Varistor question

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I have seen Epcos recommend the use of varistors in repetitive surge applications. I am therefore surprised that you say not to do it. What leads you to say that the varistors will wear out, and do you have any suggestions as to why Epcos think it is ok?

Chris

Chris Jones said

Sorry for butting in.

I work in the automotive field where transients on the vehicle battery line are pretty nasty. We quit using varistors years ago for reliability reasons. I don't know the specific details - as this was before my time. Now we use Vishay TVS's in big diesel applications. Wimpy gas controllers use simple zeners I think. (I don't work on the wimpy stuff) ;-) YMMV

It can be if you don't limit the current into them. They can, however, handle some fairly large current spikes.

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kensmith@rahul.net   forging knowledge

The 'wearing out' of a varistor is defined by the term 'degrade'. Varistors fail when threshold or let-through voltage changes. Many varistor manufacturers use 10%.

A varistor degrades as a function of transients defined by parameters pulse width, current, and number. Varistor manufacturers provide charts for each component. For example, one manufacturer lists the surge life of their series 7 varistors as 10,000 (number) 50 amp (current) transients of the classic 8x20 microsecond (pulse width).

Any varistor that vaporizes was used well bey> Thanks. I have not been able to find much info on this subject, I tried

I wouldn't use a varisotr as an OVP in any known repetitive design. (like a chopper for 200A!)

I'd use a TransZorb, which is basically a 5- or 10-watt Zener that can take surprisingly large spikes, as long as the duty cycle is small enough.

I've seen them used to protect LV circuis from HV arcs, which can happen a lot when you're experimenting with new electron gun designs and stuff in a big ultrahigh vacuum chamber. :-)

Or, in an app like that, I'd look for a designn that somehow momentarily stores the spike and gives the energy back. (resonant LC snubber? :-) )

Good Luck! Rich

I have a few questions about the Black regulator described in the above article.

-- What controls the switching frequency?

-- What determines the relationship between the two capacitors?

-- What transistor datasheet characteristics would indicate its applicability in that circuit? The article states that "hard turn-on" and "hard turn-off" are important. (Oddly, these aren't spec'ed as such.) Sure enough, playing around in SPICE, a 2N2222/2N2904 pair switches poorly, and works only under very light loads.

Also, playing around in SPICE, the power dissipated in the load is off by almost a magnitude from the RMS power entering the circuit. For a 48V battery voltage source V1, RMS(I(V1))*RMS(V(V1)) gives about 16A, but the dissipated power in the dummy-load resistor is about 2.5A. The dissipated power in all other components seem suitably small, in the micro-, nano-, and milliamp ranges. I'm at a loss to find where the losses are. The PG() and PS() results are similarly non-sensical; less power enters the circuit than the load dissipates.

Or a transzorb:

Well, it's a switch, right? So it closes, right? ;-) Reminds me of the tech school teacher that referred to "cow gates". For example, in an AND gate, a "1" at one input "opens" the gate, so that the other input can get through to the output. I think he was an idiot, or maybe he was just telling us about some other idiot. I wasn't paying that much attention - I had learned about that stuff as a scruffy adolescent. ;-)

Cheers! Rich

A TransZorb (tm)

is a Zener with a die that's designed to switch very fast, and is able to conduct a surprising amount of current, for a short spike, which it dissipates at its leisure. :-)

I don't work for the TransZorb people, I'm just a satisfied customer. :-)

Cheers! Rich

A. I've seen them wear out. On occasion, I've seen them "wear out" very rapidly. ;-) This is how I learned about Transzorbs. The brainiacs were looking for something that would last more than a couple of "spikes".

B. They're probably idiots. ;-)

Cheers! Rich

"Jim Thompson" a écrit dans le message de news: snipped-for-privacy@4ax.com...

Eh, given the currents level, if you leave enough (little is enough) inductance on the DC path between the zener and the mosfet drain it'll sure close. Definitively :-)

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Thanks,
Fred.

If energy conducted through a varistor is so large as to vaporize the varistor, that is well beyond any manufacturer specs or charts. Why? Varistor is operating well beyond what manufacturer intended. Keep those manufacturer limits in mind next time a power strip protector fails by burning out.

To protect from such unacceptable failures, Littelfuse markets a varistor line called TMOS.

Varistor must never heat so excessively as to vaporize. And yet many who recommend protectors without first learn> Thanks, I've never seen one of those graphs of lifetime in number of spikes

Thanks, I've never seen one of those graphs of lifetime in number of spikes vs. spike current or energy, though so far I haven't had the need to look into it for anything I have designed. I was already aware that varistors could overheat destructively when connected to line voltage at some time after they have been exposed to a severe transient but I had previously thought that there would be some threshold of transient energy below which the life would not be limited to any practical extent. It's always good to learn something, and I shall investigate this further should I ever consider using a varistor in this sort of situation.

Chris