DC indicators and their applications. Charging current indicators for car battery chargers and reverse polarity protection

Calculating the LED supply voltage is a necessary step for any electrical lighting project, and luckily it's easy to do. Such measurements are necessary to calculate the power of an LED, since you need to know its current and voltage. LED power is calculated by multiplying the current by the voltage. However, you need to be extremely careful when working with electrical circuits, even when measuring small quantities. In this article we will take a closer look at the question of how to find out the voltage to ensure proper operation of the LED elements.

LEDs come in different colors; they come in two and three colors, flashing and changing colors. In order for the user to program the operating sequence of the lamp, various solutions are used that directly depend on the LED supply voltage. To illuminate an LED, a minimum voltage (threshold) is required, and the brightness will be proportional to the current. The voltage across an 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 since the diode array requires much more high voltage. If U is reversed, no current flows, but for high U (eg 20V) 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 lead and the body has a cathode side plate.

Dependence of voltage on luminaire type

With the rise of high-brightness LEDs designed to provide replacement bulbs for commercial and indoor lighting applications, there is an equal, if not greater, 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 ratings, not to mention mechanical dimensions and many other features for dimming, remote control and circuit protection.

There are a large number of different LEDs on the market. Their differences are determined by many factors in the production of LEDs. Semiconductor makeup is a factor, but manufacturing 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, which determines the shape of the light beam. The color of the emitting element improves diffusion and contrast. The most common designations and forms of LEDs:

• A: Red diameter 3mm in CI holder.
• B: red 5mm diameter used in front panel.
• C: purple 5 mm.
• D: two-color yellow and green.
• E: rectangular.
• F: yellow 3 mm.
• G: white high brightness 5mm.
• H: red 3mm.
• K-Anode: A cathode designated by a flat surface in a flange.
• F: 4/100mm anode connecting wire.
• C: Reflective cup.
• L: Curved shape, acting like a magnifying glass.

Device Specification

A summary of the various LED parameters and supply voltage can be found in the seller's specifications. When selecting LEDs for specific applications, it is important to understand their differences. There are many different LED specifications, each of which will influence the specific type you choose. The basis of LED specifications are color, U and current. LEDS tend to provide a single color.

The color emitted by an LED is defined in terms of its maximum wavelength (lpk), which is the wavelength that has maximum light output. Typically process variations produce 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 shades or color variations around the yellow/orange region of the spectrum - from 560 to 600 nm. This may affect the choice of LED color or position, which is directly related to electrical parameters.

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

Some LEDs may include a series resistor, so this indicates what voltage the LEDs need to supply. LEDs do not allow a large reverse U. It should never exceed its stated maximum value, which is usually quite small. If there is a possibility of a reverse U occurring 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. Multiple 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 I-V characteristic. As a result, each diode emits a different luminous flux.

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

There are two solutions:

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

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

Advantages of LED voltage selection

Correctly calculating the supply voltage of LEDs in a lamp has 5 key advantages:

1. Safe ultra-low U possible regardless of the number of LEDs. LEDs must be installed in series to ensure the same level of current flows into each from the same source. As a result, the more LEDs there are, the higher the voltage at the LED terminals. If it is a device with an external driver, then the ultra-sensitive safety voltage must be significantly higher.
2. Integrating the driver inside the lights allows for a complete Safety Extra Low Voltage (SELV) system installation, regardless of the number of lights.
3. More reliable installation in standard wiring for LED lamps connected in parallel. The drivers provide additional protection, especially against temperature rise, which guarantees a longer service life while maintaining the LED supply voltage for different types and current. Safer commissioning.
4. Integrating LED power into the driver avoids mishandling in the field and improves their ability to withstand hot plugging. If a user only connects an LED light to an external driver that is already turned on, this may cause the LEDs to overvoltage when connected and therefore destroy them.
5. Easy maintenance. Any technical problems are more easily visible in LED lamps with a voltage source.

When the U drop across a resistance is important, you need to choose the right resistor that can dissipate the required power. A current consumption of 20 mA may seem low, but the calculated power suggests 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 - resistor value, Ohm.

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

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

Thermal control

Finding the optimal wattage for your system will help you learn more about the heat control you will need to ensure reliable LED operation, as LEDs generate heat that can be very harmful to the device. Too much heat will cause the LEDs to produce less light and also reduce operating time. For an LED rated at 1 watt of power, it is recommended to look for a heatsink that measures 3 square inches for each watt of LED.

Nowadays, the LED industry is growing at quite a fast pace and it is important to know the difference in LEDs. This 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, as a rule, they do not last long and burn out quickly due to poor parameters. When manufacturing LEDs, the manufacturer indicates characteristics with average values ​​in the data sheets. 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 find out the LED supply voltage, set the appropriate multimeter settings: current and U. Before testing, set the resistance to the highest high value to avoid LED burnout. This can be done simply: clamp the leads of the multimeter, adjust the resistance until the current reaches 20 mA and record the voltage and current. In order to measure the forward voltage of LEDs you will need:

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

The primary blue LED current was 3.356 V at 19.5 mA. If 3.6V is used, the resistor value to use is calculated as 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 high power smd leds with forward current >350mA can be a little tricky because when they heat up quickly the U drops sharply. This means that the current will be higher for a given U. If the user fails, he will have to cool the LED to room temperature before measuring again. You can use 500 ohms or 1 kohms. To provide coarse and fine tuning or connect a higher and lower range variable resistor in series.

Alternative definition of voltage

The first step to calculate LED power consumption is to determine the LED voltage. If you don’t have a multimeter at hand, you can study the manufacturer’s data and find the data sheet U of the LED block. Alternatively, U can be estimated based on the color of the LEDs, for example, a white LED supply voltage of 3.5 V.

After the LED voltage is measured, the current is determined. It can be measured directly using a multimeter. The manufacturer's data provides an approximate current estimate. After this, you can very quickly and easily calculate the power consumption of the LEDs. To calculate the power consumption of an LED, simply multiply the LED's U (in volts) by the LED's 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 power. Depending on the size and scope of the project, voltage and current readings may be measured in units smaller or larger than base current or watts. Unit conversions may be required. When performing these calculations, remember that 1000 milliwatts equals one watt, and 1000 milliamps equals one amp.

To test the LED and find out if it works and what color to choose, use a multimeter. It must have a diode test function, which is indicated by a diode symbol. Then, for testing, the multimeter test cords are attached to the LED legs:

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

To test the diode, the display must be between 400 and 800 mV in one direction and not in the opposite direction. Normal LEDs have threshold Us described in the table below, but for the same color there can be 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. LED threshold U

If the battery is fully charged, then at 3.8 V the current is only 0.7 mA. IN recent 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 packages, as well as diodes for mounting on a printed circuit board up to 1 mm long.

LEDs are generally considered constant current devices, operating on a few volts of DC. In low-power applications with a small number of LEDs, this is a perfectly acceptable approach, such as mobile phones where power is supplied from a DC battery, but other applications, such as a linear strip lighting system extending 100m around a building, cannot function with this design.

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

Lynk Labs has developed technology that allows LEDs to be powered from alternating voltage. A new approach is to develop AC LEDs that can operate directly from an AC power source. Many freestanding LED fixtures simply have a transformer between the wall socket and the fixture to provide the required constant U.

A number of companies have developed LED bulbs that screw directly into standard sockets, but they invariably also contain miniature circuits that convert AC to DC before going to 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 approximately 3.0 to 3.6 V. The table below shows some typical voltages. Values ​​in parentheses correspond to the closest normalized values ​​in the E24 series.

The supply voltage specifications for 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 a lighting project.

The charging current indicator can be assembled on a luminescent indicator, or on LEDs.

To measure current with more or less tolerable accuracy, you need to assemble a voltage amplifier from a shunt on LM358 and the indicator itself on two LM324s or on KT315s and that’s it :-). I will give a separate diagram of the amplifier, with a simple board, and separately of the indicator itself. Fastening inside is better and easier. There are two options for indicators.

Amplifier circuit. Diode D1, resistor R3, capacitor C3 is an integrating circuit, since at the input there is a pulsating voltage of negative polarity, and we need to obtain a constant voltage proportional to the current at the output. Setup: be sure to check 12 volts, defective banks are often encountered, then resistor R2 is used to calibrate the indicator readings using a multimeter. Use the current adjustment resistor to set the maximum current and adjust the resistor so that the last LED just lights up. Capacitor C3 works as an integrator and sets the smoothness of the decline in the indicator readings.

Photo of the assembled voltage amplifier boards from the shunt (the trimmers are not yet soldered).

Indicator diagram for KT 315. Of course, “last century” and all that, you say, but what if there are 3 of them? liter jar. Where do you tell me to go? Throw it away? But you have to go to the market and buy SMD transistors, but there is still a lot of space in the case. There is no need to drill holes for 315 either. But still, it’s your choice, the circuit is not critical to the choice of transistors, even if you solder the MP10, it will still work.

The number of transistors and LEDs can be reduced, for example to 6 pieces, but when there are many, it is more beautiful. Photo of the assembled line, without soldered LEDs yet.

And an earlier layout:

The emitter follower does not need to be soldered, but can be turned on directly; it works without it, only the readings drop off quickly, and not smoothly over one LED. Sometimes on some copies it was necessary to include a directly connected diode, such as KD522, between the amplifier output and the line. This was necessary when one or two of the first LEDs glowed at zero current. Setting up the line. A correctly assembled indicator without errors works immediately. We connect a variable resistor to the input - a slider to the input, the right end of the resistor to +, the left to -. We apply power, rotate the resistor and look at the LEDs, they should alternately flash and go out. This indicator has a significant non-linearity of readings (at first there is a blockage and there are humps in the middle), but it is quite suitable for a charger. When setting up, simply mark the value of each LED.

In the block diagram on the board, you need to add a 6...8V source for the LED line. For a luminescent indicator, you do not need to add this source.

Photo of the assembled charging according to the above diagrams, but on an ATX unit (there is no particular difference with the AT, the only difference is that the TL494 is powered from the standby):

Photo of the amplifier board mounting. It is soldered into the main board with pins: housing and +22V.

Below is a diagram of an indicator using operational amplifiers. It is better to use it as an indicator luminescent indicator(the diagram is simpler). If you use LEDs, you will need to add 8 more 2k resistors and connect them with cathodes to the housing. The operating principle is simple. The circuit does not need adjustment, except for selecting a resistor in the heating circuit.

This circuit uses two quad amplifiers to form eight levels of indication. The operational amplifiers used in this circuit are LM324 (Or LM393 if you are using LEDs. Then we connect their anodes to +, and each cathode to its own output). This is a fairly common IC and it won’t be difficult to find. Resistors R2:.R10 form a divider that sets the response thresholds of each amplifier. The amplifiers operate in comparator mode.

Photo of the assembled current indicator on a luminescent indicator:

Attached to the front wall using hot glue gun or soldering iron.

The above circuit has a soft charging current characteristic. The current decreases smoothly throughout the charging time (Like in a car).

The setup consists of selecting R3 depending on your shunt, and selecting R5 to limit the maximum output current to 10 amperes. Improvements to the indicator lines consist only of installing and adjusting the trimmer resistance for the current display range of 3 - 10 amperes. Setting the current channel. We temporarily replace resistor R5 with a 10k trimmer and set it to the position of maximum resistance. We connect the multimeter in current measurement mode on the 10 ampere range. We connect the unit to the network through a light bulb. If the light flashes and continues to glow brightly, it means something is wrong, check the installation. If the ammeter shows a current in the range from 0.2 to 1 ampere, then everything is fine. We set the variable resistor R6 to the maximum voltage mode with the slider, and use the trimming resistor to set the current to 10 amperes. Then we unsolder the trimmer, measure and solder in a constant resistor of the same resistance. The operation and configuration of the voltage channel is similar to the first circuit.

Let us dwell in more detail on protection against polarity reversal and short circuit. The scheme is a kind of “KNOW-HOW” in its simplicity and reliability. The advantage is that you do not need to use a powerful relay or thyristor, which has a voltage drop of about two volts. The circuit as an independent device can be built into any charger and power supply. Exit from the protection mode is automatic as soon as the short circuit or overpolarity is eliminated. When triggered, the “connection error” LED lights up.

Job description: In normal mode, the voltage through the LED and resistor R9 unlocks VT1 and all voltage from the input goes to the output. During a short circuit or polarity reversal, the current pulses sharply increases, the voltage drop across the field switch and the shunt increases sharply, which leads to the opening of VT2, which in turn bypasses the gate source. The additional negative voltage relative to the source (drop across the shunt) covers VT1. Next, an avalanche process of closing VT1 occurs. The LED is illuminated through open VT2. The circuit can remain in this state for as long as desired until the short circuit is eliminated.

A compact and simple indicator can be used to indicate the current of low and medium power heating elements. A typical example is an aquarium heater. Often such products are equipped with an LED indicator, but it is assembled according to the voltage indicator circuit. Such inclusion makes it possible for the heating coil to burn out, but the indicator continues to light. The circuit proposed below is connected in series with the load, and the LED lights up only when current passes through the heater.

With the proposed parts, the indicator can be assembled even by a novice electronics engineer. In principle, it is enough not to be afraid of a soldering iron and to know that diodes have an anode and a cathode. Below is a photograph of the assembly of the diode part of the circuit that fits on the electrical terminal block.

Example of turning on diodes

The circuit consists of only three or four diodes and uses their forward voltage, which inevitably appears on these semiconductors when forward current passes. In this case, two diodes connected in series perform the function of a stabistor; the voltage that appears on them when current passes through the load is stabilized at a level of 1.5-2.5 Volts.

Current inlicator circuit with red LED

The circuit uses elements from the Soviet period, KD105B diodes and AL307B red LED. If these elements are used and they are in good working order, the circuit will work without adjustment.

For beginners. In this circuit, it is not necessary to understand where the diode is plus and where the minus is. The elements are connected according to the principle of two consecutive ones in one direction with a mark, one in the opposite direction. A load, for example a light bulb, is connected to the output, to the input of the 220 Volt circuit. The light should light up. Next, carefully, without touching the current-carrying parts of the circuit with your fingers, connect the LED. If the LED lights up, then it should be soldered in this position; if it doesn’t light up, then it is turned over the other way around.

Possibility of changing the current indicator circuit and increasing the load power

The load power of such a circuit is limited only by the maximum forward current of the diodes. For KD105 and D226 this current is 300mA, that is, the maximum load power in this case is P 0.3 * 2 * 220 = 132 W. If, for example, we take D245 diodes with Ipr.sr = 10A, then the load power can be increased to 4400 W.

When replacing diodes from a circuit, their forward average voltage should be taken into account. For example, germanium semiconductors have a lower forward voltage, and in this case the LED will not light up, or you will have to connect three or even four such diodes in series.

Naturally, the maximum reverse voltage VD1 - VD3 must be at least 300 Volts.

When replacing a red AL307B LED in a circuit with a green one (AL307V), you need to take into account that the glow voltage of green, orange, white and others, including Chinese LEDs, can be greater than the Upr of two KD105 diodes. In this case, three or even four diodes can be connected in series.

Current indicator circuit for green LED

I practically experimented with AL307V, a Chinese yellow and bright white LED. Green and yellow lit up with three KD105s, while white required four. For the experiments, a load in the form of a 40-Watt incandescent lamp was used.

You should not overuse the amount of KD105, since in this case the voltage on the LED increases and you will have to limit its current with a resistor

Design and installation

Given the simplicity and compactness of the circuit, it can be installed in almost any electrical product. The photo shows a regular socket and a small patch panel (terminal block)

The LED is glued into the socket cover and in this case soldered to the diodes with wires from the connecting cable of the TPP (cross-connection)

Final view of the installed indicator

I have used a similar scheme many times; I was previously interested in aquarium farming and all aquarium heaters were turned on through similar indicators. When I needed to construct a warmer for a potato box on my balcony, I didn’t hesitate to use this diagram; in fact, I took all the pictures at the assembly stage. Posting this article on your website is somehow out of topic: my website is for connected cable technicians and meters, but here is everyday life and electronics.

N. TARANOV, St. Petersburg

When developing various radio-electronic devices, the problem of monitoring the presence of current in their circuits arises. Off-the-shelf measuring devices are often unavailable, expensive, or difficult to use. In such cases, built-in control units are used. For alternating current, the problem is relatively easily solved with the help of current transformers, induction magnetosensitive elements, etc. For direct current, as a rule, this problem is more complicated. The article discusses some existing devices for monitoring the presence of direct current in a circuit (hereinafter we will call them direct current indicators, or abbreviated as IPT), their advantages and disadvantages, and proposes circuit solutions that improve the characteristics of these devices.

IPTs are usually included in a break in 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 discussed in this article. IPT can be characterized by the following main parameters and features:
1) deltaU - voltage drop across the IPT over the entire range of controlled currents. To minimize the influence of IPT on the controlled circuit and reduce power losses, they strive to minimize deltaU;
2) Inom rated operating current (means the average value of the controlled current);
3) Imin, Imax - boundaries of the range of changes in 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) the presence or absence of additional power sources for IPT;
6) the presence or absence of galvanic connection of the IPT output signal with the controlled circuit.

Based on the type of current-sensing element - current sensor (CT) they are distinguished;
- IPT with series load in the circuit;
- IPT with semiconductor DTs (Hall sensors, magnetodiodes, magnetoresistors, etc.);
- IPT magnetic contact (on reed switches, on current relays);
- IPT with magnetically saturable elements.

Operating principle of IPT with series load in the circuit (Fig. 1)

It consists in the fact that a load element (LE) is connected to the break in the controlled circuit, on which a voltage drop is created when current flows in the controlled circuit. It is sent to a signal converter (SC), where it is converted into a signal indicating the presence of current in the circuit.

Obviously, deltaU for a given type of IPT depends on the magnitude of the controlled current and on the sensitivity of the PS. The more sensitive the PS, the lower the NE resistance can be used, which means the deltaU will be smaller.

In the simplest case, an NE is a resistor. The advantage of such NE is its simplicity and low cost. Disadvantages - with low sensitivity of the PS, power losses on the NE will be large, especially when controlling large currents, the dependence of AU on the magnitude of the current flowing through the IPT. It narrows the range of changes in the controlled current (this drawback is not significant when controlling the current in a narrow range of changes in its value). As an example, consider a practical IPT scheme of this type. In Fig. Figure 2 shows a diagram of the indicator for the presence of charging current for the battery. The resistor R1 acts as an NE, and the chain R2, HL1 acts as a PS.

Ballast resistor R2 has a resistance of 100 Ohms, LED HL1 has a rated current of 10 mA (for example, type AL307B), and the resistance of 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 eliminated. With a charging current of 1 A, the resistance of 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. It is possible to somewhat reduce the power losses on the IPT if you reduce the resistance of the ballast resistor R2. But it is undesirable to do this, since accidental surges in charging currents may damage the HL1 LED.

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

As diodes VD1-VD4, you can use any rectifying silicon 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 resistor R2 can in this case be reduced to a value of 30 ohms.

With this IPT scheme, the range of controlled currents expands and extends from 10 mA to Imax, where Imax is the maximum permissible 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 characteristics of an IPT with a series load in a circuit is to improve the PS. Indeed, if you increase the sensitivity of the PS and ensure its performance in a wide range of deltaU changes, you can obtain an IPT with good characteristics. True, for this you will have to complicate the IPT scheme. As an example, consider the IPT scheme developed by the author, which has shown good results in process control devices in industry. This IPT has the following technical specifications: operating current range - 0.01 mA...1 A; deltaU
The IPT diagram is shown in Fig. 4.

The 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 operational amplifier DA1 will have a voltage close to -5 V, and the HL1 LED will not light up. When a current appears between points A and B, a voltage is created on 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 light up, indicating the presence of current between points A and B. When choosing an operational amplifier with a high gain (for example, KR1401UD2B), reliable indication of the presence of current begins at 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 if there is no current in the controlled circuit. This drawback is eliminated by introducing a “zero correction” circuit of the op-amp, made according to any standard circuit. Some types of op-amps have special terminals for connecting a variable resistor "zero correction".

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 unipolar supply voltage of 5 V. (The last two requirements are due to the use of a “convenient” supply voltage IPT, although it is possible to use other supply voltages. When In this case, the resistance of the ballistic resistor R5 will need to be recalculated so that the output current of the operational amplifier DA1 does not exceed its maximum permissible value). The HL1 LED was chosen in this way for reasons of sufficient brightness at a current through it of 2.5 mA. 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 operational amplifier DA1).

This device with the KR1401UD2B microcircuit is convenient when building a four-channel IPT, for example, when controlling the separate charging of four batteries simultaneously. 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, you need to reduce the resistance of 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). In this case, deltaU did not exceed 0.1 V.

With minor modifications, 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 regulates the IPT response threshold. It is convenient to use a multi-turn resistor as R4, for example, types SP5-2, SPZ-39, etc.

If it is necessary to ensure galvanic isolation between the controlled circuit and control devices (CDs), 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.

To match the output signal of this IPT with digital control devices, Schmitt triggers are used. In Fig. Figure 7 shows a scheme for coordinating the IPT with the CC using TTL logic. Here +5 V CC is the supply voltage of the digital circuits of the CC.

IPTs with semiconductor DTs are described in detail in the literature. Of interest to radio amateurs is the use of magnetically controlled microcircuits of the K1116KP1 type in IPT (this microcircuit was widely used in the keyboards of some Soviet-made computers). The diagram of such an IPT is shown in Fig. 8.

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

The DA1 chip is placed in the gap of the magnetic concentrator. When manufacturing it, we must strive to reduce the gap. Experiments were carried out with various magnetic circuits, in particular, rings cut from ordinary water pipes, machined from dynamic head cores, and assembled from transformer steel washers were used.

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

The experimental sample had a magnetic core made from a water pipe with a diameter of 3/4 inch. 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.

The most effective were rings made from cores from magnetic systems dynamic heads, but their manufacture under amateur conditions is difficult.

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

More information about reed switches can be found in. The electrical circuit of the IPT with a current sensor (CT) on the reed switch is shown in Fig. 10, b.

Many radio amateurs probably have an old Soviet-made PC keyboard with reed switches. Such reed switches are perfect for implementing IPT. The sensitivity of IPT depends on:
- the number of turns in the winding (as the number of turns increases, the sensitivity also increases);
- winding configuration (the optimal winding is 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. The best results in terms of sensitivity were shown by KEM-2 reed switches. When winding eight turns of PEV-2 wire with a diameter of 0.8 mm without a gap, the operating current of the IPT 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 with a screw or placed in a non-magnetic bushing with an external thread, which is screwed into a coil with a winding. This method of adjusting 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 reducing 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 and a number of turns of 200 is introduced into the DT design. It is advisable to wind this winding directly onto the reed switch along the entire length of its cylinder, as shown in Fig. 11, a.

The electrical circuit of the IPT is shown in Fig. 11, b.

Winding L1 is the main winding, winding L2 is additional. If you turn on the windings L1 and L2 accordingly, then by adjusting the resistor R1 it is possible to increase the sensitivity of the IPT many times compared to the IPT version 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. An experiment was conducted 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 a reed switch type KEM-2;
- winding L2 - 10 turns of PEV-2 wire with a diameter of 0.8 mm, wound over winding L1.

The following Imin values ​​were obtained:
- when the windings are switched on in agreement -0.1...2 A;
- when the windings are turned on oppositely -2...5 A.

IPT on current relays have the qualities of: DT electromagnetic relay with low-resistance winding. Unfortunately, current relays are in very short supply. A current relay can be made from a conventional voltage relay by replacing its winding with a low-impedance 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 in its place a new winding is wound with PEV-2 wire with a diameter of 0.3 mm until the frame is filled. The sensitivity of this DT is adjusted by selecting the number of turns and changing the rigidity of the flat armature spring. The stiffness of the spring can be changed by bending it or grinding it along the width. The experimental DT sample had Imin = 200 mA, deltaU = 0.5 V (at a current of 200 mA).

If you need to calculate current relays, you can refer to.

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

IPTs with magnetically saturable elements are of particular interest. They use the property of ferromagnetic cores to change permeability when exposed to an external magnetic field. In the simplest case, an IPT of this type 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 winding L3 is detected by diode VD1 and charges capacitor C1. Then it is fed to the threshold element. In the absence of current in winding L1, the voltage created on capacitor C1 is sufficient to trigger the threshold element. When direct current is passed through winding L1, the magnetic circuit is saturated. This leads to a decrease in the transfer coefficient of alternating voltage from winding L2 to winding L3 and a decrease in the voltage on capacitor C1. When it reaches a certain value, the threshold element switches. Choke 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 conductivity of the controlled circuit.

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

The advantages of the device are ease of implementation, lack of mechanical contacts.

Its significant drawback is the penetration of alternating voltage from the IPT into the controlled circuit (however, in most applications, the 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 windings L2 and L3 to the number of turns of winding L1 and with an increase in the inductance of inductor L4.

An experimental sample of this type of IPT was assembled on a ring magnetic core of standard size K10x8x4 made of 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. Choke L4 was 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 μF. One inverter of the K561LN1 microcircuit was used as a threshold element. A rectangular voltage (“meander”) with a frequency of 10 kHz and an amplitude of 5 V was applied to winding L2. This IPT reliably indicated the presence of current in the controlled circuit in the range of 10... 1000 mA. Obviously, to expand the range of controlled currents towards increasing the upper limit, it is necessary to increase the diameter of the wire of the windings L1 and L2, and also to select a larger magnetic core.

The IPT circuit of this type, shown in Fig., has significantly better parameters. 14.

Here, the magnetic core of the transformer consists of two ferrite rings, windings L1 and L3 are wound on both rings, and 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. 15.

For clarity, the cores are spaced apart; in the actual design they are pressed against each other.

In this type of IPT there is almost completely 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 conductivity of the controlled circuit. An experimental sample of the IPT was manufactured, the diagram of which is shown in Fig. 16.

A high duty cycle pulse generator is assembled on inverters D1.1-D1.3 (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 doubling the voltage. Inverter D1.4 together with LED HL1 provides threshold indication of the presence of pulses at the output of the transformer (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 instruments 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.

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There may be a need to monitor the presence of current flowing in a circuit in two states: either present or not. Example: you are charging a battery with a built-in charging 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 this all the time. It’s easier to once build a charge flow indicator into the power supply, 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 a car that you don’t see and don’t know whether it’s on or has burned out. You can also include a current indicator in the circuit to this lamp and monitor the flow. If the lamp burns out, it will be immediately visible.
Or there is some kind of sensor with a filament. Tapa gas or oxygen sensor. And you need to know for sure that the filament has not broken and everything is working properly. This is where the indicator comes to the rescue, the diagram of which I will give below.
There can be a lot of applications, of course the main idea is the same - monitoring the presence of current.

Current indicator circuit