Dealing with mains power transients

I'm looking at protecting a power sense circuit from mains power transients, using a resistor and a zener diode.

Spice says it works well, but when the zener conducts, essentially the entire transient voltage appears across the resistor. A high voltage transient spike is likely to be very narrow, but I can find nothing to say I can put, say, 5000V, across a 1K resistor, even for a microsecond. Indeed, it seems entirely possible that a current would flow across the outside of it, rather defeating the point of having the resistor there.

Any thoughts?

Sylvia.

Reply to
Sylvia Else
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Some resistors come with pulse ratings. There's a power/time relationship that's not just as simple as dumping a constant energy pulse of any width into the thing, so you want to specify a resistor that comes with real live charts on the data sheet.

I probably don't know the whole story, but I would assume that the voltage rating on the same series' high-resistance parts would apply for transients on the lower-resistance parts.

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Tim Wescott 
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Reply to
Tim Wescott

How about a very small spark gap ?

Also, what about the Zener diode, isn't it in peril here ?

Reply to
jurb6006

It just draws enough current through the resistor for the latter to drop the transient, so I don't think it's in danger.

Sylvia.

Reply to
Sylvia Else

I believe a residential service entrance will only see about 3kv if there is a fault. Maybe the NEC has the details.

Cheers

Reply to
Martin Riddle

You can get axial-lead high-voltage resistors into the 10s of kilovolts. And there are surface-mount resistors in the 3 KV range.

Regular 1206 resistors are OK to a couple of kilovolts (I have some test data around here somewhere) and you could series a few of them, which would be cheap.

Do you expect 5KV transients? I guess lightning could do that.

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John Larkin         Highland Technology, Inc 

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John Larkin

** High voltage spikes certainly can damage resistors, both solid composition and ones that use spirals to increase the length of the conductors. High voltage types for special purposes exist but the average film resistor is rated for no more than 500V.

However, common neon indicators are used directly across 240V mains with internal film resistors of about 200kohm with 0.25W rating and typically last for decades. The failures seen are all with the resistor going open.

Connecting identical resistors in series reduces the spike and continuous voltage seen by each to a much safer level.

The best idea is to add a suitable varistor across the AC supply input to your circuit - so protecting everything beyond it.

.... Phil

Reply to
Phil Allison

Just put 5 1206 smd resistors in series. Look up the datasheet, one resistor can handle 1kV peak voltage

So 5 will provide good robustness against surges

Regards

Klaus

Reply to
Klaus Kragelund

Right. If you see one that gives a single pulse rating, you can probably-kinda-sorta-hand-wavey-extrapolate around it.

The slope is usually a goofy function of time, like t^(-1/2) (which implies diffusion, a classic thermal effect), or t^(-1/3) (..?!). Rarely, it is inversely proportional, even to fairly small time scales; this is mostly confined to the "high pulse" (bulk / composition) types.

Naturally: if the datasheet doesn't say explicitly so, you're on your own. A guess is just a guess... until you start getting field failures back (or not, as the case might be!).

Tim

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Reply to
Tim Williams

About that. IEC 60950-1 says 2.5kV is okay (though, to meet that classification, you also need single failure redundancy -- so, say if connecting resistors in series: use N+1 of them!).

What you're actually getting is: statistics. Transients over 2.5kV are very rare (decadal, perhaps?), so it's useless building most equipment to handle such events.

Tim

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Reply to
Tim Williams

Kinda depends - if you are Jeorg-pricing a solution it's rather different than if not.

My default whole-house (single-phase 120/240 VAC) mains-input regimen at this point is Delta LA 302R (SOV) and CA 302R (surge capacitor - also facility EMI filter). It's not magic, but it sure does seem to reduce issues - of course, that could be confirmation bias, since it's all been negative results so far. And it's around $100 with shopping, more without shopping. Squish it where it comes in, protect everything. You probably have something less broad and far cheaper in mind ;-)

High voltage lab methods were sometimes to spend for the high voltage / high power resistors, often to string a bunch of normal axial-lead resistors in series and stuff them in a plastic tube (tube does not help power dissipation, does help not making inadvertent contact with the high voltage junctions.) Can do the same on a PCB if you are careful that the layout is not self-defeating.

Vishay (just first place that came up looking for datasheets) offers a line of chip resistors they market as "pulse-proof" but you're still looking at a peak voltage of 500V on a 2512, or 200V on 1206. They also have medium voltage chips at 1415V in 2512, or 550V in 1206, and high-voltage but with very high resistance as a minimum (so you'd be hard pressed to get 1K from any reasonable number of those.)

Of course, you could also put a high-voltage capacitor in parallel with your resistor (or simply across the line) to take the edge off the spike. Mouser claims to have a 5KV AC/DC rated 220pF for just over a buck in onesies, 75 cents at 100. Still too rich for Jeorg, of course.

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Reply to
Ecnerwal

Right, the 1K value adds difficulties. Most HV resistors come in correspondingly high ohmic values.

5KV across 1K is 25 kilowatts. A 1 us pulse dumps 25 millijoules, which should be OK into a few 1206 or 2512 size resistors in series. I guess the issue is demonstrated survival vs regulatory approval.

There are some telecom-type resistors that come in low values and are specified to withstand lightning surges, usually 5KV peak.

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John Larkin         Highland Technology, Inc 

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Reply to
John Larkin

There must be a reason nobody suggests an inductor in seies. How come?

Reply to
Tom Del Rosso

Inductors can be useful, such as to reject a fast transient by low-pass filter action (L-C). It has to be able to handle the voltage impressed across it, which isn't always easy. Turns and cores arc!

Cheers, James Arthur

Reply to
dagmargoodboat

Well, I can tell you what happens to a 10 Ohm, 1 W axial film resistor when the line gets a lightning strike. The resistor goes POP and all the paint and much of the resistor element disappears. Sometimes I have to replace the gas tube arrestor, too, after these events, but the equipment being protected has survived several of them. The resistors are in series before the arrestor.

Jon

Reply to
Jon Elson

You really should use a resistor specifically designed to handle surges. Oh mite has several lines depending on the type of surge, one line even for li ghtning surges. Most of them are designed around IEC 64000 transient profil es, so you need to look at that and determine which one best fits your circ uit's risk exposure.

Reply to
bloggs.fredbloggs.fred

They're the lowest loss option, and it would be more of bead than an inductor= predominantly resistive dissipation of the high frequency components of the transient. Epcos has detailed design notes centered on their product line.

Reply to
bloggs.fredbloggs.fred

That, and flux.

If you've never played around with an EFT test machine (or racked your brain over a product while in the test lab), then you'll have to realize the pulses are quite tall indeed. 50ns doesn't sound like much, but it's not just a couple volts of RF, it /just keeps coming/. It's an electromagnetic tsunami. You can literally observe it washing over cables and modules, and reflecting off end nodes.

The biggest problem is the high impedance. Normally, such a test is performed at a modest distance above a ground plane, so that the cables have a common-mode transmission line behavior much like microstrip, with an impedance on the order of 150 ohms.

Even if you put a huge ferrite toroid in series with that, with many turns, all you can do is take the edge off -- a huge hump of flux still makes its way through. Indeed, if you try shorting the pulse across such a choke, you'll probably saturate it.

A 2kV pulse 50ns long contains 100uWb of flux*. That'll saturate a single turn around ferrite (Bmax = 0.3T let's say) of 333 mm^2, which is a sizable pot core, or a 75-100mm toroid, or many turns around the average ferrite clamp.

*Not really, but ballpark close. The real figure is probably less than a full rectangular 2kV * 50ns. But it's more than half, because after 50ns, the pulse is only half amplitude, and still drooling on.

As for *surges* (~us domain), that's practically DC. Those can be filtered by some iron-cored equipment (which again, largely does the job by saturating from the excess flux), but usually must be dealt with by clamping or absorption -- hence the popularity of MOVs.

Tim

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Reply to
Tim Williams

Heh. A very similar, though smaller scale, application I've seen: in high resolution CRTs (e.g., Trinitrons), the wideband video amplifier (50-100V at >100MHz) usually had protection diodes on its output, then series resistors, then GDTs, then carbon comp resistors, before finally connecting to the CRT.

The event being:

- Internal arcover from 2nd anode (30kV and ~1nF, not your grandfather's ESD!)

- Zaps the grid and/or cathode

- Carbon comp increases impedance

- GDT ignites, shunting pulse (but not after having let through a few hundred volts still)

- Second level resistor, and clamp diodes, handle the rest.

All together, probably still not all that healthy for the vid amp (which is made with ~1GHz, 100mA BJTs, usually monolithic), but that's all they can afford without killing bandwidth!

Tim

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Seven Transistor Labs, LLC 
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Reply to
Tim Williams

Automotive circuits don't have to protect against anything like 5KV (more like 50-100V) but they often use Zener (TVS) diodes with no resistor. Basically the resistor flattens out the transfer curve so the output voltage goes higher than it would without the resistor. Often a pi circuit is also used to filter the transient (though that's not its only purpose).

Reply to
krw

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