What about RC snubbers on the ON/OFF switch?

[Shannon Parks]

An RC snubber (like maybe 100 ohm in series with .05uF) across the power transformer primary or mains switch can prolong switch life and prevent transients. So why is this often omitted in circuits? Would it be useful in across a Standby switch (secondary CT ground), too?

Shannon

[erichayes]

Hi All,

Back in the days when power switches actually contolled the mains, manufacturers almost always installed a .05~.1 uF cap across the switch to suppress turn-on transients (and extend switch life).

Now, with everything turned on by a microprocessor, there are virtually no power switches, other than the ones on power entry modules (PEM).

If you're planning to use a power switch other than a PEM to control the line voltage to your project, you should install the cap. Ceramic, rated at 600V minimum is best, if you can find them; otherwise a good poly film in series with a 75~100 ohm 1/2 watt resistor will help. The same would apply to standby switches, as long as they're handling AC, but then you'd definitely want to use a ceramic rated at 1KV or higher.

Also, if you're recapping an old amp, include this one along with the coupling caps. It was the major cause of switch failure.

[EWBrown]

I had noticed the older amp "carcasses" that I have been salvaging the iron from, often had a disc ceramic cap across the power switch, but this seems to be a bygone art these days.

Would having the CL-90 in series with the PT primary make this a moot point or is it still a good practice? I suppose putting one in wouldn't hurt, the worst that could happen is that it would not do anything...


/ed brown in NH

[Tom Bavis]

The secondary circuit (rectifier and caps) snubs the transformer inductance. But it wouldn't hurt to put a MOV (metal oxided varistor) surge suppressor across the line after the switch. Use 130V rated part for 115V input.