This universal power supply circuit provides a variable voltage between 3-30 V. The power supply provides maximum current 1.5 A and with a few modifications and additions to modules can provide a higher current.
Adjustable regulator LM317 (U1) offers short-circuit protection and automatic current limiting at 1.5A. The input voltage to the regulator is supplied by DB1, a 4 A 100 PIV full-wave bridge rectifier. Capacitor C1 gives initial filtering. U1 provides extra electronic filtering included in the regulating function. The output level of the regulator is set by trim-pot R1. To prevent high-frequency oscillation Bypass capacitor added to the input and output of U1 . The current output of the transformer should be no less than 1.8 times the rated continuous-duty output of the supply. Which means that a 1.5-A supply should apply a 2.7-A transformer. For light or intermittent loads, a smaller 2.0-A transformer should be enough.
Wiring a 2nd Adjustable regulator LM317, U2, in parallel with U1 can be a quick and clean method to increase the current limiting threshold of the universal power supply to 3 A without having to sacrifice short-circuit protection. When more than 3 A is needed, the regulator module may be used to drive the base of one or more pass-transistors (see Fig. B).
