Question about MOVs for transient supression

[TheLaw]

Hey guys,

I have a question regarding MOVs...

So I want to put a transient filter on my power supply, but I don't know how to choose a MOV because their ratings don't make sense to me.

I think a transient filter (surge suppressor), should look something like this:

RV1 is the varistor (MOV). It doesn't make sense because by nature I thought the voltage rating should be around say 200V and the clamping voltage around 135V (US 120V Power).

But it turns out that if I wanted a MOV with a clamping voltage of 135V, then it would only be rated for about 40V continuous....

I'm utterly confused. What should the value(s) of RV1 be?

Thanks.

 

[Resqueline]

I trust you have read a number of the papers you find about the subject just by Googling "varistor".
Some rough guidelines though;
Remember to distinguish AC RMS values and DC absolute values. 120V RMS mains has a nominal peak voltage of 170V.
The varistor should not even begin to conduct at this peak voltage, and by a large safety margin too. Remember to add the tolerance of the mains voltage to this.
Varistors have 3 absolute voltage ratings, in order of succession; the 1mA voltage, the knee voltage, & the clamp voltage.
Varistors should never be called upon to do much continuous or repetitive tasks or else they'll wear out and change characteristics.
The 1mA level is the max safe continuous (peak) DC level and forms the basis of the marked RMS value. The clamp level is way beyond this.
Mains rated components have a high tolerance to spikes and don't need to be protected right above (close to) the nominal mains peak voltage.

 

[TheLaw]

Whoops...somehow it turned into a new topic? Don't quite remember that. But yes thank you very much for the quick response.

 

[TheLaw]

I trust you have read a number of the papers you find about the subject just by Googling "varistor".
Some rough guidelines though;
Remember to distinguish AC RMS values and DC absolute values. 120V RMS mains has a nominal peak voltage of 170V.
The varistor should not even begin to conduct at this peak voltage, and by a large safety margin too. Remember to add the tolerance of the mains voltage to this.
Varistors have 3 absolute voltage ratings, in order of succession; the 1mA voltage, the knee voltage, & the clamp voltage.
Varistors should never be called upon to do much continuous or repetitive tasks or else they'll wear out and change characteristics.
The 1mA level is the max safe continuous (peak) DC level and forms the basis of the marked RMS value. The clamp level is way beyond this.
Mains rated components have a high tolerance to spikes and don't need to be protected right above (close to) the nominal mains peak voltage.

So by voltage spikes, it is intended to suppress BIG voltage spikes? As in >200V AC?

And then I guess its the capacitors jobs to filter out other smaller spikes? Sorry not to good with power yet. Working on it.

 

[Resqueline]

That'll be steve wisely splitting it off into a new topic, as is actually appropriate since the topic changed, and I failed to do it.

MOV's are intended to absorb the energy in the spikes extending far beyond the peak value of the mains. Don't refer to that value as AC, use an absolute value instead.
I guess a clamping voltage of 400-600V for 120V mains might be ok, but notice I haven't checked this out for certain.
Spikes can be unipolar as well as bipolar with different positive & negative values, riding on top of the mains AC waveform.
The other filter components job is to smooth out and thereby further dampen the remains of any spikes. They also stop PSU generated noise from reaching the mains.

 

[TheLaw]

That'll be steve wisely splitting it off into a new topic, as is actually appropriate since the topic changed, and I failed to do it.

MOV's are intended to absorb the energy in the spikes extending far beyond the peak value of the mains. Don't refer to that value as AC, use an absolute value instead.
I guess a clamping voltage of 400-600V for 120V mains might be ok, but notice I haven't checked this out for certain.
Spikes can be unipolar as well as bipolar with different positive & negative values, riding on top of the mains AC waveform.
The other filter components job is to smooth out and thereby further dampen the remains of any spikes. They also stop PSU generated noise from reaching the mains.

Wow you guys are amazing. Really...Thanks for being on this site. It would have taken me a month to figure out just that.

So I guess I overestimated the power of a MOV. A voltage spike is not really considered 10-20V over 120V as I thought.

Right so RMS for AC....

Here's what I am thinking:

http://www.mouser.com/ProductDetail...=sGAEpiMZZMv1TUPJeFpwbiJP8jl9psy8fTyQSoWus/E=

Thanks a bunch.

 

[Resqueline]

Yep, a 20V spike on top of the mains is nothing to be concerned about at all, and varistors need to be rested and ready for the infrequent but really damaging spikes.

I figure that varistor will be a good choice/compromise.
The 175V RMS rating will ensure it survives power surges and insignificant spikes, and the 455V clamp voltage will do a good enough job in protecting even a 400V triac.

You're welcome.

 

[TheLaw]

Yep, a 20V spike on top of the mains is nothing to be concerned about at all, and varistors need to be rested and ready for the infrequent but really damaging spikes.

I figure that varistor will be a good choice/compromise.
The 175V RMS rating will ensure it survives power surges and insignificant spikes, and the 455V clamp voltage will do a good enough job in protecting even a 400V triac.

You're welcome.

I think I might have asked this before...but what are the potential hazards of wiring a power supply that gets its electricity from mains.

It doesn't seem like that difficult of a concept, but reading around, there are tons of warnings about all sorts of bad things. But what really could happen? In all honesty...

Thanks

 

[TheLaw]

Sorry to revive a dead thread...but...

The values for the capacitors and coils. Is there anyway to go about finding what they should be? I have some ideas...

Does it matter?

Eh. I'm ****ing you guys off. I'm sorry. If you don't feel like answering, that's quite alright.

 

[(*steve*)]

I think I might have asked this before...but what are the potential hazards of wiring a power supply that gets its electricity from mains.

It doesn't seem like that difficult of a concept, but reading around, there are tons of warnings about all sorts of bad things. But what really could happen? In all honesty...

Thanks

Gee, I didn't see this one. I'm glad you revived the thread!

The obvious potential hazard is death. And if any warning only saves your life once, it's probably worth it...

In all seriousness, you need to exercise extreme care when playing about with the mains.

It is a source of hazardous voltage, and is capable of delivering huge currents (at least until
fuses or circuit breakers blow).

If you're in a part of the world that has 110V mains, then it's arguably a lot less hazardous than those of us who have to deal with 240V.

OK, what can go wrong... (and this is by no means exhaustive)

* Switching neutral rather than live can result in the circuit still being live when powered off (note that this also applies to blown fuses).
* Accidental shorts between live and the chassis can cause electric shock
* Short between neutral and chasis can be a hidden danger onle exposed whn an extension or badly wired power point causes live/neutral reversal (this is less hidden if your power cables are not polarised).
* Accidental shorts between live (or neutral) and low voltage parts of the circuit can result in an operational, but dangerous device.
* Poor cable termination can result in cable being pulled out, exposing live wires. (I prefer to use a male connector (like computer power supplies use) rather than a power cable for this reason)
* You need to be much more careful about openings that people can put finges, pencils, or knives and forks into.
* Operating in damp or wet environments required far more cautions.
* Accidental shorts or overloads are potentially pyrotechnic -- you're no longer limited by the internal resistance of a battery or th maximum current from a plug-pack.
* There are often legislative requirements (in Australia you have to be a qualified electrician to do mains wiring (or to approve such wiring done by others)
* There are often design requirements - cable colours, minimum conductor spacing, methods of termination, etc.
* Your design may now be constrained by mains voltage and/or frequency.
* Your design may be dangerous and.or illegal if built or used in other counties (or indeed in your own).
* You may be subject to greater risks of legal repercussions if something goes wrong.
* You have to deal with mains-borne interference and voltage regulation issues.

An there are probably many more.

The one I always think of though is the abrupt termination of my life. I have managed to avoid that every time so far, and I encourage everyone else to do likewise.

 

[(*steve*)]

Sorry to revive a dead thread...but...

The values for the capacitors and coils. Is there anyway to go about finding what they should be? I have some ideas...

Does it matter?

Eh. I'm ****ing you guys off. I'm sorry. If you don't feel like answering, that's quite alright.

No, I don't feel ****ed at all.

I think this may become another thread though.

But first, why don't you explain more fully what you mean?

 

[TheLaw]

For the transient filter schematic I posted. i was wondering if there was a formula for determining the values of the caps and inductors.

Thanks. It seems dangerous. I run on 110V. Hopefully I won't frek it up.

 

[TheLaw]

I am considering just using a wall mount transforner but then how do you get a transiet filter in there or circuit protection?