overloading wirewound resistors

The Ohmite (apparently identical to the EBG and some other parts... same production line, it seems) uses a thin, flat alumina plate on the bottom. Its inside-side is a thickfilm resistor element and its bottom is metalized. The plastic shell is spring-coupled to the alumina thing and presses it down against the heatsink. When we whack it with enough power, the alumina cracks and the element opens. At $85 per event.

One problem is that the "heatsink" is a metal chassis, about 100 mil thick aluminum, and its thermal spreading resistance is too high... under the resistor can be 180C, and a couple of inches away the aluminum is 80C. Not good. So the best place to dump the heat is into air, which I have a lot of.

John

"John Larkin" wrote in message news: snipped-for-privacy@4ax.com...

The Ohmite wirewounds are the best choice. Once the oil burns off they roast to a nice whitish color. I've used them for loads that were air cooled, would melt the solder if they were convection cooled.

We have some Ohmite current sense resistors that pop like popcorn in PFC circuits. They might be internally similar to your Planar resistors.

Cheers

Last time I ran into this I oil-cooled the resistors. I used resistors with little material around them to make sure the thermal path to the oil was "fast".

--
Regards, Joerg

http://www.analogconsultants.com/

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It's also thermal masses and such that play into the game here. IOW how fast a massive "heat pulse" can get to a safe place.

--
Regards, Joerg

http://www.analogconsultants.com/

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The main coil should be liquid nitrogen cooled superconductor. So, you can liquid nitrogen cool the water before cooling the electronics. Direct liquid nitrogen cooling would be difficult to do for the plumbing.

So, they are not charging lead-acid batteries anymore. Precharging batteries were necessary to store enough energy for the pulses. How many coils are they driving with the amp? Can they drive with multiple amps?

Are you doing it for UCSF?

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Liquid helium. Nitrogen is used in the outer jacket, to shield the helium and reduce boiloff.

The users are not about to furnish me liquid nitrogen. It's expensive and the main magnet needs all that it has. 20C water is cheap and cools almost as well as 0C water. But adding water cooling isn't an option for this particular problem.

Imaging uses three gradient coils, three driver amps.

Among others.

John

Yeah, I guess that the alumina and the thickfilm element have very low thermal masses, so can heat up really fast. I need lots of nichrome and porcelain!

John

Any chance of making a hybrid switcher + linear thing to save most of that heat, allow high duty, yet give accuracy & low noise?

(Not in this unit of course; it'd take a new design. But what a killer product that might be...tight, small, and cool.)

James Arthur

They list a 1000W rated under the 210 series, but availability seems to be fictional, and you'd never fit it in - 20 inches long, 2.5 inches diameter.

Perhaps a pair of the 12 inch log 500 watt jobbies? Nope, they don't actually have them either...

The problem (if it is a problem) with the bare nichrome running red-hot is highly variable resistance, and the potential problem of radiant heating the rest of your circuit. Running it less than red-hot helps with both of those, to an extent.

At my first lab job, I made some serious resistors from stainless steel (not far off of nichrome) shim stock - depending on the clearances you have and the resistance you need, you might be able to fabricate copper or brass terminals to connect to the mounting holes, and (use someone with good skills, an awareness of how to work with stainless, and proper flux) braze sections of stainless shim stock to the terminals.

It's available in very consistent thicknesses, so you can get repeatable and predictable resistance. If run too hot it will burn though, but the trick would be to design so it's not getting too hot - use enough thickness and shape it (perhaps an inverted U up from the terminals into the air stream) so it never does get red. Multiple sheets in parallel to get lower resistance, narrower or longer strips for higher resistance, etc...you want the largest area to shed heat you can fit in there, and then make adjustments to get the resistance right.

The quality brazing job is needed so power goes in and out without hot spots at the end of the sheet.

--
Cats, coffee, chocolate...vices to live by

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The other side of the RF coils are very sensitive RF detectors. They would not risk it with switchers.

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In theory, it is possible to increase the coils/detectors and to lower the duty cycles. Getting the engineering changes would be impossible.

The doctors always have the final say. They are more stubborn then engineers.

I have no solutions but it sparked some Googlemania on how nichrome wire oxidizes.

I found this:

"Since chromium oxide is more stable than nickel oxide, a layer of chromium oxide will form on the surface of the nichrome wire when it is heated up."

"It (chromium) does not tarnish in air, but burns when heated, forming the green chromic oxide"

Anybody have nichrome wire go dark green when heated? Maybe I don't have the right oxide. Also, I wasn't able to find a conductivity for the oxide. I'd like to know if it's a good insulator.

As for dissipating 1kw.

This came to mind.

For a ceramic heater..

Maybe this might spark the imagination.

The stacking is interesting.

D from BC myrealaddress(at)comic(dot)com BC, Canada Posted to usenet sci.electronics.design

I've used switchers in microvolt places, so I understand that.

But I also know John, and figure he could pull it off and have a kick-ass product.

Cheers, James Arthur

That Ohmite resistor type looks really cool (pun intended). The old ceramic wire with glazing can easily tolerate red-hot conditions which is way beyond ratings (this experience was with no forced air cooling). In fact one can "safely" melt the glazing... Use of bolted on lugs is far better than fastons at those temps.

Speaking of you- hey John, didja ever get that letter I sent?

Tim

-- Deep Friar: a very philosophical monk. Website:

What actual wattage are you aimimg for? You'll have to draw a line somewhere. Running anything beyond ratings continually is going to cause grief.

Check how much heatsinking is going to be required in your airflow to keep resistor core temps within range, regardless of resistor construction. Maybe adding fins or reconfiguring mounting in the current chassis would do it.

You haven't indicated an R range. If it's not important, perhaps a PTC structure could limit power, if function isn't inhibited.

Can't you limit the rep rate in SW at detected temperatures? It's no shame to be the pinch-point in an operating cycle - just an indication of pre-specified limits (ie $).

RL

[...]

Tramcar motor resistors (non-inductive):

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~ Adrian Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk

No porcelain. Fast heat pulse ... tchk ... crack ... *BANG*

The worst I've ever had was a resistor which, in an instant, turned its body from a white ceramic into bubbly green glass. The ones that were in parallel all exploded and I found lots more bubbly green glass throughout the room.

--
Regards, Joerg

http://www.analogconsultants.com/

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