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Old 14th March 2012, 12:25 PM   #1
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Default Moderm "high watt" resistors seem too small

I recently bought a bunch of 5W wire wound resistors, ranging from 250R to 22K, and they seem way too small to dissipate that kind of wattage. In my current project I have a 5W 250R cathode bias resistor that is only dissipating about 1.8W and it's getting very hot. Its dimensions are 7.90mm x 23.80mm, which is big compared to some of the others which are 7.50mm x 18.00mm.

These seem way too small to me. Are they safe and reliable? They sure are condensed hot-spots.

-Dave
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Old 14th March 2012, 12:30 PM   #2
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Yes, I have same trouble. Rare.
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Old 14th March 2012, 12:32 PM   #3
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Quote:
Originally Posted by Osvaldo de Banfield View Post
Yes, I have same trouble. Rare.
Do you mean you're having reliability trouble with them? Comparing them against old resistors from my 1960's/70's amps these 5W are as small as the old 2W or 3W. The datasheet says they're good up to 250C ambient, with a derating curve of course.
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Old 14th March 2012, 01:02 PM   #4
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The physics of heat dissipation has not changed in the last 100 years. What has changed is the ability of modern materials to deal with and operate at high temperatures. Resistors used to be wirewound, or carbon comp. The old carbon comp resistors were famous for changing value with changing temp, so you couldn't run them too hot or they would change value, or just start smoking. The physical size was large so that they could dissipate power without becoming too hot.

Today we have ceramic cores and exotic metal films that can run at 250C without a problem. So when you see a tiny resistor rated for 2 or 3 watts, there is only one way for this to work. The little resistor will get HOT in NORMAL operation. This is usually OK, but remember this if you stuff a bunch of them into a small space or use PC board construction.

If you put a 200C resistor next to a sensitive component like a transistor you will have problems. If you are using these parts on a PC board don't pull them tight to the board, leave at least 1/4 inch between the resistor and the PC board. These resistors need air on all sides to meet their power rating spec, and hot parts can discolor the board over time.

In any of these cases I typically over spec resistors to keep temperatures down. Heat is the enemy of long term reliability.
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Old 14th March 2012, 01:14 PM   #5
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Quote:
Originally Posted by doozerdave View Post
Do you mean you're having reliability trouble with them? Comparing them against old resistors from my 1960's/70's amps these 5W are as small as the old 2W or 3W. The datasheet says they're good up to 250C ambient, with a derating curve of course.
No with these resistors itself, my trouble is that they run so hot the burn components near to them, electrolytic caps, the PCB itself, etc.
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Old 14th March 2012, 01:18 PM   #6
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Parallel two, resistors are not terribly expensive compared to other parts?

Or buy higher power rated parts.

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Old 14th March 2012, 01:33 PM   #7
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tubelab, thanks for the explanation. I assumed newer materials were the reason, but I always have concerns about mfr's cutting corners and making parts seem grander than they are.

I originally had my expensive cathode cap and inexpensive resistor wired directly together (using point to point board) but as soon as I realised how hot they R was getting I separated them and mounted the R above the board.

I will be installing a chassis fan in this amp since it's a tube-down chassis with 9 tubes. I should borrow the IR camera from work and look at the R's to see the real temperature rather than using my finger thermometer. In terms of over rating, I've got a 5W part dissipating 1.8W. I would expect that 2.7 times over rating would be plenty. What do you think?
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Old 14th March 2012, 01:44 PM   #8
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Originally Posted by bear View Post
Parallel two, resistors are not terribly expensive compared to other parts?
The trouble is that in some places there is no room to put several components.
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Old 14th March 2012, 01:59 PM   #9
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One good thing about resistors is that they give you a warning sign when they are stressed. They need to smoke. My brain has stored somewhere the smoke smell of an overheated resistor.
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Old 14th March 2012, 04:15 PM   #10
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Quote:
One good thing about resistors is that they give you a warning sign when they are stressed.
Many of the new ones don't they just go open. The resistance is a film coating on the surface of a ceramic cylinder covered with paint. The resistance film cal fail in several ways, all of which I have experienced. See below:

Quote:
My brain has stored somewhere the smoke smell of an overheated resistor.
The new metal film and carbon film resistors have a new smell. They stink more than a vintage Allen Bradley, and they smell different if slowly cooked until failure (the paint burns), or grossly overloaded until glowing red (the paint is gone and the metalization is burning), or rapidy hit with a surge well in excess of normal (the metalization goes open often with minimal indication, sometimes there is an arc), or an extreme surge (the resistor explodes leaving only a black mark on the PC board).

Quote:
I would expect that 2.7 times over rating would be plenty.
That should be fine for any resistor. I tend to use the cheap white ceramic sand filled 5 watt wirewound Xicons for cathode resistors. They will take 5 watts forever although they will eventually discolor. The blow point for long term use seems to be in the 8 to 10 watt range. I have fried two in that range and they lasted a long time before blowing. They do explode rather violently shooting ceramic and metal shrapnel in all directions if hit with a direct application of say 500+ watts (short on the B+ put 450 volts across 150 ohms).
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