>
>
>> Tom Rauch wrote:
>> >> There are ways to design around this problem, but the only reliable
>> >> way is to use a higher-rated device and/or a lower rail voltage.
>> >
>> >.......or to clamp the shunt side of the series resistor with a
>> >large zener with a capacitor across. You can use a zener clamp rated
>> >below the maximum voltage breakdown of the shunt device, and above
>> >the highest desired regulated voltage.
>> >
>> It doesn't make sense to spend money on a high-power, high-voltage
>> zener (don't forget the heatsink) when you can uprate the shunt MOSFET
>> for much less cost.
>
>Why would you need a high power zener?
// More money than common-sense?
>The zener doesn't
>regulate anything, it is just a safety device in case you get a
>millisecond or two glitch.
>
// Good point, Mr. Rauch.
>It is a hard clamp for quick transients. It will fail-short if a
>reasonable size diode is used and your fancy regulator goes bad.
>
// Amen. I have never seen a zener fail to short during a glitch.
>As a matter of fact, the LAST thing you would want is a zener
>clamp that is very large because it would allow the screen to run
>too high with poor regulation if the normal regulator failed.
>
>Another transient protection system is a conventional power
>rectifier with the cathode feeding a high-value electrolytic with a
>high resistance bleeder. This even works very well for lightning
>protection if you clamp negative transients to ground.
>
** A Lexguard® shrapnel shield is advisable for the operator.
>
cheers, Tom
- R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K,
www.vcnet.com/measures.
end
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