Capacitors across power switch

[sonicles]

Just a little extra note of caution. If you use a mains multiway socket adapter to power your amplifier fitted with a big transformer and also other delicate equipment, switching off the power at the main plug socket can send the high voltage back-emf, generated in the amplifiers transformer, through to the other equipment plugged in to it. By switching off the mains you have isolated this branch so you won't have the mains supply to absorb the spike. This doesn't happen if you switch the individual items off one at a time as each one is disconnected (in effect at the same time) before the back emf can surge into the other items. Worth thinking about I thought

 

[bucks bunny]

I've been installing 0.01 to 0.047uf 630v poly's across power switches for decades and never had an issue yet.

Instead of 630V= caps you should have taken X2-caps with 275Vac rating. These are rated for overvoltage spikes far exceeding 630V. The fact that you did not encounter any issuesl proves that you have been lucky.

 

[wiseoldtech]

You might want to tell the manufacturers that, like Pioneer, Technics, Fisher, Sansui, and a truckload of others, because they all have used common poly caps across their products' power switches, at least the ones sold in the USA.

 

[JanKarsten]

How much tolerance is there in terms of capacity? Can one replace a 0.01uF with 0.047uF and vice versa where the capacitor is placed across a power switch of a power amplifier?

Obviously we're talking only X2 275VAC or higher rated caps.

 

[wiseoldtech]

Obviously we're talking only X2 275VAC or higher rated caps.[/QUOTE]


Regardless of the "type" of capacitor, it isn't critical in terms of value.
I tend to use 0.047uF mostly

 

[Mark Tillotson]

Well if the cap is large enough there's enough current when the switch is off to be dangerous. Snubbers are generally safer across the load, not the switch, even if somewhat less effective for resistive AC loads.

If you use a mains multiway socket adapter to power your amplifier fitted with a big transformer and also other delicate equipment, switching off the power at the main plug socket can send the high voltage back-emf, generated in the amplifiers transformer, through to the other equipment plugged in to it

Only if the secondaries are all open-circuit. Normally the secondary loads help to clamp the voltage nicely and inductive kick-back isn't something you get with transformers, unless unloaded. So long as current can flow in at least one winding, opening the primary just dumps the magnetising amp-turns into the other windings, and magnetising current is usually a lot less than the load current.

The capacitor across the switch prevents arcing on switch _off_ by keeping the voltage across the gap small until the gap is large enough to handle the mains voltage without arcing. It makes switch-on worse, which is why an RC snubber is preferred over just C.

Snubbers are always a compromise I think.