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Old 12th January 2004, 09:00 PM   #1
Magura is offline Magura  Denmark
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Default High power resistors

Wouldnt it be possible to use a monster mosfet in a to-264 package rated 300W as an adjustable bias resistor that has to dissipate 200W??


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Old 12th January 2004, 09:16 PM   #2
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Sure, as long as the transistor chip not gets too hot... but this is not an easy task.
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Old 12th January 2004, 10:13 PM   #3
Steven is offline Steven  Netherlands
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Why use transistors with all their temperature problems when you can use resistors? Resistors are allowed to get a lot hotter. I think of using these new planar resistors. They do not use an additional heatsink, are small enough, can dissipate 100W at 25*C and are allowed to go to 350*C. Still you can mount them on a PCB.

Example below is from Nicrom-electronic, but such devices are also manufactured by Ohmite, BI technologies, and RCD components. Now I only have to find a distributor that sells them in small quantities. Anyone?

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Old 12th January 2004, 10:22 PM   #4
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Magura,

with that signature, why do you even ask?

Adding to Stevens comment. Caddock also makes a few types
of rather small film-based resistors. Can't remember the
model numbers now, but check ut Elfa, they carry them.
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Old 13th January 2004, 01:56 PM   #5
Magura is offline Magura  Denmark
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Quote:
Originally posted by peranders
Sure, as long as the transistor chip not gets too hot... but this is not an easy task.

How exactly would that be done...it sounds like you have an idea???

Cheers

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Old 13th January 2004, 02:14 PM   #6
joensd is offline joensd  Germany
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Quote:
can dissipate 100W at 25*C and are allowed to go to 350*C. Still you can mount them on a PCB.
If you mount a big fan directly over them, they even might not desolder themself and stay on the PCB.
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Old 13th January 2004, 02:22 PM   #7
Magura is offline Magura  Denmark
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Quote:
Originally posted by joensd

If you mount a big fan directly over them, they even might not desolder themself and stay on the PCB.

I too found it hard to implement them in something for my living room....i sure like the option to keep them sub 175C


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Old 13th January 2004, 05:06 PM   #8
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I don't want you to think that I regard light bulbs as the
solution to all problems, but they make great power resistors,
and you can vary the values by paralleling several and
unscrewing the ones you don't want. I use this to put the
Zen Lite into low power mode, unscrewing 2 or 4 of the 6
light bulbs.
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Old 13th January 2004, 06:37 PM   #9
Steven is offline Steven  Netherlands
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Quote:
Originally posted by Magura

I too found it hard to implement them in something for my living room....i sure like the option to keep them sub 175C

Magura

When it comes to heating your room, I think it makes no difference whether you dissipate 100W in a huge heatsink mounted monster mosfet with the heatsink at 100*C, or in a relatively small resistor that can be 350*C, or even in a small halogen bulb that can be 2500*C. It's just 100W.

If you care about the accuracy of the resistor over the temperature range I'm sure these thick film ruthenium oxide resistors win. They change less than 5% over the complete temperature range from 25*C to 350*C. Power derating is linear from 100% at 25*C ambient to 0% at 350*C.
Inductance is also extremely low at <50nH.

I'm not 100% sure, but I have the impression that you can use them at full power without auto-desoldering from the PCB. The stands are designed with low heat conducting properties. Maybe you need a few square cm of copper area on the PCB for each stand.

Steven
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Old 13th January 2004, 06:45 PM   #10
Magura is offline Magura  Denmark
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What i meant was that its a lot nicer to have something like a heatsink at 65C than a resistor at 300C in your living room. I would like a sealed box, in order to avoid dust and whatever else that usually resides in an old amp
A sealed box prevents me from using anything i cant mount on a heatsink or outside the box

Cheers

Magura
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