arcing mains switch -> add cap in parallel -> WHY?

[CharlieLaub]

I'd like to know more about pop suppression on the mains switch of my power supply. Its for a gainclone type of chip amp, and when I turn it on or off, there is a "pop" sound from the speaker. I'd like to get rid of this.

I have some schematics for commercial amps, and these show a 10nF (0.01uF) X2 "safety" cap in parallel with the live wire switch contacts. I did some reading about snubbers for arc suppression, but was surprised to see commercial designs only using a single cap (which I assume is for suppressing the pop that I am hearing). Usually the manufacturers don't put in extra components that aren't needed, so I am guessing that the single X2 cap on the mains switch works.

I'd like to have this confirmed. I'd also like to know more about why this works for pop suppression, if anyone out there knows the answer (or thinks they do!).

Thanks,

-Charlie

[dtproff]

The pop you are hearing is the inruch current into the capacitors when you first plg in. The eaiest method is to add a NTC thermistor inline with the AC mains. This will provide a relatively high (10-20 ohms) resistance when you first plug in and as the thermistor heats up from the current flowing though it, the resistance will decrease. Check teh current ratings on the device and read how to select teh correct value for your circuit and you should be good to go.

I hope my comments were helpful.

Tony

Tony

[zigzagflux]

The cap is intended to extend the life of the switch. An RC network would be better than a straight C in my opinion.

[DF96]

A click at the point of switch-off may be an arc at the switch. The capacitor will stop this. For a cap bypassing the switch I think I would use something a bit smaller than 10nF, as this will let enough current pass to make you jump (although harmless). 10nF across the transformer primary is fine, and will have a similar effect.

A thump a few seconds later is the amp circuits switching off as the PSU caps lose charge. Similar thump at turn-on.

[CharlieLaub]

Thanks for your replies.

What I am experiencing is a pop/snap sound at turn off. In multi-amp active speaker system, I also hear a pop (maybe through the other channels?) at turn on. I was operating under the assumption that this was RF generated by arcing across the switch contacts. Is that not right?

I am also planning to put a series RC network across the primary using 0.1uF X2 cap plus 1R 1W resistor. This reduces HF noise, no? Is the resistor sized correctly for that application?

I'm a little fuzzy how inruch current would cause these kind of noises. I am not hearing any "thump" at turn off BTW.

-Charlie

[Michael Chua]

Originally Posted by CharlieLaub

I have some schematics for commercial amps, and these show a 10nF (0.01uF) X2 "safety" cap in parallel with the live wire switch contacts. I did some reading about snubbers for arc suppression, but was surprised to see commercial designs only using a single cap (which I assume is for suppressing the pop that I am hearing). Usually the manufacturers don't put in extra components that aren't needed, so I am guessing that the single X2 cap on the mains switch works.

I'd like to have this confirmed. I'd also like to know more about why this works for pop suppression, if anyone out there knows the answer (or thinks they do!).

Thanks,

-Charlie

Yes, a 10nF or 4.7nF X2 Safety Cap or use High Voltage types (1,000~1,600Vdc).

Solder it across the power switch contacts. If it's double pole, use 2 caps, one for each pole. The caps will suppress the voltage spike when the contacts arc. Very effective. No need for RC snubber.

I am also planning to put a series RC network across the primary using 0.1uF X2 cap plus 1R 1W resistor. This reduces HF noise, no? Is the resistor sized correctly for that application?

Instead of an RC, you may want to install a MOV. This will prevent High Voltage Spikes on the Mains from passing through the power transformer.

[DF96]

A pop in other channels at turn-on is probably directly caused by momentary drop in mains voltage. Inrush limiting would reduce this.

Pop at turn-off could be RF, or could be a sudden rise in mains voltage. Maybe the other channels need better filtering?

You can sometimes get away without using a full snubber because the transformer primary resistance acts to damp the LC oscillation.

[CharlieLaub]

I don't think that there is all that much inrush current. There is only 10,000 uF of caps in the PS, and the transformer is maybe 150VA.

[DF96]

Until recently, 'only 10,000uF' would be considered a largish cap.

[CharlieLaub]

Originally Posted by DF96

Until recently, 'only 10,000uF' would be considered a largish cap.

4700uF per rail actually, so roughly 10,000 uF in total for the PS.