because they usually contain only a few elements, inexpensive and affordable, and these indicators can be installed universally in almost any home-made or industrial source of power.

A simple overcurrent indicator circuit

The simplest electronic circuit of the current overload indicator is shown in Figure 1.

Rice. one. Wiring diagram light indicator of current overload.

The operation of its elements is based on the fact that a low-resistance limiting resistor (R3 in the diagram) is connected in series with the load in the output circuit of the power source.

This node can be used universally in power supplies and stabilizers with different output voltages (tested in conditions of output voltage 5-20 V). However, the values ​​and ratings of the elements indicated in the diagram in Figure 1 are selected for a power supply with an output voltage of 12 V.

Accordingly, in order to expand the range of power supplies for this design, in the output stage of which the proposed display unit will work effectively, it will be necessary to change the parameters of the elements R1-R3, VD1, VD2.

As long as there is no overload, the power supply and the load node operate normally, the allowable current flows through R3 and the voltage drop across the resistor is small (less than 1 V). Also small in this case is the voltage drop across the diodes VD1, VD2, while the HL1 LED barely glows.

With an increase in the current consumption in the load device or a short circuit between points A and B, the current in the circuit increases, the voltage drop across the resistor R3 can reach its maximum value (output voltage of the power supply), as a result of which the HL1 LED will light up (blink) at full strength.

For a visual effect, a blinking L36B LED is used in the circuit. Instead of the indicated LED, devices similar in electrical characteristics can be used, for example, L56B, L456B (increased brightness), L816BRC-B, L769BGR, TLBR5410 or similar.

The power dissipated by the resistor R3 (at a short-circuit current) is more than 5 W, so this resistor is made independently from copper wire of the PEL-1 (PEL-2) type with a diameter of 0.8 mm.

It is taken from an unnecessary transformer. 8 turns of this wire are wound on the frame of a stationery pencil, its ends are tinned, then the frame is removed. Wirewound resistor R3 is ready.

All fixed resistors type MLT-0.25 or similar. Instead of diodes VD1, VD2, you can install KD503, KD509, KD521 with any letter index. These diodes protect the LED in overload mode (extinguish excessive voltage).

Overload indicator with buzzer

Unfortunately, in practice there is no way to constantly visually monitor the status of the indicator LED in the power source, so it is reasonable to supplement the circuit with an electronic sound accompaniment unit. Such a scheme is shown in Figure 2.

As can be seen from the diagram, it works on the same principle, but unlike the previous one, this device is more sensitive and the nature of its operation is due to the opening of the transistor VT1, when a potential of more than 0.3 V is established in its base. A current amplifier is implemented on the transistor VT1.

The transistor is selected germanium. From an old ham radio stock. It can be replaced by devices similar in electrical characteristics: MP16, MP39-MP42 with any letter index. In extreme cases, you can install a silicon transistor KT361 or KTZ107 with any letter index, but then the threshold for switching on the indication will be different.

Rice. 2. Electrical diagram of the node of the sound and light indicator of overcurrent.

The turn-on threshold of the transistor VT1 depends on the resistance of the resistors R1 and R2, and in this circuit, at a power supply voltage of 12.5 V, the indication will turn on when the load current exceeds 400 mA.

A flashing LED and a capsule with a built-in AF generator HA1 are included in the collector circuit of the transistor. When the voltage drop across the resistor R1 reaches 0.5 ... 0.6 V, the transistor VT1 opens, the supply voltage is supplied to the HL1 LED and the HA1 capsule.

Since the LED capsule is an active current limiting element, the LED behavior is normal. Thanks to the use of a flashing LED, the capsule will also sound intermittently - the sound will be heard during the pause between LED flashes.

In this scheme, an even more interesting sound effect can be achieved if, instead of the HA1 capsule, the KPI-4332-12 device, which has a built-in interrupted oscillator, is turned on. Thus, the sound in the event of an overload will resemble a siren (this is facilitated by the combination of LED flash interrupts and internal interrupts of the HA1 capsule).

Such a sound is quite loud (you can hear it in the next room at an average noise level), it will definitely attract the attention of people.

Fuse blown indicator

Another diagram of the overload indicator is shown in Figure 3. In those designs where a fusible (or other, for example, self-resetting) fuse is installed, it is often necessary to visually monitor their operation.

It uses a two-color LED with a common cathode and, accordingly, three leads. Those who have tested these diodes with one common terminal in practice know that they function a little differently than expected.

The thinking pattern is that it would seem that green and red colors will appear at the LED in the general case, respectively, when voltage is applied (in the correct polarity) to the corresponding terminals R or G. However, this is not entirely true.

Rice. 3. Fuse blown indicator light.

While the fuse FU1 is working, voltage is applied to both anodes of the HL1 LED. The glow threshold is adjusted by the resistance of the resistor R1. If the fuse breaks the load power supply circuit, then the green LED goes out, and the red one remains on (if the supply voltage has not disappeared at all).

Since the allowable reverse voltage for LEDs is small and limited, for the indicated design, diodes with different electrical characteristics VD1-VD4 are introduced into the circuit. The fact that only one diode is connected in series to the green LED, and three to the red one, is explained by the features of the ALC331A LED, noticed in practice.

During the experiments, it turned out that the voltage threshold for turning on the red LED is less than that of the green one. To balance this difference (noticeable only in practice), the number of diodes is not the same.

When the fuse blows, voltage is applied to the green LED (G) in reverse polarity. The ratings of the elements in the circuit are given to control the voltage in the 12 V circuit. Instead of the ALC331A LED, it is permissible to use other similar devices, for example, KIPD18V-M, L239EGW.

Literature: Andrey Kashkarov - Electronic homemade products.

N. TARANOV, St. Petersburg

When developing various electronic devices, the problem of monitoring the presence of current in their circuits arises. Off-the-shelf measuring devices are often missing, expensive, or inconvenient to use. In such cases, built-in control nodes are used. For alternating current, the problem is relatively easy to solve using current transformers, inductive magnetically sensitive elements, etc. For direct current, as a rule, this problem is more difficult. The article discusses some of the existing devices for monitoring the presence of direct current in the circuit (hereinafter we will call them indicators of direct current, or abbreviated - IPT), their advantages and disadvantages, proposed circuit solutions that improve the characteristics of these devices.

IPTs are usually included in the break of the controlled circuit. Some IPTs can respond to the magnetic field created by the current-carrying elements of the controlled circuit, but at low controlled currents they are complex and are not considered in this article. IPT can be characterized by the following main parameters and features:
1) deltaU - voltage drop across the IPT in the entire range of controlled currents. In order to minimize the influence of the IPT on the controlled circuit and reduce power losses, it is sought to minimize the deltaU;
2) Inom rated operating current (assuming the average value of the controlled current);
3) Imin, Imax - the limits of the range of change of the controlled current, in which the fact of its presence is reliably indicated;
4) the nature of the output indication signal (LED glow, TTL levels, etc.);
5) presence or absence of additional power sources for IPT;
6) the presence or absence of galvanic connection of the output signal of the IPT with the controlled circuit.

By the type of current-sensitive element - current sensor (DT) are distinguished;
- IPT with serial load in the circuit;
- IPT with semiconductor DT (Hall sensors, magnetodiodes, magnetoresistors, etc.);
- IPT magnetic contact (on reed switches, on current relays);
- IPT with magnetically saturated elements.

The principle of operation of the IPT with a series load in the circuit (Fig. 1)

It consists in the fact that a load element (NE) is included in the break of the controlled circuit, on which a voltage drop is created when current flows in the controlled circuit. It enters the signal converter (PS), where it is converted into a signal indicating the presence of current in the circuit.

Obviously, the deltaU for this type of IPT depends on the magnitude of the controlled current and on the sensitivity of the PS. The more sensitive the PS, the less NE resistance can be applied, and hence the deltaU will be less.

In the simplest case, NE is a resistor. The advantage of such NE is simplicity, cheapness. Disadvantages - with a low sensitivity of the PS, the power losses on the NE will be large, especially when controlling high currents, the dependence of AU on the magnitude of the current flowing through the IPT. It narrows the range of change of the controlled current (this disadvantage is insignificant when controlling the current in a narrow range of its value). As an example, consider a practical IPT scheme of this type. On fig. 2 shows a diagram of the charging current indicator for the battery. The resistor R1 acts as a NE, and the chain R2, HL1 acts as a PS.

The ballast resistor R2 has a resistance of 100 ohms, the HL1 LED has a rated current of 10 mA (for example, type AL307B), and the resistance of the resistor R1 will depend on the value of the controlled charging current.

With a stabilized charging current of 10 mA (for example, for a 7D-01 battery), resistor R1 can be excluded. With a charging current of 1 A, the resistance of the resistor R1 will be approximately 3.5 ohms. The voltage drop across the IT in both cases will be 3.5 V. The power loss at a current of 1 A will be 3.5 W. Obviously, this scheme is unacceptable at high charging currents. You can somewhat reduce the power loss at the IPT if you reduce the resistance of the ballast resistor R2. But it is undesirable to do this, since accidental surges of charging currents may damage the HL1 LED.

If you apply NE with a non-linear dependence of the voltage drop on the strength of the flowing current, you can significantly improve the characteristics of this IPT. For example, replacing the resistor R1 with a chain of four diodes connected in the forward direction gives good results, as shown in Fig. 3.

As diodes VD1-VD4, you can use any silicon rectifier diodes with a permissible operating current of at least the value of the controlled current. (For many types of LEDs, a string of three diodes is sufficient.) The resistance of the resistor R2 can in this case be reduced to a value of 30 ohms.

With such an IPT scheme, the range of controlled currents expands and extends from 10 mA to Imax, where Imax is the maximum allowable operating current of the diodes. The brightness of the HL1 LED is almost constant over the entire range of controlled currents.

Another way to improve the performance of the IPT with a series load in the circuit is the improvement of the PS. Indeed, if the PS sensitivity is increased and its performance is ensured in a wide range of deltaU changes, it is possible to obtain an IPT with good performance. True, for this it is necessary to complicate the IPT scheme. As an example, consider the IPT scheme developed by the author, which has shown good results in industrial process control devices. This IPT has the following specifications: operating current range - 0.01 mA ... 1 A; deltaU
The IPT scheme is shown in fig. four.

NE in this circuit is resistor R3. The rest of the circuit is PS. If there is no current between points A and B, the output of the operational amplifier DA1 will have a voltage close to -5 V, and the HL1 LED will not light. When a current appears between points A and B, a voltage is created on the resistor R3, which will be applied between the differential inputs of the operational amplifier DA1. As a result, a positive voltage will appear at the output of the operational amplifier DA1 and the HL1 LED will glow, indicating the presence of current between points A and B. When choosing an operational amplifier with a high gain (for example, KR1401UD2B), a reliable indication of the presence of current starts already from 5 mA. Capacitor C1 is necessary to eliminate possible self-excitation.

It should be noted that some instances of the op-amp may have an initial bias voltage (of any polarity). In this case, the LED can light up even in the absence of current in the controlled circuit. Eliminate this drawback by introducing the "zero correction" circuit of the OS, made according to any standard scheme. Some types of op amps have special terminals for connecting a "zero correction" variable resistor.

Details: resistors R1, R2, R4, R5 - any type, power 0.125 W; resistor R3 - any type, power> 0.5 W; capacitor C1 - any type; operational amplifier DA1 - any, with a gain > 5000, with an output current > 2.5 mA, allowing a single supply voltage of 5 V. (The last two requirements are due to the use of a "convenient" supply voltage of the IPT, although other supply voltages can be used. When In this case, the resistance of the ballast resistor R5 will need to be recalculated so that the output current of the operational amplifier DA1 does not exceed its maximum allowable value). The HL1 LED was chosen as such for reasons of sufficient brightness of the glow at a current of 2.5 mA through it. Experiments have shown that most miniature imported LEDs work perfectly in this device (in principle, the type of LED is determined by the maximum output current of the DA1 operational amplifier).

This device with a KR1401UD2B chip is convenient when building a four-channel IPT, for example, when controlling separate charging of four batteries at the same time. In this case, the bias circuit R1, R2, as well as point A are common to all four channels.

The device can also control large currents. To do this, reduce the resistance of the resistor R3 and recalculate its power dissipation. Experiments were carried out using a piece of PEV-2 wire as R3. With a wire diameter of 1 mm and a length of 10 cm, currents in the range of 200 mA ... 10 A were reliably indicated (if the wire length is increased, the lower limit of the range moves to weaker currents). At the same time, deltaU did not exceed 0.1 V.

With a little modification, the device is converted into an IPT with an adjustable response threshold (Fig. 5).

Such an IPT can be successfully used in current protection systems for various devices, as a basis for an adjustable electronic fuse, etc.

Resistor R4 regulate the IPT threshold. As R4, it is convenient to use a multi-turn resistor, for example, types SP5-2, SPZ-39, etc.

If it is necessary to provide galvanic isolation between the controlled circuit and control devices (CC), it is convenient to use optocouplers. To do this, it is enough to connect an optocoupler instead of the HL1 LED, for example, as shown in fig. 6.

Schmitt triggers are applicable to match the output signal of this IPT with digital control devices. On fig. 7 shows the scheme for matching the IPT with the UK on TTL logic. Here +5 V UK is the supply voltage of the digital circuits of the UK.

IPTs with semiconductor DTs are described in detail in the literature. For radio amateurs, it is of interest to use magnetically controlled microcircuits of the K1116KP1 type in IPT (this microcircuit was widely used in the keyboard of some Soviet-made computers). The scheme of such an IPT is given in Fig. eight.

Winding L1 is placed on a magnetic core made of magnetically soft steel (permalloy is better), which plays the role of a magnetic concentrator. An approximate view and dimensions of the magnetic concentrator are shown in fig. 9.

Chip DA1 is placed in the gap of the magnetic hub. In its manufacture, it is necessary to strive to reduce the gap. Experiments were carried out with various magnetic circuits, in particular, rings cut from ordinary water pipes, machined from cores of dynamic heads, and assembled from transformer steel washers were used.

The cheapest and easiest to manufacture (in amateur conditions) were rings cut from water pipes with a diameter of 1/2 and 3/4 inches. The rings were cut off from the pipes so that the length of the ring was equal to the diameter. Then it is desirable to heat these rings to a temperature of about 800 °C and slowly cool them in air (anneal them). Such rings have practically no residual magnetization and work well in IPT.

The experimental sample had a 3/4 inch water pipe magnetic core. The winding was wound with PEV-2 wire with a diameter of 1 mm. At 10 turns Imin = 8 A, at 50 turns Imin = 2 A. It should be noted that the sensitivity of such an IPT depends on the position of the microcircuit in the gap of the magnetic circuit. This circumstance can be used to adjust the sensitivity of the IPT.

Rings made from cores from magnetic systems of dynamic heads turned out to be the most effective, but their manufacture in amateur conditions is difficult.

For radio amateurs, electromagnetic IPTs on reed switches and on current relays are of undoubted interest. IPT on reed switches are reliable and cheap. The principle of operation of such IPT is illustrated in Fig. 10, a.

You can learn more about reed switches from. The electrical circuit of the IPT with a current sensor (DT) on the reed switch is shown in fig. 10b.

Many radio amateurs will surely have an old keyboard from a Soviet-made PC on reed switches. Such reed switches are perfect for the implementation of IPT. The sensitivity of the IPT depends on:
- the number of turns in the winding (with an increase in the number of turns, the sensitivity also increases);
- winding configuration (the winding is optimal, the length of which is approximately equal to the length of the reed switch bulb);
- the ratio of the outer diameter of the reed switch and the inner diameter of the winding (the closer it is to 1, the higher the sensitivity of the IPT will be).

The author conducted experiments with reed switches KEM-2, MK-16-3, MK10-3. KEM-2 reed switches showed the best results in terms of sensitivity. When winding eight turns of PEV-2 wire with a diameter of 0.8 mm without a gap, the IPT actuation current is 2 A, the release current is 1.5 A. The voltage drop across the IPT was 0.025 V. The sensitivity of this IPT can be adjusted by moving the reed switch along the longitudinal axis windings. In industrial IPTs of this type, the reed switch is moved using a screw or placed in a non-magnetic sleeve with an external thread, which is screwed into a coil with a winding. This method of adjusting the sensitivity is not always convenient, and in amateur conditions it is difficult to implement. In addition, this method allows adjustment only in the direction of decreasing the sensitivity of the IPT.

The author has developed a method that allows you to change the sensitivity of the IPT over a wide range using a variable resistor. With this method, an additional winding of PEV-2 wire with a diameter of 0.06-0.1 mm with a number of turns of 200 is introduced into the design of the diesel fuel. It is desirable to wind this winding directly on the reed switch along the entire length of its cylinder, as shown in fig. 11, a.

The electrical circuit of the IPT is given in fig. 11b.

The winding L1 is the main winding, the winding L2 is additional. If you turn on the windings L1 and L2 in accordance, then by adjusting the resistor R1 it is possible to increase the sensitivity of the IPT many times over in comparison with the version of the IPT that has a DT without an additional winding. If you turn on the windings L1 and L2 in opposite directions, then by adjusting the resistor R, you can reduce the sensitivity of the IPT many times over. An experiment was carried out with this circuit with the parameters of its elements:
- winding L1 - 200 turns of PEV-2 wire with a diameter of 0.06 mm; wound directly on the KEM-2 type reed switch;
- winding L2 - 10 turns of PEV-2 wire with a diameter of 0.8 mm, wound over winding L1.

The following Imin values ​​are obtained:
- with the consonant inclusion of windings -0.1 ... 2 A;
- when the windings are turned on in the opposite direction -2 ... 5 A.

IPT on the current relay have as: DT an electromagnetic relay with a low-resistance winding. Unfortunately, current relays are very scarce. The current relay can be made from a conventional voltage relay by replacing its winding with a low-resistance one. The author used a DT made from a relay of the RES-10 type. The relay winding is carefully cut off with a scalpel, and a new winding is wound in its place with a PEV-2 wire with a diameter of 0.3 mm until the frame is filled. The sensitivity of this diesel engine is regulated by selecting the number of turns and changing the rigidity of the flat spring armature. The stiffness of the spring can be changed by bending or grinding along the width. The experimental sample of diesel fuel had Imin = 200 mA, delta U = 0.5 V (at a current of 200 mA).

If you need to calculate the current relay, you can refer to.

The electrical circuit of the IPT of this type is shown in fig. 12.

Of particular interest are IPTs with magnetically saturable elements. They use the property of ferromagnetic cores to change the permeability under the action of an external magnetic field. In the simplest case, this type of IPT is an AC transformer with an additional winding, as shown in Fig. 13.

Here, the alternating voltage is transformed from winding L2 to winding L3. The voltage from the winding L3 is detected by the diode VD1 and charges the capacitor C1. Then it is fed to the threshold element. In the absence of current in the winding L1, the voltage created on the capacitor C1 is sufficient to trigger the threshold element. When a direct current is passed through the winding L1, the magnetic circuit is saturated. This leads to a decrease in the transfer coefficient of the alternating voltage from the winding L2 to the winding L3 and a decrease in the voltage across the capacitor C1. When it reaches a certain value, the threshold element switches. The inductor L4 eliminates the penetration of the alternating voltage of the measuring circuit into the controlled one, and also eliminates the shunting of the measuring circuit by the conductances of the controlled circuit.

The sensitivity of this device can be adjusted:
- selection of the number of turns of the windings L1, L2, L3;
- selection of the type of the transformer magnetic core;
- by adjusting the response threshold of the threshold element.

Advantages of the device - ease of implementation, the absence of mechanical contacts.

Its essential drawback is the penetration of alternating voltage from the IPT into the controlled circuit (however, in most applications, controlled circuits have blocking capacitors, which reduces this effect). The penetration of alternating voltage into the controlled circuit decreases with an increase in the ratio of the number of turns of the windings L2 and L3 to the number of turns of the winding L1 and with an increase in the inductance of the inductor L4.

An experimental sample of the IPT of this type was assembled on an annular magnetic circuit of the size K10x8x4 from ferrite grade 2000NM. Winding L1 had 10 turns of PEV-2 wire with a diameter of 0.4 mm, windings L2 and L3 each had 30 turns of PEV-2 wire with a diameter of 0.1 mm. The L4 inductor is wound on the same ring and had 30 turns of PEV-2 wire with a diameter of 0.4 mm. Diode VD1 - KD521 A. Capacitor C1 - KM6 with a capacity of 0.1 uF. One inverter of the K561LN1 microcircuit was used as a threshold element. A voltage ("meander") of a rectangular shape with a frequency of 10 kHz and an amplitude of 5 V was applied to the L2 winding. This IPT reliably indicated the presence of current in the controlled circuit in the range of 10 ... 1000 mA. It is obvious that in order to expand the range of controlled currents in the direction of increasing the upper limit, it is necessary to increase the diameter of the wire of the windings L1 and L2, and also to choose a larger magnetic circuit.

The IPT scheme of this type, shown in fig. fourteen.

Here, the transformer magnetic circuit consists of two ferrite rings, the windings L1 and L3 are wound on both rings, and the windings L1 and L4 are wound on different rings so that the voltages induced in them are mutually compensated. The design of the magnetic circuit is illustrated in Fig. fifteen.

For clarity, the cores are spaced, in a real design they are pressed against each other.

In the IPT of this type, there is practically no penetration of alternating voltage from the measuring circuit into the controlled circuit and there is practically no shunting of the measuring circuit by the conductance of the controlled one. An experimental sample of the IPT was made, the scheme of which is shown in Fig. 16.

On inverters D1.1-D1.3, a high-duty pulse generator is assembled (the use of such pulses significantly reduces the power consumption of the IPT). In the absence of excitation, a resistor with a resistance of 10 ... 100 kOhm should be included in the wire connecting pins 2, 3 of the microcircuit with resistors R1, R2 and capacitor C1.

Elements C2, SZ, VD2, VD3 form a rectifier with voltage doubling. The D1.4 inverter together with the HL1 LED provides a threshold indication of the presence of pulses at the transformer output (winding L3).

In this IPT, ferrite rings of the VT brand (used in computer memory cells) with dimensions of 8x4x2 mm were used. Windings L2 and L3 each have 20 turns of PEL-2 wire with a diameter of 0.1 mm, windings L1 and L4 each have 20 turns of PEL-2 wire with a diameter of 0.3 mm.

This sample confidently indicated the presence of current in the controlled circuit in the range of 40 mA ... 1 A. The voltage drop across the IPT at a current in the controlled circuit of 1 A did not exceed 0.1 V. Resistor R4 can be used to adjust the response threshold, which makes it possible to use this IPT as an element of circuits for protecting devices from overloads.

LITERATURE
1. Yakovlev N. Non-contact electrical measuring devices for diagnosing electronic equipment. - L .: Energoatomizdat, Leningrad branch, 1990.

2. Microcircuits of the K1116 series. - Radio, 1990, No. 6, p. 84; No. 7, p. 73, 74; No. 8, p. 89.

3. Switching devices of radio-electronic equipment. Ed. G. Ya. Rybina. - M.: Radio and communication, 1985.

4. Stupel F. Calculation and design of electromagnetic relays. - M.: Gosenergoizdat, 1950._

Radio No. 4 2005.

[email protected]

Calculating the supply voltage of an LED is a necessary step for any electric lighting project, and fortunately it's easy to do. Such measurements are necessary to calculate the power of the LEDs, since you need to know its current and voltage. The power of an LED is calculated by multiplying the current by the voltage. In this case, you need to be extremely careful when working with electrical circuits, even when measuring small quantities. In the article, we will consider in detail the question of how to find out the voltage in order to ensure the correct operation of the LED elements.

LEDs exist in different colors, they are two and three-color, flashing and changing color. In order for the user to program the sequence of operation of the lamp, various solutions are used, which directly depend on the supply voltage of the LED. To illuminate the LED, a minimum voltage (threshold) is required, while the brightness will be proportional to the current. The voltage across the LED increases slightly with current because there is internal resistance. When the current is too high, the diode heats up and burns out. Therefore, the current is limited to a safe value.

The resistor is placed in series because the diode grid needs a much higher voltage. If U is reversed, no current flows, but for high U (e.g. 20 V) an internal spark (breakdown) occurs which destroys the diode.

As with all diodes, current flows through the anode and exits through the cathode. On round diodes, the cathode has a shorter wire and the body has a cathode side plate.

Voltage dependence on the type of lamp

With the rise of high-brightness LEDs designed to provide replacement lamps for commercial and indoor lighting, there is an equal, if not more, proliferation of power solutions. With hundreds of models from dozens of manufacturers, it becomes difficult to understand all the permutations of LED input/output voltages and output current/power values, not to mention the mechanical dimensions and many other features for dimming, remote control and circuit protection.

There are many different LEDs on the market. Their difference is determined by many factors in the production of LEDs. Semiconductor makeup is a factor, but fabrication technology and encapsulation also play a major role in determining LED performance. The first LEDs were round, in the form of models C (diameter 5 mm) and F (diameter 3 mm). Then, rectangular diodes and blocks combining several LEDs (networks) came into implementation.

The hemispherical shape is a bit like a magnifying glass that determines the shape of the light beam. The color of the emitting element improves diffusion and contrast. The most common designations and form of LED:

• A: red diameter 3mm in CI holder.
• B: red diameter 5mm, used in the front panel.
• C: purple 5mm.
• D: bicolor yellow and green.
• E: rectangular.
• F: yellow 3mm.
• G: white high brightness 5mm.
• H: red 3 mm.
• K-anode: cathode, indicated by a flat surface in the flange.
• F: 4/100mm anode connecting wire.
• C: reflective cup.
• L: curved shape that acts like a magnifying glass.

Device specification

A summary of the various LED parameters and supply voltages can be found in the vendor's specifications. When choosing LEDs for specific applications, it is important to understand their difference. There are many different LED specifications, each of which will influence the choice of a particular type. LED specifications are based on color, U, and current. LEDS tend to provide one color.

The color emitted by an LED is defined in terms of its maximum wavelength (lpk), which is the wavelength that has the maximum light output. Typically, process variations give peak wavelength changes of up to ±10 nm. When choosing colors in the LED specification, it is worth remembering that the human eye is most sensitive to hues or color variations around the yellow/orange region of the spectrum - from 560 to 600 nm. This may affect the choice of color or position of the LEDs, which is directly related to electrical parameters.

During operation, LEDs have a given drop U, which depends on the material used. The supply voltage of the LEDs in the lamp also depends on the current level. LEDs are current controlled devices and the light level is a function of the current, increasing it increases the light output. It is necessary to ensure that the operation of the device is such that the maximum current does not exceed the allowable limit, which can lead to excessive heat dissipation within the chip itself, reducing the luminous flux and shortening the service life. Most LEDs require an external current limiting resistor.

Some LEDs may include a series resistor, so it specifies what voltage to supply the LEDs with. LEDs do not allow large inverse U. It should never exceed its stated maximum value, which is usually quite small. If there is a possibility of a reverse U on the LED, then it is better to build protection into the circuit to prevent damage. These can usually be simple diode circuits that will provide adequate protection for any LED. You don't have to be a professional to understand this.

Lighting LEDs are current-powered and their luminous flux is proportional to the current flowing through them. The current is related to the supply voltage of the LEDs in the lamp. Several diodes connected in series have equal current flowing through them. If they are connected in parallel, each LED receives the same U but different current flows through them due to the dispersion effect on the current-voltage characteristic. As a result, each diode emits a different light output.

Therefore, when selecting elements, it is necessary to know what supply voltage the LEDs have. Each requires approximately 3 volts at its terminals to operate. For example, a 5-diode series requires approximately 15 volts across the terminals. In order to supply a regulated current with sufficient U, the LEC uses an electronic module called a driver.

There are two solutions:

1. An external driver is installed outside the luminaire, with a safety extra-low voltage power supply.
2. Internal, built into the lantern, i.e. a subunit with an electronic module that regulates the current.

This driver can be powered by 230V (Class I or Class II) or Safety Extra Low U (Class III), such as 24V. The LEC recommends the second power supply solution because it offers 5 major benefits.

Advantages of LED voltage selection

The correct calculation of the supply voltage of the LEDs in the lamp has 5 key advantages:

1. Safe ultra-low U, possibly regardless of the number of LEDs. The LEDs must be installed in series to guarantee the same level of current in each of them from the same source. As a result, the more LEDs, the higher the voltage at the LED terminals. If it is a device with an external driver, then the oversensitive safety voltage must be much higher.
2. The integration of the driver inside the lanterns allows for a complete system installation with safety extra low voltage (SELV), regardless of the number of light sources.
3. More reliable installation in the wiring standard for LED lamps connected in parallel. Drivers provide additional protection, especially against temperature rise, which guarantees a longer service life while respecting the supply voltage of LEDs for different types and currents. Safer commissioning.
4. Integrating LED power into the driver avoids mishandling in the field and improves their ability to withstand hot plugging. If the user only connects an LED luminaire to an external driver that is already on, this can cause the LEDs to overvoltage when they are connected and therefore destroy them.
5. Simple service. Any technical problems are more easily visible in LED lamps with a voltage source.

When the U drop across the resistance is important, one must choose the right resistor capable of dissipating the required power. The current consumption of 20 mA may seem low, but the calculated power says otherwise. So, for example, for a voltage drop of 30 V, the resistor must dissipate 1400 ohms. Power dissipation calculation P = (Ures x Ures) / R,

• P is the value of the power dissipated by the resistor, which limits the current in the LED, W;
• U is the voltage across the resistor (in volts);
• R is the value of the resistor, Ohm.

P = (28 x 28) / 1400 = 0.56 W.

A 1 watt LED supply voltage would not withstand overheating for a long time, and 2 watts would also fail too quickly. For this case, it is necessary to connect two 2700 ohm / 0.5 W resistors in parallel (or two 690 ohm / 0.5 W resistors in series) to evenly distribute heat dissipation.

Thermal control

Finding the optimum wattage for your system will help you learn more about the heat control required for reliable LED operation, as LEDs generate heat that can be very damaging to the device. Too much heat will cause the LEDs to produce less light and also shorten the lifespan. For a 1 watt LED, it is recommended to look for a 3 square inch heatsink for each watt of LED.

At present, the LED industry is growing at a fairly fast pace and it is important to know the difference in LEDs. This is a general question as products can range from very cheap to expensive. You need to be careful when buying cheap LEDs, as they may work great, but usually don't last long and burn out quickly due to poor performance. In the manufacture of LEDs, the manufacturer indicates in the passports the characteristics with average values. For this reason, buyers do not always know the exact characteristics of LEDs in terms of luminous flux, color, and forward voltage.

Determination of forward voltage

Before you know the LED supply voltage, set the appropriate multimeter settings: current and U. Before testing, set the resistance to the highest value to avoid LED burnout. This can be done simply: clamp the multimeter leads, adjust the resistance until the current reaches 20 mA and fix the voltage and current. In order to measure the forward voltage of the LEDs, you will need:

1. LEDs for testing.
2. Source U LED with parameters higher than constant voltage LED.
3. Multimeter.
4. Alligator clips to hold the LED on the test leads to determine the supply voltage of LEDs in fixtures.
5. Wires.
6. Variable resistor 500 or 1000 ohm.

The blue LED's primary current was 3.356V at 19.5mA. If a voltage of 3.6V is used, the value of the resistor to use is calculated by the formula R = (3.6V-3.356V) / 0.0195A) = 12.5 ohms. To measure high power LEDs, follow the same procedure and set the current by quickly holding the value on the multimeter.

Measuring the supply voltage of smd high power LEDs with forward current >350mA can be a little tricky because when they heat up quickly, U drops drastically. This means that the current will be higher for a given U. If the user does not have time, he will have to cool the LED to room temperature before measuring again. You can use 500 Ohm or 1 kOhm. To provide coarse and fine tuning, or to connect a higher and lower range variable resistor in series.

Alternative definition of voltage

The first step to calculate the power consumption of LEDs is to determine the voltage of the LED. If there is no multimeter at hand, you can study the manufacturer's data and find the passport U of the LED block. Alternatively, you can estimate U based on the color of the LEDs, for example, the supply voltage of a white LED is 3.5 V.

After the LED voltage is measured, the current is determined. It can be measured directly with a multimeter. The manufacturer's data gives a rough estimate of the current. After that, you can very quickly and easily calculate the power consumption of the LEDs. To calculate the power consumption of an LED, simply multiply the U of the LED (in volts) by the LED current (in amps).

The result, measured in watts, is the power the LEDs use. For example, if an LED has a U of 3.6 and a current of 20 milliamps, it will use 72 milliwatts of energy. Depending on the size and scale of the project, voltage and current readings may be measured in smaller or larger units than base current or watts. Unit conversions may be required. When doing these calculations, remember that 1000 milliwatts equals one watt, and 1000 milliamps equals one ampere.

To test the LED and find out if it works and which color to choose, a multimeter is used. It must have a diode test function, which is indicated by the diode symbol. Then, for testing, fix the measuring cords of the multimeter on the legs of the LED:

1. Connect the black cord to the cathode (-) and the red cord to the anode (+), if the user makes a mistake, the LED does not light up.
2. A small current is applied to the sensors, and if it can be seen that the LED glows slightly, then it is serviceable.
3. When checking a multimeter, you need to consider the color of the LED. For example, yellow (amber) LED test - LED threshold voltage is 1636mV or 1.636V. If white LED or blue LED is tested, the threshold voltage is higher than 2.5V or 3V.

To test a diode, the indicator on the display must be between 400 and 800 mV in one direction and not show in the opposite direction. Normal LEDs have threshold U as described in the table below, but for the same color can have significant differences. The maximum current is 50 mA, but it is recommended not to exceed 20 mA. At 1-2 mA, the diodes already glow well. Threshold U LED

If the battery is fully charged, then at 3.8 V the current is only 0.7 mA. AT last years LEDs have made significant progress. There are hundreds of models, with a diameter of 3 mm and 5 mm. There are more powerful diodes with a diameter of 10 mm or in special cases, as well as diodes for mounting on a printed circuit board up to 1 mm long.

LEDs are generally considered DC devices, operating on a few volts of DC. In low power applications with few LEDs this is a perfectly acceptable approach, such as mobile phones powered by a DC battery, but other applications such as a linear strip lighting system extending 100m around a building cannot function with this arrangement.

The DC drive suffers from losses over distance, which requires the use of higher drive U from the beginning, as well as additional regulators that lose power. AC makes it easier to use transformers to step down U to 240 V AC or 120 V AC from kilovolts used in power lines, which is much more problematic for DC. Starting with any supply voltage from the mains (eg 120 V AC) requires electronics between the power supply and the devices themselves to provide a constant U (eg 12 V DC). The ability to drive multiple LEDs is important.

Lynk Labs has developed a technology that allows the LED to be powered from AC voltage. New Approach is to develop AC LEDs that can operate directly from an AC power source. Many standalone LED fixtures simply have a transformer between the wall outlet and fixture to provide the required constant U.

A number of companies have developed LED light bulbs that screw directly into standard sockets, but they invariably also contain miniature circuitry that converts AC to DC before being fed into the LEDs.

A standard red or orange LED has a threshold U of 1.6 to 2.1 V, for yellow or green LEDs the voltage is from 2.0 to 2.4 V, and for blue, pink or white it is a voltage of about 3.0 to 3.6 V. The table below lists some typical voltages. Values ​​in brackets are the closest normalized values ​​in the E24 series.

The supply voltage specifications for the LEDs are shown in the table below.

Designations:

• STD - standard LED;
• HL - high brightness LED indicator;
• FC - low consumption.

This data is enough for the user to independently determine the necessary device parameters for the lighting project.

The first circuit is the simplest current indicator, it can be used in chargers without ammeters. Another design is for discrete indication of the current consumed by the load operating in the AC mains. It is indicated by three LEDs, indicating that the consumed current has exceeded the set switch-on values.

Simple current indicator

In the role of a current sensor in this device, two diodes connected in the forward direction are used. The voltage drop across them is enough for the LED indicator to light up. A resistance is connected in series with the LED, the value of which must be chosen such that at the maximum load current, the current through the LED does not exceed the allowable one. The maximum forward current of the diodes must be at least twice the maximum load current. Any LED will do.

Mains current LED indicator

Due to its small dimensions, low power consumption and low power loss in the 220V AC circuit, the amateur radio design can be easily built into a standard household, extension cord, circuit breaker. The indication allows you to track not only the presence of an excess of current, but also quickly fix the breakdown of the windings of electric motors or an increased mechanical load on the power tool.

The current sensor is built on self-made reed relays K1 - K3, the windings of which have a different number of turns, therefore, the reed switch contacts operate at different ratings of the flowing current. In this circuit, the winding of the first relay has the largest number turns, therefore, contacts K1.1 close before other contacts. When the current consumed by the load is from 2 A to 4 A, only the HL1 LED will light. With closed K1.1, but open contacts of the remaining reed switches, the supply current of the HL1 LED will go through the diode chains VD9 - VD12 and VD13 - VD16. With an increase in the controlled parameter by more than 4 A, the contacts of the reed switch K2.1 will start to work and another HL2 will light up. The short-circuit winding has a minimum number of turns, so the contacts K3.1 close when I in a load of more than 8 A.

Since the windings of homemade reed relays have a small number of turns, there is practically no heating of the windings. The LED current indicator node is powered by a transformerless power supply made on capacitor C1, current-limiting resistances R1, R2, bridge rectifier VD1 -VD4. Capacitance C2 smooths out the ripple of the rectified voltage.

Coils of reed switches are made of winding wire with a diameter of 0.82 mm in one row. In order not to spoil the glass case of the reed switch, it is better to wind the windings on the smooth part of a steel drill with a diameter of 3.2 mm. The distance between the turns is 0.5 mm. Relay coil K1 - 11 turns, K2 - 6 turns, K3 - only 4 turns. The contact actuation current depends not only on the number of turns, but also on the specific type of reed switch and the location of the coil on the cylinder, when the coil is located in the center of the reed switch body, the sensitivity is best.

By changing the number of turns of the coils, you can select other values ​​​​of the indication of the current of the connected loads, at which the LEDs will light up. For a small correction, you can change the position of the coil on the reed switch housing. After tuning, the coils are fixed with drops of polymer glue.

Current and power indicator on 4 LEDs

The proposed amateur radio design is suitable for light indication of the consumed current (and power) by a load connected to a 220 V variable network. The device is connected to a break in one of the network wires. Design features - no power supply and galvanic isolation. This was achieved using a bright and current transformer.

The current indicator circuit includes a transformer T1, two half-wave rectifiers on VD1 and VD2 with smoothing capacitors C1 and C2. LEDs HL1 and HL4 are connected to the first rectifier, HL2 and HL3 are connected to the second one. Parallel to HL2 - HL4, trimming resistors R1 - R3 are installed. With them, you can adjust the output current of the rectifier, at which certain LEDs start to burn.

When the load current flows through the primary winding of the current transformer T1, an alternating voltage appears in the secondary, which is rectified by the rectifiers. The indicator is adjusted so that when the load current is below 0.5 A, the voltage at the rectifier outputs is not enough to light the LEDs. If the current exceeds this level, a weak but quite noticeable glow of the HL1 LED (red) will begin. As the load current increases, the output current of the rectifier also increases. If the load current reaches a level of 2 A, the HL2 LED (green) will light up, at a current above 3 A - HL3 (blue), and if the current is more than 4 A, the white HL4 LED will light up. Home experiments have shown that the device is operable up to a load current of 12 A, this is quite enough for domestic needs, while the current flowing through the LEDs is not more than 15-18 mA.

All radio components, except for the current transformer, are mounted on a fiberglass printed circuit board, the drawing of which is shown in the figure above. In the indicator circuit, tuning resistances SDR-19 are used, capacitances are oxide, diodes can be taken from any low-power rectifiers, LEDs - only with increased brightness.

The current transformer is made by hand from a step-down transformer of a small-sized power source (120/12 V, 200 mA). The active resistance of the primary winding is 200 ohms. The transformer windings are wound in different sections. For the above circuit parameters, the number of turns of the primary winding of the transformer is three, the wire must be in good insulation and designed for the mains voltage and current consumed by the load. For the manufacture of a transformer, you can take any low-power serial step-down transformer, for example, TP-121, TP-112.

To calibrate the scale, you can use an AC ammeter and a step-down transformer with a secondary voltage of 5-6 V and a current of up to a couple of amperes. By changing the value of the load resistance, the required current is set and the tuning resistances are used to achieve the ignition of the corresponding LED.

Proper operation of the car battery is the key to its long life and safe operation. Control of the battery charge-discharge mode makes it possible to take timely measures, as well as monitor the correct operation of the generator, starter and vehicle electrical wiring.

The indicator controls the voltage drop on the conductor connecting the negative terminal of the battery to the "Mass" of the car. This conductor is connected to a classic resistive measuring bridge R1-R5, which makes it possible to take heteropolar signals from it and amplify them using an operational amplifier with a unipolar supply. Diodes VD1-VD4 are connected to the negative feedback circuit of the op amp DA1, which expand the limits of the measured current, allowing you to measure even the current consumption of the starter when starting the car engine.

The recording instrument is any magnetoelectric milliammeter with a scale with zero in the middle, for example M733 with a full deflection current of the needle of 50 μA. On the scale, it is most convenient to evenly place three marks to the right and left of zero: 5 A, 50 A and 500 A. The indicator is powered by a parametric voltage regulator of 6.6 V. The right terminal of the resistance R5 is left permanently connected to the negative terminal of the battery.

To calibrate the scale, first, power is supplied directly from the battery and the trimmer resistance R4 sets the microammeter needle to zero. Then, with the ignition key turned off, we connect the positive terminal of the battery through a powerful (about 60 W) resistance of 2.4 ohms, connected to the car body and trimming resistance R7, set the ammeter needle to 5 A. After calibrating, the positive output of the indicator power supply is connected to the positive output of the on-board network car.