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July 14, 2011


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John Jovalusky

This post reminded me of my old aerospace days, at Allied Signal (now Honeywell). Useful information for protecting against similar transients in other types of equipment, as well. Thanks for the reminder!

Cor van de Water

Hi John, I believe you did not give your Vz, the ideal clamp voltage in your description.
Note that the cheap but rather simple solution of a series resistor will only work on the smallest load draw circuits.
As soon as any significant current in drawn, the power can't be sourced through such a high series resistance. The most logical way to protect a circuit and allow it to draw significant power is to give it a wide-input power supply, if necessary a DC/DC converter that can handle at least 28-80V input without changing the 28V output. You can still add protection, but the protection should clamp at voltages higher than 80V so the time duration makes the energy miniscule and therefor the dimensioning much easier.
Note that instead of only a dropping resistor, you could also employ an impedance (inductor+resistor) to avoid dissipating all energy in a resistor and relax the dissipation.

Chris Hudgins

Hi John,

I like your write-up. Could you offer this in .PDF format?



John Dunn

Hi, Cor.

The applications to which this resistor approach had been put were indeed relatively low power. One fellow I worked with did have an application of somewhat higher power though, so having read this in an intermnal memo I'd written, he found a clamping TVS diode of somewhat greater heft and asked me to do a resistor calculation for that device. It came out to be only five ohms and he walked away happy.

When I was checking the literature about this topic, I did find a single mention of using an inductance to achieve TVS protection, but there was no theoretical underpinning provided and I didn't have time to get into that analysis. Regrettably, I never did get into that. Maybe one day soon.

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