Although all circuits have an optimal voltage level at which they perform with greatest efficiency, it's often desirable to measure a particular circuit's operating parameters at voltage levels considerably higher than nominal. However, such tests should be performed with care, because too high a voltage can cause damage. This circuit (see the figure) is useful as overvoltage protection during such tests.
Upon power-up, U5A delivers a reset pulse to pin 13 of U4A. R1 is adjusted one to two volts below the actual shutdown threshold. R2 sets the trip point at which U1's output will drop to zero. When U5B switches from high to low, U4A provides a delay so that the output of U5B is held in a steady-state condition long enough to complete the feedback loop. After a 5-µs delay, U4B, which is configured as an RS flip-flop, swings high. This causes U4B to strobe the Adj terminal of U1 off, via Q1. To reset the shutdown circuit, reduce R1 below the trip threshold and depress S1 .
At first glance, it may appear that a 556 dual timer could be substituted for U4A and U4B. However, this idea was rejected when breadboarding the shutdown circuit. Instead, the 558, which is edge-sensitive and doesn't require input differentiation, also offers a smooth logic flow. Consequently, overall circuit complexity is reduced.
U2 acts as a preregulator for U3 so that the 78L12's input voltage requirements aren't exceeded. If testing applications demand higher voltages than those shown in the schematic, it's best to replace the voltage regulators with high-voltage types along with equally capable comparators for U5A and U5B.