MOV Failure Mode
MOV Failure Mode
(OP)
I am interested to hear what failure modes people have found failed MOV's in, ie open or short circuit. I am questioning the use of an MOV across a DC relay coil vs. a diode with a series resistor. I am concerned with an application (safety device) which is calling out to put an MOV across the coil. If it were up to me, I would put in a diode and series resistor for the following reasons:
- series resistor will minimize the time to de-energize vs. just a diode or just an MOV
- MOV is one device so it could potentiall fail shorted causing the safety device to not function when called upon to do so
- MOV's lose their energy dissipation abilities with each 'surge' of voltage making it less affective as it absorbs more 'spikes'
- Their is concerns with an 'short' occurring with either configuration but there is two devices that will have to fail shorted and I have not seen a resistor fail short circuited but I have seen a diode fail shorted
Any thoughts on failure modes of MOV's or other additional input will be appreciated.
- series resistor will minimize the time to de-energize vs. just a diode or just an MOV
- MOV is one device so it could potentiall fail shorted causing the safety device to not function when called upon to do so
- MOV's lose their energy dissipation abilities with each 'surge' of voltage making it less affective as it absorbs more 'spikes'
- Their is concerns with an 'short' occurring with either configuration but there is two devices that will have to fail shorted and I have not seen a resistor fail short circuited but I have seen a diode fail shorted
Any thoughts on failure modes of MOV's or other additional input will be appreciated.





RE: MOV Failure Mode
RE: MOV Failure Mode
The main concern is not the kick-back protection but the fact that a safety device uses an activated relay for the safety function. You will never get a fail-safe function there, regardless of what kind of protection you use.
Instead make sure that the relay is activated in normal situations and drops in abnormal (dangerous) situations. Use forced breaking and NO contacts. For even better safety use two relays and two NO contacts series connected.
A diode plus a resistor is probably the best choice in this application, but I would not hesitate using a MOV. I could even think of an RC combination, even if they are mostly used with AC coils.
BTW; if this is a safety device, it should not operate very often so you may be able to operate without protection (if you do not need to protect the driver).
RE: MOV Failure Mode
I strongly support skogsgurras comments re the NO relays for fail safe applications.
RE: MOV Failure Mode
To answer Buzzp's question on the failure mode of MOVs: I have only seen them fail short.
RE: MOV Failure Mode
I guess after the input I got, I am still not convinced I would be using the MOV in this application just because of the possibility it will fail shorted. I only know of cases where they fail shorted or simply burn up or explode.
Cbarn, You bring up a good point about an MOV vs just a diode. This is why I would use a diode with a series resistor. That brings up another item. If we use a diode and series resistor, the resistor will have to handle this high voltage for a short time. The question is how large will this resistor have to be as far as the transient (short time) voltage is concerned? Now I have to go check some resistor data sheets and calculate the amplitude of this voltage.
RE: MOV Failure Mode
Let's take an example: The relay coil has 500 ohms resistance and 24 V nominal voltage. You will then have 24/0,5 = 48 mA through the coil. This current will flow through the diode-resistor combination when you switch off and if you choose a 1 kohm resistor the initial voltage will be 48 mA times 1 kohm = 48 V. The current will decline three times faster than the risetime when switched on (the total resistance is now 1,5 kohms) and that is very often enough. So, a diode plus a resistor is a very good choice. The resistor wattage is almost always very low. You seldom need more than 1/4 W if you are talking normal relays.
RE: MOV Failure Mode
RE: MOV Failure Mode
buzzp: I'm still a little confused... If you are talking only about anti-kick back on the relay coil and it's DC why are you not just putting a reverse diode across the relay and being done with it?! I must be missing something... Why do we care about how fast we damp things on a relay coil unless you're trying to collapse the field extra super hyper weird fast for some pecular situation? You might want to put a mov across the CONTACTS to keep the noise of switching an inductive load down.. resistors and capacitors in "this" application will leak and drive technicians with DMM's insane tho.
RE: MOV Failure Mode
The resistor is there for more than one reason. The obvious one is that it will speed up the drop out (no pun). The other, not so obvious reason, is that diodes can also break down. If that happens, then the resistor will prevent shorting the power supply, blowing a fuse or tripping an overcurrent protection or whatever. It would actually be possible to use only a resistor parallel to the coil, but you will get a lot more power consumption that way. And the resistor has to be dimensioned accordingly.
OperaHouse mentions that resistors also have voltage ratings (not just wattage) and that is correct, but in this applications the voltage rise will be no more than 48 V and the lowest voltage limit you see in ordinary low wattage resistors is usually 200 V. The "power pulse" will be less than 2 watts during 100 milliseconds. And that will not heat an ordinary 1/4 W resistor more than temporarily and not more than, say, 50 - 80 degrees C.
RE: MOV Failure Mode
If you use a diode and 1K-ohm resistor as Skogs suggests, and if the diode fails as you suggest could happen, the resistor would have to dissipate over 1/2 watt while the relay coil is energized.
RE: MOV Failure Mode
RE: MOV Failure Mode
Well let me update everyone. This particular device is a lock-out relay (125VDC, 23 ohm coil). It is normally not energized. When a problem occurs, the relay is momentarily energized. The way this particular relay works, the coil is way underrated or the voltage applied is much higher than the rated voltage. This is done to energize the relay the quickest. Now you can see why the relay is only energized for a short time, the coil would burn out otherwise. It is de-energized with its own NC contacts. As I understand it, this is how most utilities handle their lock out relays. I do not necessarily agree with this but it is what I have to work with for now. You can see, if the MOV fails shorted, then when the relay needs to energize it may not due to too much current being routed around the relay coil.
I have done a lot of research on this and I still feel, for this application, if suppression is required that only a diode with a series resistor be used. This will be the quickest as far as the de-energize time if suppression is required. I have heard claims that if the series resistor is selected such that the value is equal to the coil resistance then this is usually the optimum value. Has anyone have any experience with this statement?
I will run through the calculations to see what kind of power rating I need on the resistor. Thanks to all for your input.
RE: MOV Failure Mode
A diode by itself offers better suppression than any combination using a resistor however it slows the deenergising time of the relay but that does not sound like a problem in your case.
RE: MOV Failure Mode
RE: MOV Failure Mode
RE: MOV Failure Mode
RE: MOV Failure Mode
RE: MOV Failure Mode
Is a lock-out relay the same thing as a "shunt release" coil in a MV or HV breaker? (I am not familiar with the word usage in your language) If so, we are having the same problem universally; Coils that are supposed to work when needed, but tested only once a year. If you are lucky.
Anyhow, in these cases, there is not much need for either kick-back protection or EMI reduction, which is the other reason for putting a MOV or diode or RC combination parallel to the coil. I think that you can live happily without any protection at all. But, of course, you shall decide that yourself.
RE: MOV Failure Mode
RE: MOV Failure Mode
There is a need for protecting the solid state relays from the high voltage transients developed when the coil is de-energized. Granted, the relays generally have some of their own protection but it is always better to handle these kinds of issues at the source. As far as EMI, the MOV would be useless for anything that is not conducted. Any radiated emissions from neighboring equipment would still affect the unit(or could). Thanks for the input.