I'm thinking about classic 50W molded-housing chassis-mount resistors, like Stackpole KAL-50, Dale RH-50, Ohmite 850, multicomp and others. They typically specify the peak power-handling capability as 5x for 5 seconds. This would be only 1250 Joules of thermal-mass energy-handling capability for a 50-watt resistor. Surely they have more thermal mass than that?!?
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Thanks,
- Win
The resistor body does but unfortunately the coiled wire doesn't :(
I've had a small incident happen some years ago, where a 20W low-ohms resistor got shorted across a power supply that was too much for it.
The resistor exploded without getting very hot. Apparently the layer of isolating material that sits between the coiled wire and the metal resistor body conducts heat only moderately well, so at high overloads it may just instantly boil and blow out an end cap from vapor pressure rather than transfer the heat to the big metal frame.
Dimitrij
Hi, Win:-
Have you sacrificed one to see what the internal construction is like? Maybe they're filled loosely with powder.
Heaters (which bear *some* resemblance to resistors) can vary greatly in surface heat dissipation (watt density) because some are thin wire loosely thermally coupled to the outside world, and others (the really good ones) are swaged to compress the mineral insulation into close theermal coupling to the element.
--sp
-- Best regards, Spehro Pefhany Amazon link for AoE 3rd Edition: http://tinyurl.com/ntrpwu8
I used some 250 watt resistors in that style, and pushed them pretty hard, pulsed loads. They tended to fail after a few years, shorts to the case. I wound up replacing them with these:
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Nope! They're a coil of wire epoxied inside a hole. They're also too small for the rating, and can only go up to 175C or whatever (for the same reason: plastic inside), so require a heatsink at 25C (or whatever the RthJC says) to even attain full ratings.
It should be no surprise why/that they are the cheapest type!
If you need energy for modest durations, anything made with ceramic (tubular/vitreous) will do. Their power scales something like t^(1/2) to t^(1/3), because of thermal diffusion from wire to body.
For shorter time scales (sub ~ms?), a bulk type looks much better. Ohmite and others have them in leaded form, or there are industrial tube, cylinder and puck shapes for weapons grade applications (so to speak).
At extremely small time scales (~ns, where skin effect, even of a bulk resistance, becomes a major hindrance), you probably just want a spool of RG-58.
Tim
-- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website: http://seventransistorlabs.com
** Makers specs claim the resistance wire is wound onto a ceramic tube and covered in silicone before fitting inside the aluminium heatsink coupler.
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MATERIAL SPECIFICATIONS
Element: Copper-nickel alloy or nickel-chrome alloy, depending on resistance value Core: Ceramic, steatite or alumina, depending on physical size Encapsulant: Silicone molded construction Housing: Aluminum with hard anodic coating End Caps: Stainless steel
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.... Phil
Interesting. Does that folded-back layout do much to keep the parasitic inductance down? What frequency are they good for?
We used them at relatively low frequencies, so I don't know. They are thick film on porcelainized steel, which should be interesting.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
Google Anaren RFP-150-50TG. It's fairly cheap per watt, and is good to
2 GHz.-- John Larkin Highland Technology, Inc lunatic fringe electronics
** The frequency characteristic depends on the resistance value, as usual.
For low values, resistor impedance rises above some frequency and for high values it falls. Just depends whether stray L or C dominates. A value may exist where the effects cancel and the part is flat to hundreds of MHz. 0.25W film resistors of about 150 ohms are like this - so you see them used in BNC dummy loads and antenna splitters.
Welwyn claim 3 to 6uH for the type - which puts them in the same ball park as 5 & 10W tubular WW resistors.
.... Phil
(sorry kinda late response)
I've take some Dales apart - and was disappointed at the poor thermal coupling between the resistive wire and the case. You haven't said anything about how "peaky" your energy dump might be - but I wouldn't recommend this kind of part for a peaky-demand situation.
-F
I once tested some ohmite 225 Watt wirewounds, which had about 1 kJ pulse r ating. When I put several times more than that into them the metal wire ex panded faster than the ceramic and you could hear crackling as the wires un bonded from the ceramic. I imagine that if I had done a few more cycles th e wires would have become so thermally isolated that the resistor would no longer be good for 225 Watts of average power and might have melted during a pulse.
** The same resistors are often used in "soft start" systems for PSUs that have large inrush currents - particularly with toroidal transformers of 1 to 2kVA rating with hefty filter caps on the secondary.
In this role, a 6.8ohm 50W part tolerates surges of over 40amps when switched to a 240VAC supply - until the event is over and a relay bridges it out.
So it can handle up to 12kW repeatedly for periods of about 40mS.
.... Phil
That part measures 620 pF element to substrate, 4.1 uH end-to-end, and roughly 28 ohms Zo, TDR'd from one end against the substrate.
Not RF stuff, but a 50 ohm version might make a decent termination.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
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