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Low inductance pulse resistors that don't blow up?

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Skogsgurra

Skogsgurra

Electrical
Mar 31, 2003
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I suddenly needed to compare a small three-phase transformer winding's reaction to a fast voltage front, similar to a Baker test but on a budget.

HV - no problem. A rectifier cascade built from PFC capacitors and standard 1N4007 diodes outputs 1.3 kV but the series resistors needed to get a reading on the scope is a problem.

Total energy in the capacitors is 1.3 Ws and I thought that ten 1 W 4.7 ohm resistors should divide the energy between them so each would see around .13 Ws. No sweat for a 1 W resistor was my reasoning.

The resistors flashed over on the first try. Same thing on second and third try. Then, there wasn't much left of the resistors and no hope to make it work either.

Obviously, the short discharge time takes momentary power up to tens of kilowatts in each little resistor and that power concentration is way too much for them.

I am sure there are standard methods and standard components for this. But I haven't found them. Isabellenhütte? No data given for this kind of operation.

Gunnar Englund
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Half full - Half empty? I don't mind. It's what in it that counts.
 
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Welwyn Components used to have a range of thick film resistors designed to have very low inductance and were specifically for high power pulse applications.


Disclaimer: I used to work for them, although not in the resistor plant (I was in thick film hybrids).

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If we learn from our mistakes I'm getting a great education!
 
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Rubber washers over every component to increase the flashover distance?

Pot the whole thing?

Submerge it in Fluorinert?

I dunno.


Mike Halloran
Pembroke Pines, FL, USA
 
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OK, I should have used some other word. It is not flash-over, but it looks and sounds like it. It is more of a concentrated high power that vapourizes the resistive material. Voltage across each resistor is no more than 150 V and that should be OK.

Scotty: I mailed Welwyn. Waiting for an answer.

Gunnar Englund
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Half full - Half empty? I don't mind. It's what in it that counts.
 
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Gunnar,

Were you using metal-film resistors on the first go around? I.e. something with lowest possible tempco?

Tungsten, by the way, has a huge, 10:1 or more, change in resistivity from room temp. to operating temp at 3000+ Kelvin. The inrush at start of a tungsten filament is the most likely cause of all failures (ok, weakened filament from 1000 hours of use also contributes).

I like the idea of spreading the load, but think you went the wrong direction, i.e. the did the leading resistor in the chain take the brunt of damage? Would ten 470-ohm resistors in parallel work, I wonder?
 
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Yes, I used what I had. Metal film, 1W, 1%. I have been destroying more passive components today. All my metal/paper capacitors are now a bleak shadow of what they used to be. Very low capacitance. PM on that coming soon on a computer near you.

PS Diodes also bad. It all happened when I tested 400 V input instead of nominal 230 V.

Gunnar Englund
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Half full - Half empty? I don't mind. It's what in it that counts.
 
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Google: globar carorundum non-inductive resistor

These are famous in the radio industry and are often used in high power dummy loads for transmitter testing. They're indestructable.

 
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Problem is initial current value; 1300/47=27.65Amp; that value is too much for usual film or solid resistors. Even this current go fast to zero, circuit have about 61 miroseconds constant time, instantaneous power disipation is very high in this very short time frame, starting at about 3.59kW going to 1.59kW at constant time; these values are for only one resistor.
 
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Try some carbon comp type resistors. Question why do you use such a low resistance? Most scopes have an input resistance of 1 Mohm (without a 10x probe) so you can use a series of 10K resistor without sacrificing accuracy.
 
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Hard to get any oomph when driving a motor or transformer via a 10 k resistor. Thanks for that other tip also. The problem is sheer power concentration over a very short time. I'll go for Welwyn or Carborundum.

Gunnar Englund
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Half full - Half empty? I don't mind. It's what in it that counts.
 
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You sound like you're having too much fun blowing up everything in sight. Better watch out for PETC (People for the Ethical Treatment of Components):)
 
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John and Ben: You are seeing right through me. Yes, I have fun.

You know what? I tried lead mines for architecht's pencils. They can take the high-powered pulse. But they only have around 7 ohms each (HB). But still..

Gunnar Englund
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
 
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