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Old 11th July 2007, 04:22 AM   #1
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Default How hot can resistor stands?

I am using a RS 820 ohms 7W resistor to drop the voltage from 266V to 208V for the B+ of a 6BQ5 SE stereo headphone amp. By calculation, it is only 4W but the resistor turns out to be very hot at 160 degree C (320 degree F).

Is this a common phenomena that resistor can turns so hot?

Is a 7W resistor sufficient for it?

Is my calculation wrong?
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Old 11th July 2007, 05:59 AM   #2
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Default Re: How hot can resistor stands?

Is this a common phenomena that resistor can turns so hot? Yes

Is a 7W resistor sufficient for it? Yes

Is my calculation wrong? No.
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Old 11th July 2007, 07:42 AM   #3
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since it is along the lines of this topic, I have a 10 watt power resistor (2.25k) from b+ to all four of the 7868 screens in my organ amp. It was mounted to the chassis with a metal clamp, and that side of the chassis gets pretty hot during normal operation.

I have always wondered about certain resistors getting so hot.. Is there no real way to fix this type of problem, other than a higher wattage resistor or a choke?
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Old 11th July 2007, 08:03 AM   #4
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Hi,

Cheap and easy remedy.

Series a string of smaller resistors which will total up to the your over heated resistor will distribute the heat generated, the total heat will be the same but each resistor will be cooler.

Why?
Ohms Law, Power = R*I^2.
Power = Sum of all resistor in series.

Cheer

KenC
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Old 11th July 2007, 08:05 AM   #5
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For the organ amp?

Add a fan and/or a huge heat sink.

Hope this help

Cheers
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Old 11th July 2007, 08:06 AM   #6
Irakli is offline Irakli  United States
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In situations like that I always buy resistors in TO-220 package (Ohmite or Caddock) and mount them on heatsink.
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Old 11th July 2007, 04:27 PM   #7
radtech is offline radtech  United States
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Quote:
Originally posted by ThSpeakerDude88
since it is along the lines of this topic, I have a 10 watt power resistor (2.25k) from b+ to all four of the 7868 screens in my organ amp. It was mounted to the chassis with a metal clamp, and that side of the chassis gets pretty hot during normal operation.

I have always wondered about certain resistors getting so hot.. Is there no real way to fix this type of problem, other than a higher wattage resistor or a choke?
The amount of heat generated is dependant on the amount of wattage dissipated, so if you have a 10W resistor dissipating 5W, and a 20W resistor dissipating 5W they'll both generate the same amount of heat.. 5 Watts worth. The only difference is in how much heat they can handle before burning up, and how well they can transfer that heat to the air or a heatsink. In air you might feel a difference if one has more surface area to shed the heat, clamped to a metal chassis the chassis will probably feel about as hot with either size resistor.
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Old 12th July 2007, 12:09 AM   #8
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Quote:
Originally posted by ThSpeakerDude88
I have always wondered about certain resistors getting so hot.. Is there no real way to fix this type of problem, other than a higher wattage resistor or a choke?
If the 10W resistor is running within its rating, then you already did what you can do: clamp it to the chassis so that it can serve as a heatsink.
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Old 12th July 2007, 02:27 AM   #9
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There are even small power resistors with slightly reduced size that rely on very heavy PCB traces, or maybe big solder lugs, to dissipate the heat. If mounted on a PCB with thin traces, they'll melt the solder and fall out, even though the power is within their published rating. Please don't ask how I know this
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Old 12th July 2007, 02:36 AM   #10
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At work we bought some so called 1/2w resistors from digikey but their physical size was the same as a 1/8w. Well needless to say they made great fast blow fuses.
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