Adjustable Power Supply

Posted on Feb 6, 2014

As I carry out a lot of repairs on C. B. and audio equipment, I need a variable power supply with very good current limiting and protection. The circuit I designed for this purpose is shown in the diagram below. This power unit will deliver a current from zero to about 4. 25A at a voltage from zero to about 23V. With the component values shown, the

Adjustable Power Supply
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short circuit protection is 400 mA, but it can be set below this figure; for instance, an LED can be connected directly to the power supply and the current adjusted to light it without destroying it. The unit is not difficult to build, and in my opinion it outperforms many commercial power supplies. Mine has been in use for more than a year, and has given no trouble at all in that time. The first op-amp (IC1) acts as a comparator, comparing the voltage drop across R10 with that set by the wiper of VR1, derived from the Zener diodes ZD1 and ZD2. Two diodes are used in series instead of a single one because Zener diodes below 5V have a negative temperature coefficient. ZD2 has a positive temperature coefficient, so the two tend to cancel out and the reference voltage is more stable. The current available from the unit can be increased or decreased by adjusting VR1. The output from IC1 is applied to VR2, and because it is constantly reacting to variations in the current through R10, ZD3 is placed in the circuit to provide a reference voltage for VR2. The second op-amp (IC2) compares the voltage at the junction of R14 and R15 with the voltage set by the wiper of VR2, and by varying this setting the output voltage can be increased or decreased. Diodes Dl and D2 together with R5 provide "foldback" current limiting and in the event of a short circuit the current output is limited to about 400mA. If the output is short circuited,...

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