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Old 22nd October 2012, 12:56 PM   #1
alecu7 is offline alecu7  Canada
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Default PSU resistor temperature

Hi all,

I have a test rig for my kt88 set amplifier.
The configuration is:
Edcor XPWR105 (360V) 82uF 250R/13W 820uF 250R/13W 820uF 10k/5W 8uF
After the second 250R I get ~400Vdc for kt88 and after 10k I get ~300Vdc for 6N1P driver. Voltages are very similar with PSUII program.
My question is: is it normal that both 250R resistor reached ~150C in about 5 minutes? After 5 more minutes temperature is the same. Resistors are Dale wire-wound silicon coated (# CW010250R0JS73). From their spec sheet it says derating power at 150C is 60% (13Wx0.6=7.8W) I'm dissipating 7.5W so I should be ok from the wattage point of view. But what about temperature? It seams excessive to me. Apparently these resistors can go to max 350C.
Should I replace them with caddock 25-30W?

Thanks,
Alex
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Old 22nd October 2012, 02:22 PM   #2
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7 + watts into a component just 45mm by 9.5mm will no doubt make that component very hot. Probably not an issue for such a resistor, but make sure it's kept a distance from other sensitive components such as electrolytics. Also they must be mounted a bit off the circuitboard, not right onto it, if you use pcb that is.
I think u can get heatsinks for those resistors, or make your own.
Kinda waste dropping so much power in a resistor. How bout using a choke?
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Old 22nd October 2012, 02:34 PM   #3
alecu7 is offline alecu7  Canada
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Resistor is about 10mm higher than the pcb. and it's ~10mm to the 82uF cap. From what I've been reading solder melts @182C which seams too close to the 150C. As a test load I have 2.8K/100W(2x 5.6K in parallel) in place of kt88 and 33k/25w caddock for 6n1p.
Other results are 45C on 10k/5w and 41C on 33k/25w. 10k resistor is also dale and similar to 250R. 33k has a small heatsink.
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Old 22nd October 2012, 02:50 PM   #4
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I'd put in a 10W W/W as 7.5W is sailing very close to that resistors rating.
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Old 22nd October 2012, 03:13 PM   #5
alecu7 is offline alecu7  Canada
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Quote:
Originally Posted by KatieandDad View Post
I'd put in a 10W W/W as 7.5W is sailing very close to that resistors rating.
The resistor is 13W.
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Old 22nd October 2012, 03:27 PM   #6
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The problem is the amount of heat you are dissipating relative to the size of the component. Its thermal resistance to ambient will be quite high so you get a large temperature rise ( a quick calculation shows it is about 17 degrees per watt). One possibility is to use two 500 ohm parts in parallel but physically separated from each other so they share the dissipation. This should nearly halve the temperature rise.

Cheers

Ian
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Old 22nd October 2012, 03:45 PM   #7
DF96 is offline DF96  England
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The derating spec is probably for ambient temperature, not surface temperature. If you put a straight line between 13W at around 20C and 0W at 350C you will get around 60% at 150C. The resistor should be happy at 7.5W, but adjacent components might not be. Thermal cycling could affect solder joints eventually. I would ensure that the resistor has long legs to lose a bit of heat. In the olden days very hot resistors sometimes had screw terminals for connections so solder melting (or oxidising) was not an issue.
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Old 22nd October 2012, 04:16 PM   #8
alecu7 is offline alecu7  Canada
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Quote:
Originally Posted by DF96 View Post
The derating spec is probably for ambient temperature, not surface temperature. If you put a straight line between 13W at around 20C and 0W at 350C you will get around 60% at 150C. The resistor should be happy at 7.5W, but adjacent components might not be. Thermal cycling could affect solder joints eventually. I would ensure that the resistor has long legs to lose a bit of heat. In the olden days very hot resistors sometimes had screw terminals for connections so solder melting (or oxidising) was not an issue.
Since I need to buy new resistor (you suggest to get 2x 500) should I not get caddock and mount then on the case? The sides are 1/2" aluminum.

Alex
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Old 22nd October 2012, 04:48 PM   #9
alecu7 is offline alecu7  Canada
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Quote:
Originally Posted by DF96 View Post
The derating spec is probably for ambient temperature, not surface temperature. If you put a straight line between 13W at around 20C and 0W at 350C you will get around 60% at 150C. The resistor should be happy at 7.5W, but adjacent components might not be. Thermal cycling could affect solder joints eventually. I would ensure that the resistor has long legs to lose a bit of heat. In the olden days very hot resistors sometimes had screw terminals for connections so solder melting (or oxidising) was not an issue.
derating spec is for the case temperature not ambient. Ambient is 25C. And this is exactly what I did/explained in post #1. 60% from 13W is 7.8W.

I'm more concerned with the high temperature that I measured.. I think I'm gonna go with caddocks for my piece of mind.
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Old 22nd October 2012, 05:47 PM   #10
DF96 is offline DF96  England
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Quote:
Originally Posted by alecu7
derating spec is for the case temperature not ambient. Ambient is 25C. And this is exactly what I did/explained in post #1. 60% from 13W is 7.8W.
No, this is not what you explained in post #1. You did not say the derating figure you gave was for case temperature. Derating could be for case or ambient temp, with the former more likely for a metal cased resistor intended for a heatsink and the latter more likely for other resistors. You said "silicon coated" which I assumes means not metal cased.

If you are right, then 7.5W for a resistor rated at 7.8W is asking for trouble. If I am right, then the resistor is fine but the things around it might not be.

150C surface temperature for a resistor OK to 350C is fine. Nothing to worry about.
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