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Putting a MOV into my power supply?

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Chris_P

New Member
Dumb question, I have searched everywhere and can't work out where I install an MOV into a power supply. My power supply is similar to this shematic.



I wish to install an MOV just in case, where would I install it in that circuit?
 

dknguyen

Well-Known Member
Most Helpful Member
You can stick it between the outputs of the bridge rectifier, the inputs of the transformer, or the outputs of the transformer, depending on the breakdown voltage rating of the varistor. You could also stick it between the outputs of the power supply. Best place to stick it is wherever the transient is most likely to enter first and damage something first (make sure the breakdown rating is appropriate for where you stick it).
 

Torben

Well-Known Member
dknguyen said:
You can stick it between the outputs of the bridge rectifier, the inputs of the transformer, or the outputs of the transformer, depending on the breakdown voltage rating of the varistor. You could also stick it between the outputs of the power supply. Best place to stick it is wherever the transient is most likely to enter first and damage something first (make sure the breakdown rating is appropriate for where you stick it).
What would the pros and cons be of sticking them in all those spots?

Don't worry, I plan to read up on it too, but pointers are always welcome. :)


Torben
 

Nigel Goodwin

Super Moderator
Most Helpful Member
I would stick one across the primary of the transformer, that way it protects the entire circuit (assuming I was going to add one at all?) - and it's a standard value designed for that exact use. If you put it in the secondary you're going to have to try and source a specific perculiar value - which may be very difficult.
 

dknguyen

Well-Known Member
Most Helpful Member
Transients are most likely to occur on the input power (from the AC mains), which is the intput of the transformer. It's also the largest voltage in the circuit which makes it easier to get the value you need. So...basically...what nigel said.

The only other place you might want to actually add one is the +5V output. To protect from static damage, since loads usually aren't a source of transients. It's a much lower voltage value, a MOV might not be the best choice. There are other devices that do the same thing and behave almost the same way, but they aren't MOVs.

http://www.electro-tech-online.com/custompdfs/2007/03/Chapter1IntroductiontoCircuitProtection.pdf
 

Hero999

Banned
I don't normally bother. The transformer is inductive so high frequency spikes are likely to be heavily attenuated and if the transformer doesn't stop them the regulator certainly will and is unlikely to be damages unless it's a really high voltage for a long period of time.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Hero999 said:
I don't normally bother. The transformer is inductive so high frequency spikes are likely to be heavily attenuated and if the transformer doesn't stop them the regulator certainly will and is unlikely to be damages unless it's a really high voltage for a long period of time.
I've never bothered either, but the UK has exceptionally good quality mains! - on lots of TV boards you find loads of empty holes for mains filtering used in some other countries.
 

Chris_P

New Member
Hmmm .. maybe I don't need it then. I am guessing that our power in Australia would be good quality. Thanks for the info.
 

audioguru

Well-Known Member
Most Helpful Member
They sold "surge arrestors". The good ones had powerful gas tubes rated for thousands of Amps for a fairly long time. The cheap ones had a MOV or 3 of them.

One cheap one had a window to see the MOV and it said, "If it turns black, send it back". They all turned black because the MOV was too small and a very small spike on the power line caused it to blow open like a fuse. Your refrigerator turning off would blow up the MOV and make it useless.

The powerful surge arrestors were banned by the electricity authorities because they were too heavy to be plugged into and hanging from an electrical outlet but they were good and would arrest a lightning strike not too far away.
 

RODALCO

Well-Known Member
Australia and New Zealand have good quality powersupplies based upon the British systems.

Because of lots of overhead wires in some suburbs, it can't do any harm to fit a 240 Volts rated (275 Vrms) MOV across the primary of your TX.
Put it just after your appliance fuse, between the phase and neutral.

In case of a spike because of lightning it will clamp the mains and blow your appliance fuse.

Jaycars sells these MOVs for a couple of dollars each.

As hero999 says, the transformer will attenuate out most spikes, because its core will saturate during brief over voltages.
 
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