blowing resistor

[panelman]

A slightly unusual one………

We have some 1 meg 0.2W smd resistors that are encased in potting compound and need to blow them open circuit down the wires from the outside.

Equipment we have available includes a 0-450V Varic and a 0-5kV 5mA flash tester.

As most of my previous contact with resistors has involved trying to not blow them up I’d appreciate any input.

 

[Skogsgurra]

Just a couple of wires and a resistor? Or other circuitry also connected? If other circuitry, I would try and locate the component and then drill down to it.

Would it work to blow open one wire instead of the resistor? A 200 mW SMD resistor will probably fail temporarily shorted if you put high voltage across it. Put a capacitor parallel to your flash tester to get a nice heavy current pulse to blow the wire clean. You will probably not need all the 5 kV, so capacitor choice is somewhat easier.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[panelman]

Thanks…..

No other circuitry, two wires go into the potting compound, each is soldered to a piece of copper mesh, the resistor is soldered between the two bits of mesh.

They want to blow the resistor but keep the connections to the mesh

 

[DickDV]

Well, .2 watts times 10 to blow it and 1,000,000 ohms is how much voltage? V^2 = P x R. = 2 x 1,000,000 = 2,000,000.
The square root of 2Meg is 1414 so that's the voltage needed to get 2 watts of power in that resistor. While it won't be "blown apart" by that wattage, I would expect it to burn open in 10 seconds or so. More voltage would make it happen faster but eventually you will get some "collateral damage" which you don't seem to want.

Good luck. This even sounds like a bit of fun. Let us know how it works out.

 

[waross]

5kV across 1 meg = 5 ma.
5kV and 5 ma = 25 Watts.
That should do it.
Try one and let us know how much collateral damage you suffer. grin

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[panelman]

I'm leaning towards the 25W short sharp option rather than the 2W 10s one as well.

I did think of a capacitor but the flash tester trips at 5mA so I didn't think I'd be able to charge it up without lot of messing about with series resistors

We have a couple of samples to practice on before the real thing which is lucky as if I can't pop them my client ends up spending a week digging all the potting out and redoing it

 

[Skogsgurra]

The problem, I think, is that a 1206 SMD resistor isn't specified for more than around 700 V pulse. And that is quite short a pulse, around 100 microseconds maximum. Then again, there is probably a safety margin, so it is not easy to predict the outcome.

Not sure if one can guarantee a flash over across the resistor, but that is the most likely outcome. When that happens, no thermal effects from a 5 mA power supply can do any damage because the voltage goes to zero. Or almost.

That is why you should use a capacitor to get enough punch to blow either the resistor or the wire away. If you don't want to hurt the wire, a drill is probably the safest way to go. I would chose that in any case.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[Skogsgurra]

Forgot about the tripping: If the flash tester trips at 5 mA, you can easily bring it up 'nice and easy' by starting at zero volts and adjust manually until full voltage across capacitor.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[sreid]

We tried it with 100 Ohm 1206 Resistors [charged capacitor]. Those blew cleanly with no damage to the package what-so-ever. The resistors were used in series with IGBT Gates. In through hole versions, Fusible Resistors wer used so the circuit board wouldn't be damaged if the Power IGBT blew. In surface mount designs, we simply used the 1206 resistors since they blew cleanly.