MOV Question 1

[PUMPDESIGNER]

I am reading a manufacturer's spec's on an MOV. Part number is VA150L10a.
Chart says:
"Varistor voltage @ 1mA DC Test Current"
The directly under that are two columns as follows:
V-Nom Min. and V-Nom Max.

The rating under the MIN column for the MOV I am looking at is 212, and the rating for that same MOV in the MAX column is 268.

How do they arrive at those figures, what is the test procedure?
What do those figures mean?

My reason is simple. A competitor uses that MOV on all their products working on 115 VAC system. Their stuff blows up all the time, way too often with the MOVs burning out. We believe the clamp voltage is way too tight, they are catching normal common little spikes which are just too common to attempt dealing with at the MOV position, should be handled elsewhere.

Any insight would be helpful.

PUMPDESIGNER

 

[BrianG]

The reason there are V-Nom Min and V-Nom Max voltages quoted is due to plain old production tolerance. Unlike zener diodes the "knee" voltage, i.e. the 1mA test current where the device starts to conduct, is not all that well-defined in varistors.
Varistors come in various different power ratings, specified for total pulse energy in Joules. You need to have some idea of the kind of transient you are trying to damp out to select the correct one. You may find that the combination of an R-C snubber circuit plus an MOV works better.

 

[OperaHouse]

Could be. Years ago 130V MOVs were common and then they found that they were being destroyed over time. Now they have gone to 150. Anything you can put in front of them helps. You can even go to lower power if they are attached to a long power cord. I also like RC networks on power lines. Had a friend tripping his alarm system about once a week that was connected to a GFI outlet. He asked we if I could figure what the problem was. Noticed that it was plugged into a surge ptotection module. Popped the cover off and it had 130V movs in it. Replaced that and not a problem since. A lot of industrial areas see a rise in voltage on weekends. You really don't see any los in protection if you go to 270V.

 

[Comcokid]

GE used to have a good MOV design guide that had quite a few pages. I'm not sure who has the GE line now, but a quick search showed LittleFuse had some good MOV data.

It's been a while since I've designed in a MOV, but I seem to remember that a MOV that is subjected to repeated spikes over time and age slowly develops a lower trip voltage as the zinc oxide grain sturcture degrades. The typicial, eventual failure mode of a MOV subjected to repeated transients is a short circuit. Early surge suppressor outlets used just a MOV. Later, in this application manufacturere began using a fuse in series with the MOV, possibly due to UL regs.

I suggest you search the literature from the different MOV manufacturers.

 

[PUMPDESIGNER]

Thank you guys for your comments.
Comcokid - You are correct about that GE book. D23 send me a scan of the relevent pages from that book.
Your comments were helpful.

PUMPDESIGNER

 

[rumbaldking]

Eagle made (?makes?) a duplex outlet for installation in homes, I guess, with little replacable MOV plugin elements. It beeped when the MOV had been "Consumed" by arresting enough surges until you removed the thing, but I don't know if it beeped until you put a new one in.

 

[hillwilliam]

An MOV acts like a Double Anode Zener (a very rare device) or two zeners back to back.
However there are very important differences. The impedance (slope) of the I vs V at avalanche is very large for a MOV. This is why the voltage max is so much larger that the min.
Unfortunately, each time an MOV avalances (protects) the PIV at rated current (say 1 ma) decreases. They will eventually short over time. (A small path is burned through the ZnO structure).
This is also why surge suppressors have a rated lifetime.
If you look at protection, you will see that a diode is sufficient for DC but an MOV (essentially two diodes back-to-back) is recommended for AC.

 

[twodogs]

Hey, pumpdesigner - what's burning out? which MOV - or is it all 3? We had an offshore job, nuking the L-G MOV almost continually. Floating ground on the vessal! Up to 350V float, no way to stop that on a 120VAC line. Removed it, left L-N in place, problem solved, welders think I'm a genius (don't tell!) <G>