MOV ratings? Can't understand them.... - Page 1
Forum section: electronics / beginners

author: Chris Wilson
author_rank: Super Contributor
timestamp: February 20, 2017, 10:48:08 pm
content: 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.Seehttp://uk.rs-online.com/web/p/metal-oxide-varistors/2897244/Would this MOV work for me? What perplexes me is:Varistor Voltage 100VMaximum AC Voltage Rating 60VMaximum DC Voltage Rating 85VClamping Voltage 165VClamping Current 100AEnergy 33JSeries StandardDiameter 21.5mmMaximum Surge Current 6500AThe 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.

author: Seekonk
author_rank: Super Contributor
timestamp: February 20, 2017, 10:57:41 pm
content: It starts conducting a couple ma around 100V. These don't have a nice knee like a zener.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 21, 2017, 12:04:23 am
content: 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

author: Chris Wilson
author_rank: Super Contributor
timestamp: February 21, 2017, 12:16:08 am
content: Quote from: Seekonk on February 20, 2017, 10:57:41 pmAhh, 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!

author: Chris Wilson
author_rank: Super Contributor
timestamp: February 21, 2017, 12:23:40 am
content: Quote from: T3sl4co1l on February 21, 2017, 12:04:23 amNods 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.

author: Brumby
author_rank: Unknown
timestamp: February 21, 2017, 12:26:04 am
content: 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.