MOV ratings? Can't understand them....

[Chris Wilson]

If you hop onto this page for example, at RS, you can see a spec list for a typical metal oxide varistor. I am a relative beginner and don't understand these specs. Let's say I want to clamp a supply that runs at 50V DC at 30 Amps. I don't want any spikes to go over 165V.

See http://uk.rs-online.com/web/p/metal-oxide-varistors/2897244/

Would this MOV work for me? What perplexes me is:

        Varistor Voltage   100V
   Maximum AC Voltage Rating   60V
   Maximum DC Voltage Rating   85V
   Clamping Voltage   165V
   Clamping Current   100A
   Energy   33J
   Series   Standard
   Diameter   21.5mm
   Maximum Surge Current   6500A

The varistor voltage is 100V, yet clamping voltage is 165V. To me it sounds as if it's under rated? And the varistor voltage is shown as 100V yet max AC volts is only 60V and max DC volts is 85V? Could someone just run through what this means and how to rate one please? Thanks.

[Seekonk]

It starts conducting a couple ma around 100V.  These don't have a nice knee like a zener.

[T3sl4co1l]

For your 136kHz amp, right?

Why not control the input in the first place, so it doesn't produce transients?  I think there was a buck controller in there, or something..?

Tim

[Chris Wilson]

It starts conducting a couple ma around 100V.  These don't have a nice knee like a zener.

Ahh, right, I wrongly assumed they *were* like a huge Zener, I understand now, they start conducting early and "lock up" at the clamp voltage... Understood now, thanks a lot!

[Chris Wilson]

For your 136kHz amp, right?

Why not control the input in the first place, so it doesn't produce transients?  I think there was a buck controller in there, or something..?

Tim

Nods sheepishly, the dreaded 136 kHz amp.... When it's behaving it's great, I made my first trans Atlantic contact the other day, UK to East coast USA. I wish I could stop the transients. I have rebuilt it to two different designs, different FET's, different driver chips, different output transformers, different filters. But I am 90% sure the signal feeding it, which is a sine wave, then pre amplified, then doubled in frequency is the issue. It neither starts, nor, stops, cleanly. the square waves start and stop noisily with none 180 degree separation. I am now getting a bit desperate, hence MOV's but they seem to have a lot of capacitance, which may upset the snubbers. The drain wave forms, when running, are very good. I am also thinking if it's feasible to awit the waveform settling before powering the PA FET's, but it would be complicated timing it right I guess.
 

[Brumby]

I haven't designed with MOVs, but I do appreciate one or two things.

The maximum AC voltage of 60V would be RMS which, for a sine wave, means the peak voltage would be 60 * 1.414 = 84.84V ... your DC maximum.

As I understand it, these are the voltage limits for the normal operating state of the circuit in which the MOV is used, so as to minimise the effect of it being there and maximise its life.  When you read about MOVs, they mention "survival" a lot.