another resistor killer

English is a very messy language.

In my example above

"the greengrocer's apostrophe"

I think having or deleting the apostrophe would both be correct, but with different meanings.

It might have been "the greengrocers' apostrophe" too.

But I'm just an electronics design engineer. Sloman is our grammarian and spelling enforcer.

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics

I don't expect to have much average power dissipation. The resistor on my prototype is rising about 15c at 180 watts and 1 us/1KHz pulses, according to my thermal imager. Do the math on that.

But a MELF can have a longer and wider resistance track compared to a planar equivalent.

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics

Sloman might enjoy this:

I play safe:

I do that all the time, using phonetics for fun, so US becomes YouAsh Will become real soon if King Kong stays in the war-house.

There was a warning from France today about EU - YouAsh relations collapsing due to Kong's Greenland remarks.

We are all stardust

That's enormous, and probably inductive.

I've blown up that type of resistor, from high power cycling within its ratings. I replaced it with a Welwyn planar porcelain-on-steel thick film.

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics

Samuel Johnson wrote the first popular English dictionary, which preceded Webster's effort by quite a while. Webster was a spelling reformer, and changed a number of Johson's spellings, claiming that he was offering a dictionary of American English, presumably hoping to squeeze English publishers out of the American market.

My dyslexic engineer colleague always wrote what he thought sounded right - and couldn't see the point of using specific spellings to distinguish words that sound the same. Presumably he always sub-vocalised.

John Larkin aspires to be seen as an electronic design engineer. He doesn't know enough to realise that he doesn't make the cut there either.

I'm certainly not a grammarian. My wife started off as a linguist before she became a psycholinguist, and she did know grammarians. I've got a copy of Donaldson's "Dutch Reference Grammar" on my bookshelf that he gave to my wife when we dropped in on him at Melbourne University. I need to read it whenever I try to write Dutch.

Grammar is actually a rationalisation of what speakers of the language actually do. Large Language Models are the only reliable authority on that subject.

You'd need to identify the resistor so that I could get the thermal resistance of the resistor to ambient before I could do that. If you knew what you were talking about you'd know that

But most resistors dissipate most of the heat they generate into the printed circuit board on which they are mounted. Planar surface mount resistors do that a lot better than MELFs. If you are less worried about the average power dissipation and just want high peak dissipation, MELFs may look attractive, but that long track is likely to be an inductive helix, which may be less attractive in your application.

Do you have any data for that? In both cases the substrate is alumina which has a high thermal conductivity. MELFs have crimped on end caps which probably provide good thermal conduction into the pcb pads.

If you are less worried about

I'll reveal the secret mathematics:

Please keep this confidential.

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics

Degrees per watt.

Jeroen Belleman

It's a broad generalisation, and only really applies to small surface mount resistors, but they are the resistors that sell in the largest numbers.

But not as good as the flat solder pads on planar resistors.

The secret you reveal isn't in the mathematics. It's the thermal resistance from the dissipating element to ambient

Why? It's on the data sheet of whatever resistor you happen to be using, which you haven't specified. You may not realise this.

Oops. You knew what I meant.

The 83 K/W is reasonable, considering that measuring a 1206 temp with a cheap thermal imager isn't an exact science.

After several days pulsing at 300 volts, 180 watts, the thickfilm resistance was bobbling around in the 5th decimal place. Boring.

I stepped it up to 350v, about 250 watts. That's 1000x the part's DC power rating. I'll check that now and then.

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics

I've been abusing Welwyn SMD resistors in a similar manner. Never a problem. Good to know.

On the other hand, I've tried to use 10W Radiall SMA attenuators with ns kW-level pulsed power, and they'd die, even though the average power was very much below 10W. 15W attenuators held up fine, fortunately. I never tried to find out why that was. Maybe the geometry of the 10W attenuators was small enough to cause a spark discharge.

Jeroen Belleman

It was hard to find attenuators that would survive 700v sub-usec pulses, with low average power dissipation.

We blew out a bunch of the MIniCircuits VAT parts.

I made one atten using the Caddock dpak resistors. It wasn't beautiful for step response but did survive.

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics

Indeed, the same experience here, although mine was a splitter rather than an attenuator.

Jeroen Belleman

I guess I'll try 500. That will be 500 watts peak.

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics

No; average power is low.

What matters is microsecond-range heat capacity in the resistive element. The ceramic substrate may help a little.

The data sheet of a cheap thickfilm resistor does not characterize it for pulsing at 1000x rated power.

Or 2000x, which I'm running now.

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics