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thick film resistor questions




 
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Author Topic: thick film resistor questions  (Read 1400 times)
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ssbothwell KJ6RSG
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« on: December 28, 2011, 06:18:05 PM »

hi everyone. i have a couple questions about thick film resistors.

1. would they make good dummy loads? it costs about ten bucks for a 1% 100w 50ohm 'non-inductive' thick film resistor on digikey. that is way less then i have seen 100W dry loads go for at the local hamfest.

2. i bought some 50W 0.5ohm thick film resistors to replace the metal oxide resistors i am using on the sources of my IRF510 AB amp. here is the datasheet, they are the tr-50 model: http://www.seielect.com/catalog/SEI-TR.pdf

i thought i was buying to-220 packages but these dont have a screw hole for mounting a heatsink. do you think i might get sufficient cooling if i just mount them flat against the copper board my amp is built on? the copper board is mounted directly to a large heatsink which i have the FETs mounted on.

what sort of heatsink are these resistors supposed to be used with?
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WA1GFZ
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« Reply #1 on: December 28, 2011, 07:49:52 PM »

You will need to make some sort of a clamp to hold them against the heatsink for best thermal transfer. A screw either side with a metal strap.
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WBear2GCR
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« Reply #2 on: December 28, 2011, 10:44:50 PM »

Yeah, but it is a ceramic substrate and they caution against over torquing, they want you to use a spring washer on the ones with a hole and a spring clip on the ones without... so if you use a clamp, I'd put some Berquist pad between the clamp side and the housing to suck up the pressure a bit and
use caution, remembering that things expand when hot! :d

Spring washers on the screws when using a clamp is not a bad idea either.

the more dead flat you make the mating surface on the heatsink the better it will work too...

I'd ask the company what their freq vs resistance graph looks like. It might be
there in their master catalog or elsewhere online.

But yeah, ought to make a fine 100w class dummy load...

                   _-_-bear
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_-_- bear WB2GCR                   http://www.bearlabs.com
KM1H
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« Reply #3 on: December 29, 2011, 08:48:59 PM »

Ive blown up Caddocks by trying to push things a "bit". CAREFULLY read their heat derating curves and add 50% at least for hammy hambone construction.
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