Power supply 0-12 volts 400 milliamps with short circuit protection. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Power Supplies

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This circuit is a simple transistor power supply equipped with short circuit protection (short circuit). Its scheme is shown in the figure.

The main parameters:

The scheme works as follows. The input voltage of the 220V network is converted by a transformer to 16-17V, then it is rectified by diodes VD1-VD4. The rectified voltage ripple is filtered by capacitor C1. Further, the rectified voltage is supplied to the VD5 zener diode, which stabilizes the voltage at its terminals up to 12V. The rest of the voltage is quenched on the resistor R2. Next, the voltage is adjusted with a variable resistor R3 to the required level within 0-12V. This is followed by a current amplifier on transistors VT2 and VT3, which amplifies the current to a level of 400 mA. The load of the current amplifier is resistor R5. Capacitor C2 additionally filters the output voltage ripple.

Defense works like this. In the absence of a short circuit at the output, the voltage at the terminals of VT1 is close to zero and the transistor is closed. The R1-VD1 circuit provides a bias at its base at a level of 0,4-0,7 V (voltage drop at the open pn junction of the diode). This bias is enough to open the transistor at a certain collector-emitter voltage level. As soon as a short circuit occurs at the output, the collector-emitter voltage becomes different from zero and equal to the voltage at the block output. Transistor VT1 opens, and the resistance of its collector junction becomes close to zero, and, therefore, on the zener diode. Thus, zero input voltage is supplied to the current amplifier, a very small current will flow through the transistors VT2, VT3, and they will not fail. Protection is disabled immediately when the short circuit is eliminated.

Details

The transformer can be any with a core cross-sectional area of ​​\u4b\uXNUMXbXNUMX cm² and more. The primary winding contains 2200 turns of PEV-0,18 wire, the secondary - 150-170 turns of PEV-0,45 wire. A ready-made vertical scan transformer from old tube TVs of the TVK110L2 series or similar is also suitable. Diodes VD1-VD4 can be D302-D305, D229Zh-D229L or any for a current of at least 1 A and a reverse voltage of at least 55 V. Transistors VT1, VT2 can be any low-frequency low-power ones, for example, MP39-MP42. More modern silicon transistors can also be used, for example, KT361, Kt203, KT209, KT503, KT3107 and others. As VT3 - germanium P213-P215 or more modern silicon powerful low-frequency KT814, KT816, KT818 and others. When replacing VT1, it may turn out that the short circuit protection does not work. Then one more diode (or two, if necessary) should be connected in series with VD5. If VT1 is silicon, then it is better to use silicon diodes, for example, KD209 (A-B).

In conclusion, it is worth noting that instead of the transistors indicated on the pnp circuit, npn transistors similar in parameters can also be used (not instead of any of the VT1-VT3, but instead of all of them). Then you will need to change the polarity of the diodes, zener diode, capacitors, diode bridge. At the output, respectively, the polarity of the voltage will be different.

Publication: cxem.net

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