[Amps] Hotswitching

[Bill Turner]

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. 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