Why is there a radial cap on my ac plug?

[Gibson924]

Can somebody explain the purpose of this radial cap on the ac receptor.
I remember reading about this somewhere, but can't remember where or why. Thanks

[Alexo]

I believe it is there to prevent a "pop" when you switch the unit on or off.

I used to think they were there to filter the AC and shunt any noise over 60hz to ground but I don't think that's its purpose.

[Zippy]

That's not a cap, it's a MOV for surge/spike protection.

[mlp-mx6]

That is most likely NOT a capacitor, but a MOV - Metal Oxide Varistor. http://en.wikipedia.org/wiki/Metal_oxide_varistor

It is a surge protector of sorts. If the AC voltage exceeds 130V or 135V (usually the value used) it prevents the over-voltage from frying your amp.

I put them on every build I do. Just a little inexpensive insurance.

[Gibson924]

Thanks

[Alexo]

Shows what I know!

[van_wylde]

Hi all

The value of the MOV is 130v. no considering that this is for american voltages would i have to put a 250v MOV in my amp as i am in Australia and the voltage is 140v?

regards brendon

[jem]

Hi all

The value of the MOV is 130v. no considering that this is for american voltages would i have to put a 250v MOV in my amp as i am in Australia and the voltage is 140v?

regards brendon

I assume you meant your voltage is 240 (not 140) so, yes 250 would work.

You don't want the voltage to be so close that it would affect normal operation, and not so high that it never "kicks in" and provides protection until it's too late.

[Beerman]

I'm in Melbourne and use 260V (get 255V if you can) cause my home mains is between 238 - 252v measured twice a day over a week.

[mhuss]

+1, you don't want to be too close. MOVs tend to fail silently, and then you have a dead but perfectly good-looking MOV doing nothing useful!

--mark

[Drumslinger]

WOW! Learn something every day.

[sliberty]

Mark, Under what conditions do they tend to fail? When the spike exceeds some rating? Is there any way to determine that they have failed (take them out and perform a measurement???)?