PSU surge protection - Metal Oxide Varistors

[UKRobotics]

I've been told that I should use metal oxide varistors in my linear power supply to protect the transformer and capacitors from voltage spikes.

My limited understanding is that they should be put after the fuse but before the transformer and caps; positioned between live and neutral.

Under normal useage (At rated working voltage) the MOV has a very high resistance and does not effect the rest of the circuit. If the input voltage increases to a predefined level (clamping voltage?) then the MOV will gain a much lower resistance, effectivly shorting live and netutral and blowing the fuse much faster than it would without an MOV in place.

Without the MOV in place, the fuse wouldnt blow until every component in the PSU had recived a high enough voltage to increase the current to a level where the rating of the fuse was exceeded. Of course the danger here is that the components could be damaged by the over voltage before the fuse blows; even with a properly rated fuse.


First of all, to all the electronics experts out there, is my understanding correct and if not, where am I going wrong ?

Secondly, please could someone point me in the right direction of a suitable MOV device for my application? an RS (or similar) part no. would be great. I have had a look on the websites and catalouges but there are so many different ratings and types i'm not sure which one i'd need. Electrical spec of my PSU is as follows: input voltage 240vac , output voltage 160vdc (no load-unregulated), maximum continous output 10a@140vdc.

[Al_The_Man]

Are you talking the output for protection of voltage spikes reflected back on the output, or on the Incomming supply?
I would put a RLC filter on the 240v ac mains at the supply input, these can be obtained in a small metal case, if the supply does not already have, there are several manufacturers.
If you want protection on the outgoing side, it may be wiser to use a RC snubber .5 ~ 1µfd in series with a 220 ohm resistor, if it is DC you do not require an AC rated cap, also if DC the resistor is optional, but a non-electolytic type cap is prefered.
RC snubber is typically fast than a MOV.
Al.

[pminmo]

Is you incoming power prone to power disturbances? If so the MOV is probably a good idea. For example if your in a shop that has machines that are turned on and off an pull a significant current. Select a MOV at a value of 20% over the nominal working voltage. For 240 that would be 288V. You probably won't find a 288V MOV, so the next higher voltage.

[UKRobotics]

I'm talking about protection from the incomming supply.

I wouldnt say our area is particularly prone to power disturbances but an engineer at the company that did my spindle retrofit suggested that power spikes are more common than you'd imagine and that he'd seen many machines where transformers got wiped out by power spikes that could have been prevented by an MOV.

Considering i've spent about $500 on transformers it seems silly to quibble over the insiginificant cost of fitting some basic surge protection.

Would this component be suitable ?

http://uk.farnell.com/jsp/Passive+Co...sp?sku=1057204

[rancherbill]

Hi

I used to sell industrial electronics and industrial computer systems.

My approach to the situation would be to buy a built "power conditioner".

They come in a number price ranges. They come in price ranges from $8 to $140 CDN

The entry power conditioner is a cheap power bar it has MOV, glass discharge tube and fuse. The problem is that they "wear out" as they take hits and become less effective over time to the point that they are not working. They are not fast to respond (Milliseconds). They are not very good.

The more expensive the power bar becomes, better technology is used in the poweer conditioner. It is faster to respond (nanoseconds), and remain effective longer.

The added benefit is that they are designed by someone who knows about transient suppression technology. It's a buy and forget decision, vs. a research, design, source, build and then worry decision.

I would buy the most expensive power conditioner that I could find, and if I wanted to really go over the top I would buy a true UPS.

Here is a link to a general surge suppression technology guide.

http://www.stealthlabs.com/surgeguide.pdf

That's my 2 cents

Bill