Capacitor Across Power Switch to Control Switch On Noise?

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#7
Michael Chua said:
No problem with 110V switch. What is important is to use CLASS X or X2 caps. These are specially manufactured (approved) for Mains use.
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Because (and this is especially true with bigger power amplifiers) if you switch off at the wrong time in the power line cycle (when the transformer field is at maximum) The transformer field collapses, but the current has no-where to go (the switch is now open) so the primary voltage heads towards infinity! Only the circuit leakages limit this voltage, but it's all across the capacitor.

Now in real life things aren't this bad, but it does stress the capacitor.
 
#9
DreadPirate said:
I have a jfet based preamp that really makes a racket on turn on. It runs off 24vdc. Would putting a small value .22uf/630v capacitor across the power on switch be a way to control this noise?
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Have you tried a slow start (power transistor/resistor and cap) as you say it runs off 24 Vdc, to get the rail to rise slowly.

Or connect a normally closed relay across the output shorting to ground, and again arrange for small delay before it opens.

Is the circuit "oscillating" as the DC conditions stabilise ?
 
#10
I'm going to try the capacitor trick, if not, where is a power trans/resistor/cap circuit I can try? Would it power off the 24vdc, I don't want to provide another power source just for it. Using the 555 and relay is a bit more complicated than can be justified for this project.
 
#11
It can be as simple as this for a single supply. Input to the collector, output on the emitter. Use a highish gain transistor, even a darlington. Choose R and C to give a slow rise. You would have to experiment with values but try say a 1k and a 1000 uf for starters. A darlington transistor and you could use a higher value R and lower C as the base current needed would be a lot less.

Same type of circuit (it's basic but would work) could be used to power a relay too.

Arcing mains switch contacts give a very brief pulse of noise... if your preamp make a noise that lasts a second or too I reckon it's something else going on... hence the slow start and mute idea.
 

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#13
DreadPirate said:
Slow rise, but will it kill the noise?
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That depends on what's causing the noise. It may well not.

What I would do to test, is leave the AC power on and just switch the DC and see if that alters anything. That proves immediately whether it's the AC switch arcing or not.
You have to narrow it down... and circuits often do make noises as the rails rise. The surest way is to mute the output with a relay that shorts the signal output to ground, and that only opens after the supply has been present for a few seconds. And it's so easy to do...

The L200 is way over the top and may well not achieve anything.
 
#14
Mooly said:
It can be as simple as this for a single supply. Input to the collector, output on the emitter. Use a highish gain transistor, even a darlington. Choose R and C to give a slow rise. You would have to experiment with values but try say a 1k and a 1000 uf for starters. A darlington transistor and you could use a higher value R and lower C as the base current needed would be a lot less.

Same type of circuit (it's basic but would work) could be used to power a relay too.

Arcing mains switch contacts give a very brief pulse of noise... if your preamp make a noise that lasts a second or too I reckon it's something else going on... hence the slow start and mute idea.
Click to expand...

I'll give this a try if the cap across mains does not work. What would be a good candidate for the transistor available at Radio Shack?

Am I looking for a Vcb<24V and Ic of a few amps?
 
#16
mains rated caps and snubbers

A good source for the hobbiest for mains rated caps and Metal Oxide snubbers is a dead PC power supply (switcher). These are full of these things, up near the fuse. You can see the little UL CSA VCO label on the caps. The Black or blue things are snubbers, the last supply I scrapped out had double diode head on symbols printed under the snubbers. Snubbers important to suppress lightning on the mains. Lots of improvement in components in the last 20 years. If you don't have a dead PC,charity resale shops usually have underpowered supplies for a couple of dollars.
 
#18
What's causing the noise? is it mains bourne interference (so arcing across the mains switch thats causing radiated or conducted noise thats causing pops/crackles etc on the circuits output signal) or is it your circuit thats' making a noise as it powers up or down and the DC operating point is not stable during these events. These problems require completely different solutions. If its th e firs t one, then som e kind of snubber across the switch may well help. Also try a cap acorss th emains lines (make sure you select one thats suitable for the job!!!). If its the second issue, use a relay to mute the output during turn on and turn off events.
 
#19
When wiring an ac pulse suppressor x cap across a dpdt switch, do i wire it across live and neutral on the input side of the switch or output side?

Im -guessing- input side... If I'm right...I assume it would cause the psu to draw power even when off ... If so should i install a second power switch for total power off, or just pull the plug when leaving the house for an extended time.
 
#20
the noise suppressor is across the load, in this case the load is the transformer.
If the switch is a long way away from the load then suppressors at both Load and at switch may give a further improvement.

I am part way through reading H.Ott
He shows a diode added in parallel to the R of the R+C of a conventional noise suppressor. I have not see this before and I don't think it is available as a standard integrated 2pin package.
 
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