[Amps] Hotswitching

[R at contesting.com]

On Apr 8, 2006, at 7:18 AM, Bill Turner wrote:

> ORIGINAL MESSAGE:
>
> At 12:44 AM 4/8/2006, R at contesting.com;Measures wrote:
>
>> Some years ago when testing a 24vdc relay, I noticed a healthy spark
>> whenever I disconnected the 24v PS from the coil.  Curiosity led me to
>> use my oscilloscope and a 10x probe to measure reverse potential as
>> current through the coil stopped and the magnetic field collapsed.  To
>> my surprise, the reverse potential was c. 19x the PS voltage - which 
>> is
>> way above the DC rating on common toggle switches.  This readily
>> explained the fireworks.
>
> *********** REPLY SEPARATOR ***********
>
> There is a similar effect with choke-input power supplies. If you'd
> like to see this, observe the DC output with an oscilloscope while
> the load is suddenly removed. You may be surprised at how much the
> voltage can spike.

Indeed, Bill, I've seen 50%.   The fix is to use a resonant-choke 
instead of a fixed-choke.

> The spike amplitude will depend on the value of
> the both the choke and the filter capacitor. Unfortunately, many
> designers of choke-input supplies use a relatively small value
> capacitor, believing that to be one of the advantages of choke-input,
> and are blissfully unaware of the negative side effect.
>
> Bill, W6WRT
>
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Rich Measures, 805.386.3734, AG6K, www.somis.org