Doohickie question: spike absorption.

NoEcho

NoEcho

2[H]4U
Joined
Aug 14, 2001
Messages
3,250
Heard there's a component in PSU's, a metal that oxidizes when voltage exceeds regular, that helps them absorb power hits. After awhile it gets burned out. What is this component? Where does it reside? How can one replace it?
 
A MOV, which is a metal-oxide varistor.

It is a component who's resitance varies in inverse proportion (usually geometrically) with the current across it.

There are very small ones (PDF) in your power supply. There are very large ones in the downstation nearest your home.
 
You can also get external versions of these in spike strips or UPSes. Those are a lot easier to replace ;) I don't know how you'd go about replacing the MOV, other than to RMA the supply if it's in warranty.

Any reason in particular you ask?

 
unhappy_mage said:
You can also get external versions of these in spike strips or UPSes. Those are a lot easier to replace ;) I don't know how you'd go about replacing the MOV, other than to RMA the supply if it's in warranty.

Any reason in particular you ask?

Click to expand...

This is the biggest replaceable part in a psu, near as I know, after the cooling fans. That is, an otherwise wonderful PSU can be rendered worse than useless by the failure of this component - and over time they are designed to fail by absorbing the spikes... strikes me as worth knowing how to service.
 
NoEcho said:
This is the biggest replaceable part in a psu,
Click to expand...
That's a pretty slippery claim. Most PSUs aren't designe to have any user-replaceable parts. The volume of the supply is mostly the transformer and the capacitors.

Ferrite beads don't provide any protection. They're intended to filter high-frequency noise. An EMI filter is a very different device than a MOV.
 
think a little more clarification is in order
EMI filtering isn't exactly surge protection, like youd see from the fuse options, but it is akin to it which is why I included it. More of a matter of scale.

MOV's do wear out
http://www.dansdata.com/sbs9.htm
http://www.brickwall.com/nofail.htm#mov

fuses blow but ferrites absorb and convert much smaller spikes (in this case think noise) into heat, obviously at some point they are simply overwhelmed

so they are related but also fundementally different

if you want protection on the cheap


gee said:
Ice Czar said:
promises promises
Id just be happy if you hung out here more and kept me from being a fool a little more often :p
Click to expand...
I check in every once in a while. If there's an interesting thread, i'll post in it.

And having designed AC protection for very expensive equipment... you can't beat a circuit breaker upstream from a MOV. For spikes they act fast and absorb a huge amount of energy, and during big surges, they'll just pop the circuit breaker.

promises promises, eh? - http://dkc3.digikey.com/PDF/T052/1135.pdf

For 120VAC operation, you want part # ROV20-201K-S... they offer 114J apiece. They're $4.70 for 10 at Digikey, and to meet the $25 minimum order you're basically buying 60 of them - enough to make a 6840 joule power bar. Put 1 from ground to neutral, 1 from ground to hot, and 58 between line and neutral. This will take up a *lot* of space. :D

Another way to protect your stuff is to use a PFC power supply - Most power supplies with a 120/240 volt switch can't handle any more than 140 volts on the input, before they pop their main input cap. But a PFC supply running at 120 volts can get a 230 volt surge (which will make a regular supply explode) and it'll just alter its duty cycle and carry on its merry way, powering your computer off the surge. :D
Click to expand...

Id want to verify with jonnyGURU but I seem to think most PSUs will have a fuse rather than an MOV as their primary surge protection device

mikeblas said:
Ferrite beads don't provide any protection. They're intended to filter high-frequency noise. An EMI filter is a very different device than a MOV.
Click to expand...

damn you got that in before my post :p
 
The goal is to have something you can swap out for piece of mind and performance. I figured it already existed but wasn't advertized (since it's failure would typically force the purchase of a new PSU).
 
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