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Old 20th Feb 2017, 11:47 pm   #1
Chris Wilson
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Default Please explain MOV specs!

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-...stors/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.
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Old 21st Feb 2017, 12:28 am   #2
Craig Sawyers
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Default Re: Please explain MOV specs!

The clamping voltage is what the varistor will clamp at for short transients. And depending on the transient duration and clamping current (ie the pulse energy), the life of the varistor is derated.

Have a look at the specs (graphs) at the back of the datasheet for those varistors, and see if that helps.

http://docs-europe.electrocomponents...6b813c2454.pdf

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Old 21st Feb 2017, 12:39 am   #3
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Default Re: Please explain MOV specs!

If you are using a MOV for spike suppression on a DC system, then need to start by identifying the max DC voltage that could occur for any significant time - that could be from a turn-on surge, or a load on/off response, or a change in mains voltage, or a twiddle of some regulating voltage pot.

The 1mA DC voltage rating of the MOV then needs to be above any likely DC operating level - the 1mA voltage spec is a worst-case min, so gives some margin.

What MOV clamping performance occurs above the 1mA DC spec level then depends on the MOV model (joule rating, and voltage clamping characteristic curve with current), and also on the energy available from the source of the disturbance that is going to generate a spike, and the output impedance of your power supply (as any disturbance may need to raise an output capacitors voltage).

And of course if you have connecting wires between power supply, and MOV, and any load you may be wanting to connect, then all those connections may have impedance and may influence a transient voltage level.
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Old 21st Feb 2017, 1:05 am   #4
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Default Re: Please explain MOV specs!

So, this part is intended to be run with normal operating voltages of 85v DC or 65v RMS AC and it will take very little current. The moment the voltage reaches 100v it will start to take a little current... enough current to upset signal connections.

Along comes a lightning transient, injecting say 100A. Now if the varistor was linear and took 1mA at 100v, it would be 100k Ohms, so at 100A it would have 10 megavolts across it. Bu it's non linear and it would only have 165v across it. Much better!

I use these things for real, fighting induced surges when lightning hits and passes through aircraft. Sure 165v is more than most things will survive, so Ineed a second layer of defence to isolate and clamp that 165v, but it isn't as bad as having to absorb the whole of the transient in one go.

MOVs are good for brute force energy absorption. It's rated to swallow 33 joules without indigestion. and they take the real sting out of a transient, but I can then have something lighter butmore precise to get the remainder down to what the circuits being protected will survive.

David
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Old 21st Feb 2017, 6:13 am   #5
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Default Re: Please explain MOV specs!

Surge suppression looks like quite a complicated thing to have to design for not to mention quite a few calculation's. Those surge specs look a bit excessive 100A! 6500A max surge! That's a heavy duty PSU that can supply those currents albeit briefly.

Arn't there zener type diode's designed for this sort of thing (transient suppression) that might be easier for Chris to get his head round and use? Maybe entering "transient suppression" into Farnell or Digikey's search box might throw up some solution's.

Andy.
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Old 21st Feb 2017, 6:36 am   #6
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Default Re: Please explain MOV specs!

Re the OP, if the 50 volts psu is mains supplied, what's the worst spike that anyone would expect from it?

I recently came across some devices called "Tranzorbs", which were being advocated for protecting PICs; not sure whether they offer anything new.

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Old 21st Feb 2017, 9:38 am   #7
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Default Re: Please explain MOV specs!

MOV s are highly nonlinear resistors made out of metal oxide powder as the name implies. Their voltage is a bit high ar peak current but their ability to take punishment is as good as it gets.

Tranzorb is a trademark name for TVS Transient Voltage Suppressor diodes. Essentially monster zeners. Be aware that they come in two flavours. Unidirectional look like a diode one way, and a zener the other. Bi-directional are cathode-to cathode zener pairs and allow the allowed voltage either way before starting to conduct.

They don't take the hammer an MOV will survive, but their voltage increase at rated current is less.

Horses for courses.I use both for protecting different signals in the same equipment.

A heavy duty PSU, Andy ? Yup, planet Earth no less. When lightning zaps happen, big transients get induced in all sorts of systems. This iMac I'm typing on has a dead LAN port (Apple quoted £600 for a new motherboard as the only possible repair, so I use an appropriately named thunderbolt to 1000 base-T adaptor) This was an induced transient on the radio shack HF antenna putting a surge on the house and the LAN port saw a better ground via the ASL modem and the buried phone line.

When you get flashovers on high voltage power lines or even transients when switchgear operates, you can get evil surges on mains, and that can couple all over the place.

Those smelly Rifa capacitors are designed to flash over internally without risk of going short-circuit (well, at least for the first several years)

David
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Old 21st Feb 2017, 8:37 pm   #8
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Default Re: Please explain MOV specs!

I suspect that "Tranzorb" might be a Motorola trademark but I could be wrong!
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Old 28th Feb 2017, 12:18 pm   #9
Chris Wilson
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Default Re: Please explain MOV specs!

Sorry for the late response, been poorly and then trying to catch up with work....

Some great info here that's duly noted, I now see how those specifications work, it's quite simple once you know!

I was suffering huge voltage spikes on the drains of a Class D push pull 1kW 136kHz amplifier i had built, and was having huge issues sorting. I think I now have it sussed without the crutches of MOV's or Tranzorbs or whatever. It seems the flip flop IC and the FET driver IC were not sufficiently de-coupled on their power pins and the levels of RF, especially when the output transformer had its signal removed at the end of a transmission were getting into these devices. In the end 100nF ceramic caps soldered directly across the pins to ground fixed it. I also learnt that film caps are no good as decoupling caps, I should use ceramic... Every day's a school day!

Thanks again, some great info as always from the treasure trove that is this forum!
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