What's a good rating for a MOV suppressor?

[leadfootdriver]

That little disc that goes on the power switch... I don't really understand how to select a rating.

[tedmich]

they are usually 120-130VAC but a few people here recommend using 150VAC minimum as they last longer, see RG article here
The Immortal Amp Mods, Pt. 3 - Premier Guitar

RG also recommends very high voltage MOVs on the OT as protection

[leadfootdriver]

Thanks for that link! I did pickup a few 150V MOV's at a 395 clamping voltage. But I wasn't sure if the clamping spec would be right. It's amazing to me that the AC line could ever get up to 395v. But it must happen really fast, eh?

V150LA10AP Littelfuse Varistors

Does the MOV short itself out when it fails?

[tedmich]

I usually think of them as shorting to ground at high voltages; they don't conduct until their clamp voltage gets exceeded. They are said to "fail" after repeated surges move they clamp voltage >10%, and parallel MOVs will take more surges.

[leadfootdriver]

That layout shows 3-MOV's installed after the power switch. Do you see a problem mounting then on the back of the IEC? It would be easier to ground them out that way.

[Steve Conner]

I like to put these things after the switch and certainly after the fuse. The reason being they wear out and eventually fail short. If you leave your amp plugged into an outlet, you don't want the voltage on the MOV all the time when you're not using it.

However, there are many IEC outlets and power strips out there with built in MOVs. I remember seeing a lot of burnt ones in a UL bulletin, though.

[R.G.]

Thanks for that link! I did pickup a few 150V MOV's at a 395 clamping voltage. But I wasn't sure if the clamping spec would be right. It's amazing to me that the AC line could ever get up to 395v. But it must happen really fast, eh?

Transients of up to 1kv and more are fairly common. They're spikes, but they can puncture insulation. That's one reason that the 1.4kV test for primary-secondary insulation has moved up to 2.5kV, then 4kV.

Does the MOV short itself out when it fails?

MOVs conduct when the voltage exceeds their clamping voltage. Each breakover eats up a little of the insides of the MOV, and the breakover voltage gets a little lower. Eventually, if it conducts often enough, the breakover voltage drifts down to the peak of the normal AC power line. Then it breaks over 100 (or 120) times per second, and the voltage drifts lower, and lower, and soon it's conducting all the time, the power from the conduction gets huge, and you have yourself a Light Emitting MOV (LEMOV). Well, some people would call it a DED (Darkness Emitting Diode).

Whether it opens or shorts in its Light Emitting period is open to discussion. Eventually it opens or the breaker does. This is why fuses in series between the MOV is a good idea. Using 150Vac instead of 130Vac (on a 120Vac nominal line) is a good idea because it gives you more room to drift down before the MOV's lifetime is over. What you want is for the line to open before the resistive (volts times amps) power generating phase of the death can start fires.

[leadfootdriver]

Thanks for the practical answers!

I couldn't search anything that made sense to me for an amp build.

[Enzo]

They only short to ground if they are wired to ground. Neutral isn't ground, though it may be connected to it at the service panel.

[Jazz P Bass]

Hear Ye! Hear Ye!
Read All About It.
Here ia a well written AppNote from Littlefuse, on voltage suppression devices and applications.

[GainFreak]

I read somewhere that MOVs are known to explode and set equipment on fire which doesn't sound like something that I need in my equipment...

[trobbins]

Put a short section of heatshrink tubing over a MOV on the AC input if the position of the MOV cannot cope with the risk of a 'failed' MOV. This is also appropriate to NTC surge thermistors.

The PT primary can also cause transient voltages on the primary circuit after the mains power switch is turned off - the MOV across the primary alleviates any stress on parts that result (eg. stress on an old power switch).

The MOV across the Output transformer half-primary is effectively a shunt capacitor - so the MOV size should be kept as small as possible to reduce the capacitance (eg. 7mm disk size) - and preferably a resistor is placed in series with each MOV to make an RC zobel network (eg. resistance similar to OT primary loading). The DC 1mA voltage rating of that OT MOV should be somewhat above the DC HT level of the amp, and so would vary with the amp type/model. MOVs can be connected in series to achieve high DC voltage ratings. I reckon this form of protection is more effective than using a 'speaker side' pre-load resistor for making an amp more rugged to open-circuit output conditions.

Ciao, Tim

[leadfootdriver]

All Boogies have them, there Ralphie boy! I remember that was part of their advertising. I didn't even think to look at a Boogie schematic...

[GainFreak]

Not only Boogie has them. I don't know how often they explode but since it was mentioned I would assume they were not talking about a couple of cases only.
The other thing about NTC surge thermistors was they work as specified only if your device has a constant current draw which is not the case with a guitar amp so at this point I'm not really convinced I should use both.

[leadfootdriver]

Thanks for the reply.

You mentioned capacitance in the OT mod. But what about just the AC lines? Does the capacitance of the MOV have a snubbing cap effect for the whole amp?