The lab supply in Fig 1 automatically sets its output voltage to zero whenever you switch it on. Thus, the lab supply can't fry your circuit if the previous user left the voltage turned up too high.
The lab supply in Fig 1 automatically sets its output voltage to zero whenever you switch it on. Thus, the lab supply can't fry your circuit if the previous user left the voltage turned up too high.
The circuit supplies 1 to 25V dc because it uses an LM317 regulator. When you turn on the supply, the bridge develops a voltage across C1. This voltage causes IC1A and IC1B to trigger, turning on Q1 via R1. In this case, the voltage at the control input of the three-terminal regulator, IC3, is essentially zero, and the regulator outputs its minimal voltage, approximately 1.25V.
Because Q1 has shorted potentiometer R2, you cannot adjust the supply's voltage unless you first rotate the pot to its off position, so that normally closed switch, S2, opens. When switch S2 opens, the bridge's high voltage, via R3 and D1, resets IC1A and IC1B. The resetting turns Q1 off, permitting potentiometer R1 to control the lab supply's output voltage. (DI #1618)