Normal temp of a 2 watt resistor? - diyAudio
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Old 26th May 2008, 09:13 PM   #1
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Default Normal temp of a 2 watt resistor?

Just curious but does anyone know roughly how hot a 2watt resistor should get if it's operating at just under 2watts?

I have an amp where there's a resistor heating up to 120 degrees Celsius (22 degrees C ambient). It appears the resistor is inline with a 24VDC relay coil. The resistor currently measures 928 ohm and there's 40VDC across it so it's dissipating around 1.7Watts. Power dissipation seems ok assuming it's a 2 watt resistor so is it normal for the resistor to be getting so hot?

btw, I don't know the nominal value for the resistor because the color bands appear to be faded (they all appear to be some different shade of brown). It's a 5% resistor so i guess it should be 910 ohm even though the first band appears more grey/brown than white.
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Old 26th May 2008, 09:28 PM   #2
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The first band being brown tells me it's a 1K resistor.

1.7 watts is cutting it close for a 2 watt. I'd up that to 3 or even 5 if you have room. It's not just the resistor, but surrounding component life too.

Keep the leads as long as possible and wrap the resistor in fibreglass or high temp silicone spaghetti (if your local surplus store doesnt have it, rip apart a dead coffee maker/heater/kettle for the stuff).

Cheers!
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Old 26th May 2008, 09:44 PM   #3
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I don't know the exact temperature, but I've seen the smaller body MOX 2W resistors melt the solder right off their leads and fall out of a board while dissipating less then 2W. A large amount of the heat has to get dissipated through the leads and the only time I've ever come near the 2W rating was with fan cooling. As said above, go with a larger wattage resistor, or spread the load across a couple 2W resistors. Always pay extra attention to what's attached to the leads and traces- if it's an electrolytic capacitor or a semiconductor circuit, take some trouble to keep the heat out. This is high on the list of things that make caps fail.
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Old 26th May 2008, 10:24 PM   #4
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Quote:
Originally posted by Geek
The first band being brown tells me it's a 1K resistor.

1.7 watts is cutting it close for a 2 watt. I'd up that to 3 or even 5 if you have room. It's not just the resistor, but surrounding component life too.

Keep the leads as long as possible and wrap the resistor in fibreglass or high temp silicone spaghetti (if your local surplus store doesnt have it, rip apart a dead coffee maker/heater/kettle for the stuff).

Cheers!
i'd swap the resistor for a bigger one but i'm currently still trying to find out whether there's a problem with the amp or the hot resistor is due to a crappy amp design... so far it appears that its resistance seems to be what the manufacturer intended it to be because, the power supply is around 68 vdc and needs to power a 24vdc relay. the resistor currently drops 40vdc so the relay sees 28vdc or less.

it doesn't look like a 1k resistor.... see photos below (note the main board is crooked because it's not screwed down). in the photo, it's the resistor (the one w/ bands that all appear to be brown) next to the 6.8k ohm one.

Click the image to open in full size.
Click the image to open in full size.
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Old 27th May 2008, 07:34 PM   #5
Nordic is offline Nordic  South Africa
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Click the image to open in full size.

Click the image to open in full size.
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Old 28th May 2008, 12:33 PM   #6
glennb is offline glennb  Australia
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Quote:
Originally posted by Geek
... 1.7 watts is cutting it close for a 2 watt. I'd up that to 3 or even 5 if you have room. It's not just the resistor, but surrounding component life too....
Agree. For long term stability and reliability, I don't go over 50% of the wattage rating of resistors. Any discolored resistors are a concern.
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Old 28th May 2008, 12:47 PM   #7
Mooly is offline Mooly  United Kingdom
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Hi,
Talk to any repair tech in the TV trade and they will tell you what hot means for resistors. One major manufacturer had a major problem with this on two cermet type resistors. They were about 7 watts rated I think and got so hot that they actually de soldered themselves. And the official cure. Order some special high melting point solder.

Edit-- Can I ask what the item is in the pic.
There are a couple of well known tricks for cutting down on the power dissipated by relay coils by the way, usually easy to implement.
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Old 28th May 2008, 12:59 PM   #8
AndrewT is online now AndrewT  Scotland
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a 24Vdc relay does not need 28Vdc across it to hold it in.
It will probably hold in with less than 15Vdc Maybe much less.
Measure it.
Add a second resistor in series to drop the operating voltage to well below 24Vdc.

Add a 22uF to 47uF cap between the resistor junction and the other side of the switching transistor to provide a high voltage pulse to fire the relay on.

All will run much cooler if you can get the relay coil down to around 15 to 18Vdc.
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Old 28th May 2008, 02:24 PM   #9
Nordic is offline Nordic  South Africa
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As you can see from the second graph for ceramic resistors...

Even up to an 8W package you are dealing with 50C at 1W dissipation.
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Old 28th May 2008, 07:39 PM   #10
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Quote:
Originally posted by Mooly
Hi,
Talk to any repair tech in the TV trade and they will tell you what hot means for resistors. One major manufacturer had a major problem with this on two cermet type resistors. They were about 7 watts rated I think and got so hot that they actually de soldered themselves. And the official cure. Order some special high melting point solder.

Edit-- Can I ask what the item is in the pic.
There are a couple of well known tricks for cutting down on the power dissipated by relay coils by the way, usually easy to implement.

are you asking what the relay is used for? it's used to switch the amplifier on/off (but not the power supply).

btw, the power supply voltage is actually 63.5V, not 68V as what I originally stated. i think i'll just replace the resistor with a higher wattage 1kohm one. i don't want to be adding too much bulk to the device because the resistor sits relatively close to the enclosure.
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