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Old 13th September 2010, 02:34 PM   #1
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Default Snubbing power switch properly

I'm building a chip-amp and thinking it can't hurt to use a C+R snubber over the power switch to reduce RF at turn-on. When using a DPST so both active and neutral are switched, is it necessary, pointless or bad practice to put in snubbers for both active and neutral sides (diagram attached)?
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Old 13th September 2010, 02:44 PM   #2
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If you're switching both sides of the line, you'll need two snubbers.
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Old 13th September 2010, 02:46 PM   #3
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Good questiion.

Both contacts are effectively in series - 'though either side of the Tx primary.

You are protecting against contact arcing/erosion as much as anything. Since at that level the contacts will have different switching times and bounce, it would be better to snub both.
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Old 13th September 2010, 04:10 PM   #4
DF96 is offline DF96  England
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You could put one snubber on the transformer side of the switch. Are you aiming to protect the equipment or the switch?
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Old 13th September 2010, 06:26 PM   #5
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By a few orders of magnitude, put the snubber across the contact which opens first. If you can't tell which, then you need one for each.

I usually use a 2200pF Y1 type capacitor. This is sufficient to soften the remarkably fast switching edge (mechanical switches / arcs break much faster even than MOSFETs, so they generate lots of RFI). It probably still rings quite a bit at one frequency, but that frequency is much lower than the wideband RFI of an unsnubbed switch, so it causes a lot less interference.

You can, of course, add a resistor, making some guess at stray inductance, in order to properly estimate what resistance is required. The resistor will worsen performance somewhat (because it can allow the switch to arc), but will shorten the ringing, so if you have an application that cannot tolerate any narrowband interference (idunno what), it would be wise.

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Old 14th September 2010, 08:46 AM   #6
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Quote:
Originally Posted by Sch3mat1c View Post
By a few orders of magnitude, put the snubber across the contact which opens first. If you can't tell which, then you need one for each.

I usually use a 2200pF Y1 type capacitor. This is sufficient to soften the remarkably fast switching edge (mechanical switches / arcs break much faster even than MOSFETs, so they generate lots of RFI). It probably still rings quite a bit at one frequency, but that frequency is much lower than the wideband RFI of an unsnubbed switch, so it causes a lot less interference.

You can, of course, add a resistor, making some guess at stray inductance, in order to properly estimate what resistance is required. The resistor will worsen performance somewhat (because it can allow the switch to arc), but will shorten the ringing, so if you have an application that cannot tolerate any narrowband interference (idunno what), it would be wise.

Tim
Thanks.

Resistor in series with the capacitor (as shown in the attachement) or across active and neutral - I've seen resistors used across mains power and thought they were either part of a pi filter or something to do with passive power factor correction - I can find almost nothing on this on the internet (probably looking in the wrong places).

The audio amplifier application in question features a momentary self-mute on start-up (a capacitor pulling up the mute pin of the LM3886 for half a second) which may successfully suppress any switch EMI popping. In any case, protecting the switch contacts has got to be worthwhile.

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Originally Posted by DF96 View Post
You could put one snubber on the transformer side of the switch. Are you aiming to protect the equipment or the switch?
From active to neutral? Um, both equipment and switch if possible. It's not a critical thing.

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Originally Posted by cliffforrest View Post
Good questiion.

Both contacts are effectively in series - 'though either side of the Tx primary.

You are protecting against contact arcing/erosion as much as anything. Since at that level the contacts will have different switching times and bounce, it would be better to snub both.
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Originally Posted by Frank Berry View Post
If you're switching both sides of the line, you'll need two snubbers.
Thanks. I was momentarily worried that any leakage from the capacitors would cause some current in the rest of the circuit but if that were the case, one snubber on one switch would be just as bad and I've seen plenty of those designs.


Thanks, everyone, for your comments!
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Old 14th September 2010, 11:16 AM   #7
DF96 is offline DF96  England
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Putting a single snubber on the transformer side of the switch puts the snubber right where the main inductance is, and automatically snubs whichever switch contact happens to open first without needing two snubbers. With two snubbers, one across each contact, one of them is probably unnecessary but you will never know which one.

A single snubber at the transformer ensures that there is no leakage current when the switch is open. The only thing it won't do is protect the switch from any spike arising from the inductance of a long mains lead, but in most cases this will be smaller than the leakage inductance of the transformer. I usually just put a mains-rated 5nF or 10nF capacitor from live to neutral after the switch - no resistor as the transformer copper losses provide enough damping in most cases.
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Old 14th September 2010, 11:38 AM   #8
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Hello Random007

An important point is the voltage of the cap. It should be at least a 600 volt device and it’s possible that the CSA (Canadian Standards Association) even asks for a 1200 volt device when placed across a 120v AC line. The function of this cap is more useful as an arc suppressant that helps prolong the life of the switch. I believe that most people that has repaired amps must have seen (quite often in my case) amplifiers with power switches that have seized / welded contacts.


Cheers

Philip
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Old 14th September 2010, 06:27 PM   #9
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Originally Posted by Max Caliber View Post
Hello Random007

An important point is the voltage of the cap. It should be at least a 600 volt device and itís possible that the CSA (Canadian Standards Association) even asks for a 1200 volt device when placed across a 120v AC line. The function of this cap is more useful as an arc suppressant that helps prolong the life of the switch. I believe that most people that has repaired amps must have seen (quite often in my case) amplifiers with power switches that have seized / welded contacts.


Cheers

Philip
I can do either an X2 (fails with open circuit at extreme conditions) or
a 640V capacitor (no guarantee what it will do at extreme conditions but it can take a pounding re voltage). If the high voltage cap is more likely to provide a graceful and sustainable spark suppressant + RFI, I'll pick that.

I'm still a bit ignorant as to (a quite possibly imaginary) capacitor leakage causing a very un-green constant power drain. I've run a SPICE simulation at it showed a 2mV constant supply beyond the switch when snubbers were installed and after that it just got crazy (I think I've discovered the solution to infinite worldwide electricity - a Marsbar - um, don't quote me just yet) and the computer wanted a rest and have a cool sponge on its forehead (no, really!) - I don't trust a simulation with 'ideal' components.
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Old 14th September 2010, 07:18 PM   #10
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"I'm still a bit ignorant as to (a quite possibly imaginary) capacitor leakage causing a very un-green constant power drain........"

Any cap (OK - modern) rated for the voltage will have absolutely negligable leakage resistance. Electrolytics possible excluded, but they are not for use here.

So this is something NOT to worry about!

BTW - a larger (> ~1uF) cap will pass a measureable reactive current - just calculate its impedance at 50/60Hz. Again, we are not talking about that here.
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