another resistor killer

A few hours now at 500 volts, and I'm finally seeing some action. Resistance has dropped about 0.1%.

The 499r thickfilms should be fine at 250 volts and a few hundred ns pulse width.

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics

The impulse load ratings for the Vishay thin films that I posted suggested that for them the thermal mass of the substrate stops mattering below about 300usec.

You may have to worry about the skin effect as well, which restricts very high frequency components to the edge of any track. For higher resistance metal film resistors the tracks tend to be pretty narrow anyway.

So you probably shouldn't use them for that job.

And where you are starting to see changes in resistance with time.

Should be fine at 250 volts per resistor.

After a day of pulsing at 500v, 500 watts, the resistance has dropped about 0.1%.

It takes me about a minute a day to run this experiment, so I'll let it run some more.

I might try the thinfilm 1206 just for fun.

I should have bought a higher voltage version of this power supply. The mosfet is good for 1500 volts.

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics

A prudent engineer would try a part that was specified for some kind of short pulse use, rather than one which wasn't at all.

All sorts of nasty stuff starts happening at high voltages. Cambridge Instruments didn't like photomultipliers where the photocathode was more that 1kV away from ground. The glass widow of the photomultiplier tube and the glass window of the sample chamber both started conducting current by ion migration at that sort of voltage, with a little electroluminescence, which was an inconvenient source of noise.

It's within the tempco range of a cheap thickfilm.

If the resistor is being damaged, we may have fused a few weak spots, and things may stabilize. So I'll let it keep running.

A sensible engineer will test parts that are available. Maybe you can find a small surface-mount 499 ohm resistor that is specified to tolerate kilowatt pulses.

Being prudent isn't fun, or a good way to invent things. It's good to get crazy and reckless once in a while, destroy things.

The final design stages of a sellable product should be very prudent and reviewed by careful and prissy people.

Numbers under a kv or so aren't too scary on a surface-mount PCB. Much higher ones are.

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics

Why test parts when you can buy something that is guaranteed to work?

I've got three patents and you've got your name on one that was taken out by other people. Being moderately prudent has worked better for me than your slap-dash habits have worked for you.

A lunatic might think that.

You need design reviews at every stage of a design. Other people can sometimes see possibilities that you have missed.

Exactly the point I was making.

I ran the cheap thickfilm 1206 at 250v, then 350v, and then at 500 volts, days at each voltage. Resistance value changes were down at the

So I tried a Panasonic ERA-series thinfilm. I figured I could start at

Another is running OK at 100 volts.

So much for theory.

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics

Apparently so. But a link I posted above says that thinfilms are about twice as tough as thicks for short pulses.

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics

I see.

I’ve been working on a very fast temperature control system that uses that effect, so I’m uo on it. (Patent filed yesterday—details to follow. )

Cheers

Phil Hobbs

But you haven't posted a link to the part data sheet, or articulated the reasoning that lead you to believe that you could start at 350V.

Not that you can articulate the theory you implicitly claim to be testing.