On Monday, March 15, 2021 at 12:38:27 PM UTC-4, Hem Jung wrote:
Been using a little square wave generator for experiments, has input
values from 3-29 VDC. However, at 24 VDC input, it burned out. I have
a replacement on the way, but vender suggested using a clamping circuit
to limit voltage to 25 VDC if my power supply is prone to spikes and I
suspect it is. Can someone suggest a suitable circuit with component
values needed? Thank you.

24V Zener diode to system ground? The shipping will cost more than the device! I am not sure if you mean an actual 'clamping circuit' as much as a voltage limiter. There is no AC that you are trying to limit, just the DC input maximum voltage.

Peter Wieck
Melrose Park, PA