clamping circuit help

[Hem Jung]

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.

[Peter W.]

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

[Hem Jung]

Well, it's a cheap, Chinese 24 VDC switching supply so just trying to
avoid spiking at start-up. Is it as simple as a 24 V zener across the
input (+ to -)? If so, I have a few of those here.

[Peter W.]

> Well, it's a cheap, Chinese 24 VDC switching supply so just trying to
> avoid spiking at start-up. Is it as simple as a 24 V zener across the
> input (+ to -)? If so, I have a few of those here.

Just that simple, yes. Not sure how much current goes into this device, but you want the diode to be rated at least 2x that current so it does not fail under load.

[amdx]

So your point is, if he has a 1 amp 24V power supply, he needs a 2 amp
24V Zener Diode?

Ya, I found one, it is a 1.980 amp 24 Volt Zener at Mouser. Better order
it soon it has a

40 week lead time and the price is $802,40.

> https://www.mouser.com/ProductDetail/Microchip-Technology/JANTX1N2820B?qs=CP1DtB0LfQcCGfenu8oJbg%3D%3D

  In this case, putting a Zener directly across the output of a power
supply output is not the way to do this.

As long as you don't require a precise 24 volts, find out what the
current draw is of the square wave generator with your

load. Calculate the resistance needed to drop 2 volts. Put the resistor
in series with the Positive output and Zener from the resistor

to ground. This way any voltage that is over 24 volts at the Zener will
cause it to conduct and the over voltage will be dissipated in the resistor.

  You could also use a 20V Zener and calculate the proper resistor
value so the Zener is actual Zenering all the time and regulating the
voltage,

rather than just limiting any over voltages. Adjust resistor value to
not exceed Zener dissipation limits.

Mikek






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[Peter W.]

His goal is to limit the surge at power-up. A diode of the correct rating will do that just fine without a resistor. KISS