Varistor question

I have a question about varistors. Actually two.

Question 1.

Are varistors the sort of devices that cannot be expected to conduct many pulses of electricity?

Or can I expect a varistor to happily conduct many short pulses of extremely short duration, into a resistor so that energy dissipates in a resistor and not in a varistor?

In other words, are they more like one shot devices, wearing out with every breakdown, or can they be expected to conduct a tiny amount of electricity 1000 times per second?

I realize that I cannot exceeed the power limit that the varistor is rated for, for any duration like seconds or minutes. But what about very many pulses that, cumulatively, are below the varistor's rated power on the per minute basis.

Question 2. Once a breakdown voltage of a varistor is exceeded, and it starts conducting, when would it return to a non-conductive state? When current falls below some threshold? Or when?

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Reply to
Ignoramus10725
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Yes, when operated within their specifications, varistors can be expected to last as long as any other solid-state device.

Varistors are not like that. They do not suddenly start conducting. They do not latch. In fact, they have a very gradual current-vs-voltage curve. Take a look as some data sheets. You will see that in order to conduct enough current to protect a circuit at

400 volts, it must also conduct quite a bit of current at 100 volts too. Even at 50 volts, the current does not drop to zero. A varistor would be very poor as a voltage regulator.

-Robert Scott Ypsilanti, Michigan

Reply to
Robert Scott

Varistors can stand a great many conduction cycles, as long as the peak and average power dissipation is kept within certain bounds. Here is an information document from Epcos that contains a wealth of info on varistors.

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Reply to
John Popelish

Can you put some specific numbers in there? For example, when you say "quite a bit of current at 100 volts" and "enough current to protect a circuit at 400 volts" it's impossible to tell what you have in mind.

Also, the "very gradual voltage-vs-current curve" doesn't paint a proper picture. For example, looking at the datasheet at

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for the 420 V MOV. (Figure 6) Note that the scale of the graph is not linear. If it was, the curve would not look as gradual as it appears. Here are some numbers taken off the chart:

Below the 420 clamping voltage, leakage current ranges from

1 uA at 100 V to over 100 uA as V approaches the clamping voltage. (There is a discontinuity on the chart, so the maximum leakage current is not shown.)

The charts starts again at the clamping voltage, where current is 1 mA. That's 1000 times the leakage current at nominal 120 volts. And the current range shown when the voltage exceeds the clamping voltage is from 1 mA at 420 volts, to over 1 amp at 1000 volts, to about 10000 amps at around 1200 volts.

I'll put that in a table: (deltas with respect to previous) Voltage Current deltaV DeltaI 120 1uA - - 420 1mA 3.5x 1000x

1000 ~1amp 2.9x 1000x 1200 ~ 10000 amp 1.2x 10000x

That is hardly a very gradual voltage-vs-current curve.

Ed

Reply to
ehsjr

Thanks.

Thanks. I want to use a varistor to clamp voltage spikes on a DC rail.

See my today's pictures about how I used a varistor for my tig welder inverter:

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i
Reply to
Ignoramus10725

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Thanks... I saved the file and will print it out tomorrow.

i
Reply to
Ignoramus10725

Ehhh? "Closes"? You need to think a little more about that.

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

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Do _not_ do that.

Varistors are designed to clamp once-in-a-while spikes, not
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Reply to
John Fields

Yes, but 1 mA is not going to protect anything. To serve its function of protecting against overvoltage, you've got to get up to some serious amps. According to the table that you found, that doesn't happen until about 1000 volts.

As long as you understand this table and what it means to protection from overvoltage, you can decide which varistor is right for you.

-Robert Scott Ypsilanti, Michigan

Reply to
Robert Scott

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They _are_ Zeners! :-)
Reply to
John Fields

how about TVS diodes, they seem to work much faster than zeners ?

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Reply to
Jamie

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I made a mistake; the circuit should look like this:

DC>-----+------------+--->
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Reply to
John Fields

This is an interesting idea! I will think about its implication for snubbing, esp. what happens when MOSFET closes.

i
Reply to
Ignoramus29530
[.. a bunch of stuff ..]

You may want to have a look at Sidactors from Teccor. They really do "turn on" and go to a lower drop when fired.

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kensmith@rahul.net   forging knowledge
Reply to
Ken Smith

Hmm, that's strange, the last time i used those they didn't behave exactly like Zeners. are we sure we are talking about the same thing here?

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Reply to
Jamie

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They\'re not supposed to.  In the one case you\'re talking about
reverse biasing the diode until, say, with a tame 20mA of current
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Reply to
John Fields

I used to work on supplies that used varistors, and they were one of the least reliable parts. However I'm fairly positive this was simply because they were being pushed so hard: they always showed deterioration due to high temperatures, ie blackened, cracked, and sometimes burned metallisation. I guess you can push them like that for occasional surges, but not all the time.

NT

Reply to
meow2222

they are designed to clamp on over loads that is correct how ever, on heavy over loads, they will clamp to blow the fuse and most times short them self's out permanently with out physically falling apart so that the next time you put a fuse in place! it will simply blow it!. to hold that much current back while its waiting for the fuse to blow it does get hot and destroys it self and remains shorted. that main not sounds like a very good idea to you how ever, it has protected components after it that are much more expensive..

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Reply to
Jamie

TVS diodes are good for that, they start their latching process early once the set voltage starts the clime on the low side. because they don't act in a linear way like a zener would when the avalanche voltage is being approach, it makes this little device a nice thing to have for a fast clamp, and when it does clamp they clamp to a very low R.. and to top it off, because of the low R in the clamp state they come in some large amp sizes in small packages. and the fast time recovery is good so that you don't see a dip below the min voltage. the only problem i have seen with these items is in a case where we used a TVS that apparently was clamping alot on a circuit, the trace on the board was generating pulses to the adjacent trace there for inducing a pulse where we didn't want it. in that project we had to redesign the board to remove the noise.

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Reply to
Jamie

That sounds like diode-suicide in this application.

Chris

Reply to
Chris Jones

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