Stabilizer protection. Encyclopedia of radio electronics and electrical engineering

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

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Overloading a stabilized rectifier due to a short circuit in the load or for another reason usually leads to failure of the regulating transistor. You can protect the stabilizer from overload using the proposed device.

The protective device included in the stabilizer of the power supply, the circuit of which is shown in the figure, has high speed and good "relay", i.e., a small effect on the characteristics of the unit in operating mode and reliable closing of the regulating transistor V2 in overload mode. The protective device consists of a trinistor V3, diodes V6, V7 and resistors R2 and R3. In operating mode, the trinistor V3 is closed and the voltage at the base of transistor V1 is equal to the stabilization voltage of the zener diode chain V4, V5. When overloaded, the current through the resistor R2 and the voltage drop across it reach values ​​sufficient to open the trinistor V3 through the control electrode circuit. The opened trinistor closes the chain of zener diodes V4, V5, which leads to the closing of transistors V1 and V2.

In order to restore the operating mode after the cause of the overload has been eliminated, press and release the S1 button. In this case, the trinistor will close, and the transistors V1 and V2 will open again. Resistor R3 and diodes V6, V7 protect the control transition of the trinistor V3 from current and voltage overloads, respectively.

The stabilizer provides a stabilization coefficient of about 30, the protection is triggered at a current exceeding 2 A.

Transistor V2 can be replaced with KT802A, KT805B, and V1 - P307, P309, KT601, KT602 with any letter index. Trinistor V3 can be any of the KU201 series, except for KU201A and KU201B.

The power supply stabilizer, the circuit of which is shown in the following figure, can be protected from overloads and short circuits of the load by adding just two elements - the trinistor V3 and the resistor R5.

The protective device is activated when the load current exceeds the threshold value determined by the resistance of the resistor R5. At this moment, the voltage drop across the resistor R5 reaches the opening voltage of the trinistor V3 (about 1 V), it opens, and the voltage at the base of the transistor V2 decreases almost to zero. Therefore, the transistor V2, and then V4 is closed, turning off the load circuit.

To return the stabilizer to its original mode, briefly press the button S1. Resistor R3 serves to limit the base current of transistor V4. Resistor R5 is wound with copper wire. The output impedance of the stabilizer can be reduced if R5 is turned on as shown in the dashed line diagram. If false positives are observed when the stabilizer is turned on, capacitor C2 should be excluded from the device. The maximum load current is 2 A.

Instead of the transistor P701A, you can use KT801A, K.T801B. Transistor V2 can be replaced with KT803A, KT805A, KT805B, P702, P702A.

The protective device shown in the following figure is assembled on transistors V1 and V2 (it also includes resistors R1-R4, a zener diode V3, a switch S1 and an incandescent lamp H1).

(click to enlarge)

The required value of the operating current is set by switch S1. In operating mode, due to the base current flowing through the resistor R1 (R2 or R3), the transistor V1 is open and the voltage drop across it is small. Therefore, the current in the base circuit of the transistor V2 is very small, the zener diode V3 is connected in the forward direction, and the transistor V2 is closed.

With an increase in the load current of the stabilizer, the voltage drop across the transistor V1 increases. At some point, the zener diode V3 opens, followed by the opening of the transistor V2, which leads to the closing of the transistor V1. Now almost the entire input voltage drops on this transistor, and the current through the load drops sharply to several tens of milliamps. Lamp H1 lights up to indicate that the fuse has blown. It is returned to its original mode by briefly disconnecting it from the network. The stabilization coefficient is about 20.

Transistors V1 and V7 are mounted on heat sinks with an effective heat dissipation area of ​​about 250 cm2 each. Zener diodes V4 and V5 are mounted on a copper heat sink plate measuring 150 x 40 x 4 mm. The establishment of an electronic fuse is reduced to the selection of resistors Rl-R3 for the required operating current. Lamp H1 type KM60-75.

The electronic-mechanical protection device works in two stages - first it turns off the power of the electronic device, then it completely blocks the load with the contacts K1.1 of the electromechanical relay K1.

It consists of a transistor V3 loaded with a two-winding electromagnetic relay K1, a zener diode V2, diodes V1, V4 and resistors R1 and R2.

The cascade on the transistor V3 compares the voltage across the resistor R2, which is proportional to the load current of the stabilizer, with the voltage on the zener diode V2, connected in the forward direction. When the stabilizer is overloaded, the voltage across the resistor R2 becomes greater than the voltage across the zener diode, and the transistor V3 opens. Due to the action of positive feedback between the collector and base circuits of this transistor, a blocking process develops in the transistor V3 - relay K1 system.

The pulse duration is about 30 ms (in the case of using the RMU relay, passport RS4.533.360SP). During the pulse, the collector voltage of transistor V3 drops sharply. This voltage is transmitted through the diode V4 to the base of the regulating transistor V5 (the voltage at the base of the transistor becomes positive relative to the emitter), the transistor closes, and the current through the load circuit decreases sharply.

Simultaneously with the opening of the transistor V3, the current through the collector winding of the relay K1 begins to increase, and after 10 ms it operates, self-locks and turns off the load circuit with contacts K1.1. To restore the operating mode, the mains voltage is turned off for a short time. The protection is triggered at a current of 0,4 A, the stabilization coefficient is 50.

The circuit diagram of the protection device shown in the following figure uses a V6 dinistor optocoupler, which improves the performance of the protection.

When the load current is less than the threshold, the electronic key on transistors V1-V3 is open, the indicator lamp H! is on, and the optocoupler is off (the LED is off, the photothyristor is closed). As soon as the load current reaches the threshold value, the voltage drop across the resistors R5, R6 increases so much that the optocoupler turns on, through the photothyristor of which a positive voltage is supplied to the base of the transistor V1, and the electronic key closes. The device is returned to working condition by briefly pressing the button S1. The voltage at the load increases slowly, with the rate of charging of the capacitor C1. This eliminates current surges that cause either false tripping of the protection or failure of load parts when the power is turned on.

The threshold is set by resistor R5. Transistors V2, V3 require a heat sink with an area of ​​100-200 cm2. The maximum load current is 5 A, the minimum operating current is 0,4 A. This stabilizer can be used to power audio frequency amplifiers.

See other articles Section Protection of equipment from emergency operation of the network, uninterruptible power supplies.

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