current limit indicator

Encyclopedia of radio electronics and electrical engineering / Power Supplies

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When operating laboratory power supplies, it is often necessary to control the current consumed by the load. In most cases, for this purpose, a low-resistance resistor (current sensor) is introduced into the output circuit of the unit, and a pointer device (milli- or microammeter) is connected in parallel with it. For a complete deflection of its arrow, 0,3 ... 0,5 V is often required, therefore, no less voltage should fall on the sensor. If the radio amateur has a pointer device at his disposal that requires more voltage, this control option is unacceptable. Firstly, because a noticeable part of the output voltage will drop on the current sensor, and secondly, due to the significant release of heat on it at currents of more than 1...2 A.

A possible way out in such a case is the use of the device described below, which makes it possible to reduce the resistance of the current sensor to an acceptable value. In addition, in this device it is easy to implement a light or sound signaling that the output current exceeds a predetermined value.

The schematic diagram of the device is shown in fig. one.

As you can see, it was made on a dual op-amp LM358, capable of operating with a unipolar supply, and a transistor VT1. A current converter is assembled on one of the op amps (DA1.1) and a transistor, and a comparator is assembled on the other (DA1.2). The device is connected to the output circuit of the power supply in accordance with Fig. 2. At the same time, a voltage exceeding the voltage at its inputs is supplied to the power output of the op-amp DA1.1, which ensures its normal operation.

The converter works as follows. When the output current flows through the resistor R3 - the current sensor - a voltage drop is created. As a result, a voltage appears at the output of the op-amp DA1.1, which opens the transistor VT1, and current begins to flow through the resistors R1 and R2. Its value is set such that the voltage drops across resistors R1 and R3 equalize. In other words, a current flows through the transistor, approximately R1 / R3 \u1000d 1 times less than the output current of the power supply Iout. For example, if the latter is 2 A, a current of 1 mA flows through the resistor R1. With a resistance of this resistor equal to 2 kΩ, the voltage drop UR1 in this case will be 1 V, i.e. the current / voltage conversion ratio is 2. In general, UR3 = Iout (R1 / R2) RXNUMX. By changing the values ​​​​of the resistors, you can implement different conversion factors.

The output voltage of the converter UR2 is supplied to the non-inverting input of the op-amp DA1.2, and the exemplary voltage Uobr is supplied to the inverting input from the engine of the tuning resistor R6. If UR2 does not exceed Uobr, a voltage close to zero is maintained at the output of the op-amp DA1.2, and the HL1 LED is off. When UR2 exceeds Uobr, the voltage at the output of the op-amp will become equal to the supply voltage and the LED will start to glow, signaling that the output current has exceeded the set value.

The device is designed to work with power supplies, in which the voltage at the rectifier output is in the range from 5 to 32 V.

In the presence of a KT3130B-9 transistor, small-sized parts for surface mounting (for example, a K10-17v capacitor, P1-12 or similar foreign-made resistors and a tuning resistor of the POZ3AN RVG3A, RVG4A type), the device is mounted on a printed circuit board made of double-sided foil fiberglass, made in accordance with from fig. 3a (the foil of the second side is used as a common conductor). The placement of parts on the board is shown in fig. 3b. Printed conductors of different sides are connected by wire jumpers through the holes.

If you use a transistor of the KT3102 series (with index A, B or E), fixed resistors MLT, C2-33, trimmer SDR-19 and capacitor K10-17a, the dimensions of the board will have to be increased accordingly. Resistor R3 can be made from a piece of high-resistance (for example, constantan) wire. LED HL1 - any with a working current of up to 25 mA.

Establishing the device is reduced to the selection of resistors R1-R3 to obtain the required conversion factor. The value and power of the resistor R7 is selected based on the required current through the LED at a given voltage at the output of the power supply. Trimmer resistor R6 sets the ignition threshold of the LED.

When using a pointer device to control the current (it is connected to pins 3 and 5, the resistor R2 can be excluded in this case), it is necessary to adjust the current / voltage conversion factor so that the current values, up to the maximum allowable, can be easily read from the scale of the device . If you intend to enter a sound indication of exceeding the permissible output current, the sound generator is connected directly to the output of the op-amp DA1.2.

Author: I. Nechaev, Kursk

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