Protection of the power supply against short circuit. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Power Supplies

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To power the assembled structures, radio amateurs often use the simplest blocks, consisting of a step-down transformer and a rectifier with a filter capacitor. And, of course, in such blocks there is no protection against short circuit (short circuit) in the load, although it sometimes leads to the failure of the rectifier and even the transformer.

It is not always convenient to use a fuse in such power supplies as a protection element, and, moreover, its speed is low.

One of the options for solving the problem of short circuit protection is to connect a medium power field effect transistor with a built-in channel in series with the load. The fact is that on the current-voltage characteristic of such a transistor there is a section where the drain current does not depend on the voltage between the drain and the source. Therefore, in this section, the transistor works as a current stabilizer (limiter).

Fig. 1

The connection diagram of the transistor to the power supply is shown in fig. 1, and the current-voltage characteristics of the transistor for various resistances of the resistor R1 - in fig. 2. Protection works like this. If the resistance of the resistor is zero (i.e., the source is connected to the gate), and the load draws a current of about 0,25 A, then the voltage drop across the field-effect transistor does not exceed 1,5 V, and practically all the rectified voltage will be on the load. When a short circuit appears in the load circuit, the current through the rectifier increases sharply and, in the absence of a transistor, can reach several amperes. The transistor limits the short circuit current to 0,45...0,5 A, regardless of the voltage drop across it. In this case, the output voltage will become zero, and the entire voltage will drop across the FET. Thus, in the event of a short circuit, the power consumed from the power source will not more than double in this example, which in most cases is quite acceptable and will not affect the "health" of the power supply parts.

Fig. 2

You can reduce the short circuit current by increasing the resistance of the resistor R1. It is necessary to choose a resistor such that the short-circuit current is approximately twice the maximum load current.

This protection method is especially convenient for power supplies with a smoothing RC filter - then the field effect transistor is turned on instead of the filter resistor (such an example is shown in Fig. 3).

Since almost all of the rectified voltage drops on the field effect transistor during a short circuit, it can be used for light or sound signaling. Here, for example, is a diagram for switching on a light signal - fig. 7. When everything is in order with the load, the green LED HL2 is on. In this case, the voltage drop across the transistor is not enough to ignite the HL1 LED. But as soon as a short circuit appears in the load, the HL2 LED goes out, but HL1 flashes red.

Fig. 3

Resistor R2 is selected depending on the desired short-circuit current limitation according to the above recommendations.

The connection diagram of the sound signaling device is shown in fig. 4. It can be connected either between the drain and the source of the transistor, or between the drain and the gate, like the HL1 LED.

When sufficient voltage appears on the signaling device, the AF generator, made on a unijunction transistor VT2, comes into action, and a sound is heard in the BF1 headphone.

The unijunction transistor can be KT117A-KT117G, the phone is low-resistance (can be replaced with a low-power dynamic head).

Fig. 4

It remains to be added that for low-current loads, a short-circuit current limiter on a KP302V field-effect transistor can be introduced into the power supply. When choosing a transistor for other blocks, its allowable power and drain-source voltage should be taken into account.

Of course, such automation can also be introduced into a stabilized power supply that does not have protection against short circuits in the load.

Author: I. Nechaev, Kursk; Publication: cxem.net

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