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-   -   A serious question about sizing Metal Oxide Varistors. (http://www.diyaudio.com/forums/everything-else/225436-serious-question-sizing-metal-oxide-varistors.html)

 freax 11th December 2012 04:09 AM

A serious question about sizing Metal Oxide Varistors.

Hi, I've just bought 10x of Vishay MOV's PN: RMV10-230K and now I'm stuck in a bind on how to use them.

Here is the datasheet: http://www.datasheetcatalog.org/data...shay/rmv10.pdf

I've read that wiring them in series is fine if you want to double or triple the clamping voltage, is this true?

And that wiring them in parallel requires matched sets.

Now what I want to know is, what clamping voltage should these 230K models be set to if I am going to be using them on the quite variable 210v-258v AC mains that I have here?

I've used the calculations on this site: How to Size a MOV for Surge Protection | eHow.com

And come up with a clamping voltage of 1414.427 v

Here is my working:
250v RMS / 0.707 = 353.6067892503536
353.606 x 4 = 1414.427

Now does that mean that I will need to use 3x wired in series of the 230K model for a clamping voltage of 1785v which I think is too high, or should I consider buying other types/models? Or maybe even just using 2x in series for a clamping voltage of 1190v? Or will that be too low?

 davidsrsb 13th December 2012 12:14 PM

That model is designed for direct connection live to neutral on 220V nominal mains (with a fuse).
What is your actual mains voltage?

 AndrewT 13th December 2012 01:05 PM

A MOV does not operate like a Zener.
It does not "clamp" at a voltage.
The MOV presents a very high resistance at normal or rated voltage.
When the voltage exceeds the rated voltage by the specified amount, the resistance plummets to a very low value that allows the kA snubbing current to pass.

Will this type of resistance behaviour actually work when series connecting two or more devices?

230k is for supply voltages that never exceed 230Vac. That implies usage on supplies that are less than a nominal 216V .

275k would be used on 240Vac. These nominal 240Vac supplies can rise to 254Vac. The MOV must not operate when the supply reaches 254Vac. 250k is not suitable for 240Vac supplies.

 DF96 13th December 2012 02:44 PM

The eHow page suggests 4x peak. The manufacturers datasheet works on the basis of 2x peak. On that basis even using two in series you are still clamping at a much higher peak voltage than the manufacturer suggests. I guess it depends on what you are protecting; how sensitive is it to overvoltage?

The usual problem is things like rectifier diodes, which are usually recommended to be chosen to cope with a 2x normal peak on the mains. This fits with the MOV datasheet.

I think the correct MOV is the 250k. Given that you have already bought the 230k, you could use two of them provided that the equipment you are protecting can cope with 3x or 4x overvoltage. Or use one 230k (protected by a fuse), and expect to have to replace it from time to time.

 freax 13th December 2012 02:55 PM

no matter, I'm sure I can reuse the 230K's for radio antenna protection and just buy some 250K's

The load to be protected is a 20 amp 13.8v linear power supply with a 600va transformer and soft start circuit based upon project 77: http://sound.westhost.com/project77.htm

 tvrgeek 15th December 2012 08:45 PM

As an aside, folks should remember that MOV's are very slow devices so they will not protect against fast transients, and they are sacrificial, shorting when failing. So they should always be fused.

 Speedskater 15th December 2012 10:52 PM

Well hundreds of nanoseconds is not slow motion, not as fast as a TVS diode that's true, but not real slow either. They are not good for ESD or EFT protection.

 AndrewT 16th December 2012 10:50 AM

Movs are designed for short transients.
They are not designed for long over voltage incidents.

 freax 16th December 2012 10:59 AM

Quote:
 Originally Posted by AndrewT (http://www.diyaudio.com/forums/everything-else/225436-serious-question-sizing-metal-oxide-varistors-post3287079.html#post3287079) Movs are designed for short transients. They are not designed for long over voltage incidents.
I should imagine that a fuse placed before the MOV would blow in the event of a MOV conducting if its flow current is sufficiently high for a long enough period?

What would be designed for a sustained overvoltage? A Gas Discharge Tube?

Surge protector - Wikipedia, the free encyclopedia

 dmills 16th December 2012 11:07 AM

For radio antenna protection I usually favour a GDT with a static bleed resistor or (if not massively broadband) an inductor, MOVs can be rather capacitive at HF.

You can usefully place a high voltage GDT at the aerial socket with a much lower voltage one on the rx path after the T/R relay (if a trancever).

A good place for further protection is after the rx BPF bank and just before the first mixer as the BPF will limit the rise time of the energy at this point.
Setting things up so that the recever is disconnected from the aerial socket when the rig is powered off is a good move (The BPF switching relays will help here), as is setting up the T/R relay such that when it is powered off the aerial is connected to the PA, this usually being more robust then the first mixer.

On MOVs, dont use ones rated too close to the nominal mains supply, and remember that in a conventional linear power supply, transformer leakage inductance and possibly saturation will tend to limit the level of transients making it through the iron anyway.

Regards, Dan.

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