# 13.8V 20A power supply

Posted on Feb 6, 2014

It delivers safely around 20Amps at 13. 8V. For lower currents, a separate current limiting output, capable of 15ma up to a total of 20A has been added. Let us see what we have got here. The power transformer should be capable to deliver at least 25A at 17. 5 to 20V. Th

e lower the voltage, the lower power dissipation. The rectified current will be ironed  by the C1, whose capacity should not be less than 40. 000uF, (a golden rule of around 2000uF/A), but we recommend up to 50. 000uF. This capacity can be built up by several smaller capacitors in parallel. The base of this design is a simple 12V regulator (7812). The output voltage can be brought to desired value (here 13. 8V) by two external resistors (R5 and R6) using this formula: The low currents (here 15mA) will keep the 7812 in its regular function. As soon as the current rises over 15ma, the voltage drop on R4 will open  the Q3, actually handling the high output current. This is a PNP transistor (Ic>25) and current amplification factor of at least 20. The one that has been tested and proven here is the 2N5683. The current limiting resistance RL, for the maximum output of 20 Amps should be 0. 03 Ohms, rated at least 15W. You can use the resistance wire or switch several resistors in parallel, totaling the resistance/power values. Values for other currents can be calculated by the rule: The RL and Q2 (3A PNP such as BD330) form a short circuit automatic fuse . As soon as the maximum current reaches 20Amps, the voltage drop over the resistor RL will open Q2, and thus limit the B-E Current of Q3. Parallel to Q2 is Q1, which lights the LED 1 whenever the current limiting circuit is active. When the fuse  is active, the Q2 bridges the...

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