Testing a ZNR and other

On Tue, 14 Apr 2009 21:02:28 -0700 (PDT), Uriah wrote:

:Thanks to everyone for all of the help that I have received through :this group. This one and the Pinball newsgroup are the most helpful :newsgroups there are. : :I have a few components that I am having trouble finding out how to :test. One is a zinc oxide nonlinear resistor. It is labeled on the :board as ZNR1 the symbol is a Z on its side. It is has two legs and :is orange, kind of shaped like a capacitor. On it is written 10N471K :under that is 66+. It is some kind of protection device. If I test :it on a Huntron it shows a capacitor like signature. It is not :shorted. How do I know that this thing is OK. How would I test to :know that it is OK. Everything I read requires equipment I don't :have. Does this operate on AC or DC in the circuit. Is it to protect :for over voltage or reversed voltage? How is it supposed to work? Is :it a one time thing that goes short or open when it is in action or is :it something that works by keeping a voltage in check? : :The other thing is a blue square two legged device. The symbol on the :board where it is mounted shows a capacitor and a resistor and is :labeled R63 which implies that it is a resistor of some sorts. But why :is there a capacitor symbol along with the resistor symbol on the :schematic and the board. Written on it is 100nM +120K, Under that is :PCRC 420 250V~2, and under that is PILKOR WK0626. What am I testing :this thing for? Resistance and capacitance? How do I know when it is :failing or has failed? All of these new components always leave me :guessing. : :Thanks very much :Russ

It is a 470V varistor. You'll find it mentioned in the Fairchild app note.

An MOV (ZNR1) can be tested by applying a variable DC voltage through a limiting resistor of about 100k. You can read the leakage current with a DVM across the resistor. When you see the current start to rise quickly with increased applied voltage, that will be close to the rated breakdown voltage of the device, which is in this case is about 470V although the actual breakdown may be about 600VDC.

R63 is a snubber which has a 100nF capacitor and probably a 120 ohm (not

120k) resistor in series with it. You can test it with an RCL meter that gives ESR, or apply an AC current and observe the phase shift with a scope. At low frequencies it will act as a capacitor, and at high frequencies it will act as a resistor, so you can test it that way as well.

Paul

Thanks for the help. Could you clarify a few things?

which is in this case is about 470V although the

Do I need to get a 400-600 volt DC source to do do this test? Lets say I have this device. Would I need a meter to handle this voltage also right? Then I would put a DVM in line with it to monitor the current. Should it not allow any current to flow at lower voltage and then when it gets to 600VDC I would see a rush of current flow at its breakdown point. Does that then fry the device? All of this makes sense. Could I use a Megger type tester to do this?

You can test it with an RCL meter that gives ESR,

With the Snubber, I have a RCL meter, one of those smart tweezers. Would that 120 ohm resistor always show up when reading ESR? Would that interfere with the low ohm leakage that ESR usually reports? If it was only a cap, and I got something like 4 ohms with an ESR meter, would this device then add the 120 ohms to that? I am not sure I would know what is a correct reading would be when using a scope to watch the phase. I don't have a known working one of these to compare with. Same with the ZNR. I would have to order a few of them to be able to compare a known working one to the ones I have.

Thanks for the help Russ

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Don't bother. In normal operation, this kind of protection device passes little or no current, and you've already verified that behavior. Like a fuse, it does very exciting things when a power surge comes in, but if you DO apply such a surge, it (like a fuse) might change behavior.

Low-power testing is possible, but that takes (at minimum) a HV transformer and appropriate safety precautions, and your test will only cover a small subset of the specification sheet behaviors.

Yes, a Megger should work. Or a hipot. As long as the current is limited, the device should not be damaged or degraded. You should be able to apply the rated working voltage plus a bit more and see a high resistance and no breakdown. These devices usually fail shorted or leaky at lower voltages, so you should be able to detect a bad one. It will usually test as a capacitor.

It may depend on the specs of the LCR meter to see if it will discern a 120 ohms resistance in series with the capacitance. The ESR of the capacitor should be in milliohms. You could use a known good capacitor and resistor in series and compare the waveforms. The Huntron tracker would work well, or a Lissajous pattern on a scope.

You're welcome,

Paul