Ul, an LM317 adjustable regulator provides short-circuit protection and automatic current limiting at 1.5 A. The input voltage to the regulator is supplied by DBl, a 4-A 100 PIV full-wave bridge rectifier. Capacitor Cl provides initial filtering. Ul provides additional electronic filtering as part of the regulating function. The output level of the regulator is set by trim-pot Rl. Bypass capacitors on the input and output of Ul prevent high-frequency oscillation. The current rating of the transformer must be at least 1.8 times the rated continuous-duty output of the supply.
3-30V-universal-power-supply-module - schematic

This means that a 1.5-A supply should use a 2. 7-A transformer. For light or intermittent loads, a smaller 2.0-A transformer should suffice. Wiring a second LM317, U2, in parallel with Ul is a quick and clean way to increase the currentlimiting threshold to 3 A withoutsacrificing short-circuit protection. When more than 3 A is required, the regulator module can be used to drive the base of one or more pass-transistors (see Fig. 75-2B).

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