Diodes for transients

Thread Starter

shortbus

Joined Sep 30, 2009
7,460
When should diodes be used in a circuit to suppress transient voltages? And when used, what are the selection criteria? Or should you just add one for good luck in any circuit?
 

#12

Joined Nov 30, 2010
18,190
Pretty wide question. It mostly depends on how sensitive the parts are. Start with input protectors for MCUs and maybe op-amps. "Small" is the word for that application. 1N4148 or a low current Shottky. We recently had a thread about using a diode connected jfet because it has such low leakage current (pico amps). The datasheet says a nanoamp, but real world measurements were in the pico range. Dump the positive surges into the power supply and dump the negative surges into common. In a car, a zener across the input power lines after some sort of resistance or impedance. Once upon a time, I used a small transformer secondary for an inductor. When inductors live in the same neighborhood, the current through the inductor is the surge rating for your diode. When the input is a small transformer, the impedance of the transformer is pretty good protection except for lightning strikes and primary to secondary capacitive coupling. That's where you can place a much larger capacitance like 0.1 uf from the secondary to neutral, or a TVS from each powered line to neutral on the primary.

Wide question, this is not all the answers.
 

Thread Starter

shortbus

Joined Sep 30, 2009
7,460
Thanks guy's, this has to do with the 'logic' circuit for my electrical discharge machine. Ronv was helping me and his circuits had reverse biased diodes in the logic circuit. Since seeing them I've noticed other circuits posted online that have them. My circuit has only logic(4xxx series) and 3 half bridge drivers and the mosfets. Where I see them there is either an inductor, relay or motor in the circuit. What I'm doing and most logic type circuits don't use them.

So was wondering when and how to use them.
 
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