This circuit is a simple power supply transistors, equipped with protection against short-circuit. His scheme shown in the figure.
Basic parameters:
- The output voltage is 0..12V;
- Maximum output current is 400 mA.
The scheme works as follows. Input voltage 220V converted by the transformer 16-17V, then rectified by diodes VD1-VD4. Filtering of ripple of the rectified voltage is performed by the capacitor C1. Further rectified voltage is supplied to the Zener diode VD5, which stabilizes the voltage at its findings to 12V. The remainder of the tension was dissipated on the resistor R2. Further, by adjustment of the voltage variable resistor R3 to the desired level in the range 0-12V. Then, the current amplifier transistors VT2 and VT3, which increases the current to 400 mA. Load current amplifier serves as a resistor R5. The Capacitor C2 additionally filters the output voltage ripple.
Protection works. In the absence of Short-circuit on the output voltage at conclusions VT1 close to zero and the transistor is closed. Circuit R1-VD1 provides a bias on its base at the level of 0.4-0.7 (fall the voltage on the open p-n junction diode). This displacement enough to open the transistor at a certain level voltage collector-emitter. As soon as the output is short circuit, voltage kolektor-emitter becomes great from zero and equal to the voltage at the output of the block. Transistor VT1 open, and his resistance of the collector junction becomes close to zero, and, hence, the Zener diode. Thus, the current amplifier receives a zero input voltage, via transistors VT2, VT3 will leak very little current, and they are not will be damaged. Protection is disabled immediately upon removal of short circuit.
Details
The transformer can be any of the cross-sectional area of the core 4 cm2 or more. The primary winding has turns 2200 of PEV-0,18, secondary - 150-170 turns of wire sew-0,45. Fit and ready transformer frame scanning old tube TVs series TCL or similar. Diodes VD1-VD4 can be D-D305, DI-DL, or any at least 1 A and reverse voltage not less than 55 V. the Transistors VT1, VT2 can be any low-power low-frequency, for example, MP39-MP42. Can to use and more modern silicon transistors, for example, KT361, CT, CT, CT, CT and others. As VT3 - germanium P-P or more modern silicon powerful low frequency CT, CT, CT and others. When replacing VT1 can to be that a short circuit protection does not work. Then consistently with VD5 to include another diode (or two, if will be required). If VT1 will be silicon, and the diodes is better to use silicon, for example, CD(A-b).
In conclusion, it is worth noting that is indicated in the diagram of p-n-p transistors can be used similar parameters transistors n-p-n (not instead of any of VT1-VT3, and instead of all of them). Then you need to change the polarity of the diodes, Zener diode, capacitor, diode bridge. At the output, accordingly, the polarity of the voltage will be different.
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