Trying to cheat ohm's law

Never mind. I was put off by 2$ and 3$ resistors, but I found 3.9 ohm 10W for $0.64.

Almost certainly yes. Find a resistor that is specified by it's manufacturer for transient loads; the ones that I've designed in will have a graph in the data sheet that plots the load power against duration (or average power, or energy, or something -- you'll have to do some math to get it into the form you need).

I can't give you any part numbers or manufacturers off the top of my head, but they're out there, and for that brief of a load you probably won't need much more than something rated for your average power consumption.

--

Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com

Yes - use a 7-10W resistor.

30W for a few mS is of the order of 0.1J of energy. The question is: how much will 0.1J heat up a resistor body?

Answer - not much. If you want to get technical, look at the mass of the body and guesstimate a specific heat capacity and work out a pulse temperature rise.

There are extremes to this - if the pulse is very high for a very short time, it could heat the resistor material (wire?) to melting point before the energy dissipates throughout the body of the resistor.

Hence suggesting a 7-10W resistor as these are cheap, not very big and very common. My gut instinct says that will be fine and in fact you could go lower still, possibly.

Just a power MOSFET, an OpAmp and a few resistors and a capacitor. ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
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I love to cook with wine.     Sometimes I even put it in the food.

So here's a cool thing, around room temp everything has about the same volumetric(sp) heat capacity. ~3 J/(K*cm^3).

George H.

Cool, but a 1206 resistor may not have good transient thermal coupling from the thin resistive element into the alumina substrate.

30 w for a few ms will damage a conventional thickfilm or thinfilm 1206 resistor; trust me on that, we've proven it recently. Surface-mount wirewounds are the way to go for big power peaks.

--

John Larkin         Highland Technology, Inc 
picosecond timing   laser drivers and controllers 

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

** 10 watt wire-wound resistors are tough components. Most are rated to acc ept 10 times rated power for 5 seconds and can usually stand up to 100 tim es for a few milliseconds.

So in your app there is no problem at all.

Ones like the 6.8ohm example below are often used in line with the AC suppl y to large transformers to reduce inrush surge currents at switch on - whe re peak values might be up to 40amps during first half cycles. Normally the resistor is bypassed by a pair of relay contacts after a short time.

0p.jpg

.... Phil

I don't doubt it - those things are pretty small.

That's why I suggest a 7-10W to the OP :)

Well not many cm^3 in a 1206... (one here is ~0.02 inches tall so ~2.5E-3 cm^3, ~7E-3 J/K, 0.1 J is about a 14 degree rise... Assuming I punched in all the numbers correctly. )

OK poor coupling... so 30 W for ~3 ms blows up a film 1206?

George H.

OK so

But they have a lot of active resistance material, and it's hard to damage nichrome wire. The thermal interface surface, wire to epoxy or whatever, is huge. The steady-state cooling is limited by air-exposed surface and the small PCB pads, but transient energy absorption is excellent on wirewounds. They can be had with food TCs, too, unlike bulk conductors like carbon resistors.

Sure, but big resistors tend to be, well, big. Maybe that's OK.

--

John Larkin         Highland Technology, Inc 
picosecond timing   laser drivers and controllers 

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

One specific case was a 1206, 50 ohm precision resistor pulsed at 30 volts, which is 18 watts. Pulsed for 2 ms at 1 Hz is 36 mJ per shot,

36 mW average power. Average lifetime to hard failure was around 30 minutes, but just a few shots changed the value out of the 0.1% tolerance. On an IR viewer, each shot made a central thermal hotspot - poor coupling! - which has to be doing mechanical damage. We think that laser trimming creates hot/weak spots on the surface. It looks like four of those resistors connected series-parallel would be OK, but we'll probably go with with one wirewound. For the 5 ohm range, we have to go wirewound.

I'd expect that 30 watts for 3 ms will kill a normal 1206 resistor in one shot. Try it!

--

John Larkin         Highland Technology, Inc 
picosecond timing   laser drivers and controllers 

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

Hence why I suggested finding a resistor whose data sheet lists pulse power vs. time -- because the manufacturer has tested it for all the various thermal time constants, and you don't have to do things b'guess and b'gosh.

--

Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com

Phil Have you moved across the ditch? lol

--
John G Sydney.

It's hard to know what's going on at the surface. The different TEC (Thermal Expansion) could rip things apart, with each pulse.

Well I'm not sure I have time. (I'm using electronics to find my math mistakes in a sqrt circuit. :^) But I can believe that the volume of the resistive material is pretty small.

Having the resistor taking up all the volume.. like old carbon composites, makes a lot of sense for pulsed power.

Or wire wound, as you said.

George H.

** John is alluding to the gold coloured coin shown alongside the 10W resistor in the JPG, the Kiki bird depicted indicates a New Zealand dollar.

Australia is separated from new Zealand by the Tasman Sea - aka "the Ditch".

The 10W resistor has "IRH Australia" printed on it, so it was likely made here in Sydney.

The resistor is 48 x 9.5 x 9 mm - which seems to be standard size.

... Phil