Power supply overload protection device. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Power supply overload protection device

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Encyclopedia of radio electronics and electrical engineering / Protection of equipment from emergency operation of the network, uninterruptible power supplies

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It is advisable to use the proposed device in conjunction with an adjustable voltage stabilizer that does not have special protection units.

The device is designed to protect the regulating element of the voltage stabilizer from current and temperature overloads. Protection is triggered when:

exceeding the permissible (set) value by the load current;
short circuit at the output of the stabilizer;
exceeding the permissible dissipated power by the regulating element (heating of its body above 50...70°C).

Temperature sensor - thermistor RK1 (Fig. 1), mounted directly on the regulating element of the stabilizer. As the temperature increases, the resistance of the thermistor decreases, therefore, the voltage at the inverting input of the comparator at the op-amp DA1 also decreases. Resistors R1 and R2 form a voltage divider that sets the threshold for thermal protection - it is regulated by a tuning resistor R2.

Power supply overload protection device

If the voltage at the inverting input of the op-amp becomes less than at the direct one, the signal at its output will abruptly change from a low level to close to the supply voltage. As a result, the HL1 LED will turn on, the trinistor VS1 will energize the relay K1. which will work and disconnect the load with its normally closed contacts, the HL2 LED will turn on. The buzzer will emit a short beep.

On transistor VT1, a protection unit is assembled against exceeding the load current of the set value and against a short circuit at the output. Resistor R8 - current sensor. At a current value of about 1,5 A, a voltage drop across it opens the transistor, which, in turn, turns on the trinistor VS1.

The SB1 and SB2 buttons allow you to disconnect and connect the load to the power source, which is necessary in the process of establishing a powered device. If the protection is triggered as a result of overheating of the regulating element, the load will not be connected until its temperature decreases, as judged by turning off the HL1 LED.

The design of the protection device depends on the size of the power supply used. If necessary, the sound indicator can be omitted.

In the proposed version, resistors MLT-0,125, S1-4-0,125, S2-33-0,125 and the like are applicable. Resistor R8 is made of nichrome or manganin wire with a diameter of 0,5 mm. its resistance is determined based on the required protection operation current according to the formula

R8 \u0,7d XNUMX / Iprotect., Where Iprotect - protection operation current.

You can also install several resistors of different resistances for different values of the protection operation current (Fig. 2).

Power supply overload protection device

Switch contacts SA1 must be rated for the maximum load current. Relay K1 should be selected so that it operates reliably at the rated voltage at the input of the stabilizer. It is important that its contacts can withstand the maximum current. The resistance of the resistor R10 is selected depending on the relay used. Of the domestic ones, for example, the KUTs-1, REK-53 relays, installed in televisions of the fourth and fifth generations, are applicable.

If the operating voltage of the relay is within 5 ... 15 V, the sound indicator is connected, as shown in fig. 1. if more - you should use a parametric stabilizer for a voltage of 9 ... 12 V.

Oxide capacitors - K50-16 or similar, the rest - KM-6 and the like. Diodes - any low-power silicon, transistor - also any low-power silicon, for example, the KT361 series. KT3107 Thermistor - MMT-4. it is fastened with a bracket directly to the regulating element of the stabilizer, using an insulating gasket if necessary, since the thermistor body is connected to one of its terminals.

A properly assembled device does not require adjustment. It is only recommended to experimentally select the resistance of the resistor R8 for a certain protection trip current. Resistor R2 sets the thermal protection threshold by measuring the temperature of the control element with a thermometer or, in extreme cases, by touch. However, be careful and careful not to get burned.

To protect the transistor VT1 from overload, it is advisable to include a resistor with a resistance of 220 ... 510 Ohm in the circuit of its base output.

Author: L. Morokhin, village Makarovets, Moscow region

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