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 Post subject: Replacement 2 watt resistor - really?
PostPosted: May Mon 17, 2021 1:03 am 
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Can this tiny 2 watt wire wound resistor really dissipate 2 watts? It would have to be glowing RED! Dale CFP 2 10K 1% (Mouser)
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10K 2 watts.jpg
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 Post subject: Re: Replacement 2 watt resistor - really?
PostPosted: May Mon 17, 2021 1:50 am 
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I don't know, for more than a hundred years we've had nichrome on ceramic in heaters, electric stove elements, and other appliances. Not that unusual really except now they can make resistors that are relatively stable and precise out of the same stuff.

The thing to keep in mind is, for the same power dissipation, as physical area is decreased, surface temperature increases. If you run that little resistor at its full two watts, the temperature rise from the spec sheet is 125 degrees C above ambient. So at room temperature (25 degrees C), it can be expected that the resistor will reach 150 degrees C, or 302 degrees F. It may get even hotter under a chassis as the temperature there is usually at least a few degrees above ambient. Above 70 degrees C ambient you have to start de-rating the power to avoid overheating the resistor. Just be careful of how and where you mount it as it will definitely burn wires and other components if it is up against any.

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 Post subject: Re: Replacement 2 watt resistor - really?
PostPosted: May Mon 17, 2021 7:31 am 
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Today I use >=50% higher rating in a modern resistor when replacing a vintage high wattage resistor. Only expection are the sand resistors where I've used the equal rating.

Previously I had used the exact rating but found out later color changes and paint charring on some of them. Could be a quality issue or tolerance was too close for comfort. Especially one 2W was running on higher current and was closer to 2.5W in actual use inside the hot valve oscilloscope cabinet.


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 Post subject: Re: Replacement 2 watt resistor - really?
PostPosted: May Mon 17, 2021 7:46 am 
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Location: British Columbia
tikonen wrote:
Today I use >=50% higher rating in a modern resistor when replacing a vintage high wattage resistor. Only expection are the sand resistors where I've used the equal rating.

Previously I had used the exact rating but found out later color changes and paint charring on some of them. Could be a quality issue or tolerance was too close for comfort. Especially one 2W was running on higher current and was closer to 2.5W in actual use inside the hot valve oscilloscope cabinet.


I've noticed that certain makers liked to use 1.4 Watt resistors wherever they could get away with it, even if it was just barely adequate, whilst others used 1.2 Watt or better, depending on usage. With power resistors I've usually ended up using 5-10 Watt jobs because I have a lot of surplus wire wounds.
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Arran


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 Post subject: Re: Replacement 2 watt resistor - really?
PostPosted: May Mon 17, 2021 11:51 am 
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Chris108 wrote:
The thing to keep in mind is, for the same power dissipation, as physical area is decreased, surface temperature increases. If you run that little resistor at its full two watts, the temperature rise from the spec sheet is 125 degrees C above ambient.


Yep. Heavy emphasis that, that little resistor will have a higher temperature than the older bigger one. Both will burn the same heat (power), but that same power in a smaller volume, translates to higher localized temperature.

Also, I've found a few resistors spot welded, instead of being soldered, in place. That's because many power resistors can handle temperatures greater than the melting point of solder. I've actually had a resistor fall out of a dummy load, due to the solder melting. The resistor was otherwise fine. Nichrome, itself, can handle temps over 2000°F. Its the other parts of the resistor that break first.

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 Post subject: Re: Replacement 2 watt resistor - really?
PostPosted: May Mon 17, 2021 12:03 pm 
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ON PC boards I have seen 1-2 watt resistors mounted with clearance from the board and also have seen them mounted against the board with discoloration to the surface of the board. I always like to mount them up off the board. That resistor in the photo looks like the size of an RN60 half watt. I guess it is the construction/materials as much as the size. I bought a variety pack of resistors from Ed Jones in Fla. Values up to 1000 ohms, all rated 2W. They are bigger than that one. I have not run them at their full ratings in anything. They are for stuff like cathode resistors in radios, where the small CC jobs tend to run hot and change value.

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 Post subject: Re: Replacement 2 watt resistor - really?
PostPosted: May Mon 17, 2021 1:43 pm 
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I"ll always space higher watt resistors off the board, and sometimes in circuits that won't degrade due to a "coil" I will put a loop, single turn in each lead to prevent as much heat transfer to the board and solder connection. Another trick is to put a lot of solder on the pad where it connects, to provide as much contact area and thermal mass as possible. Normally you won't see more than 3 watt resistors directly mounted to a PCB.... but some manufacturers did it and the boards always get discolored and burn with age.

A lot of the time, the solder joint degrades or 'goes away' as well. I've fixed any number of things where the resistor unsoldered itself, and that was the only issue. Bad design, but happened too often.

Then again, I suppose no one ever envisioned these things lasting over 50 years .....

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 Post subject: Re: Replacement 2 watt resistor - really?
PostPosted: May Mon 17, 2021 3:12 pm 
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The radio is a GE E-61. http://www.nostalgiaair.org/Resources/778/M0007778.htm. The resistor is the screen grid dropper R7. B+ is 250, screen voltage is 98. I calculate 2.31 watts! So the replacement should be at least 5 watts. I will likely use a 10 watt sand type wire wound.

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 Post subject: Re: Replacement 2 watt resistor - really?
PostPosted: May Mon 17, 2021 4:53 pm 
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Location: Santa Rosa, CA
Chris makes good points.

Data sheet: https://www.mouser.com/datasheet/2/427/cpf-1762878.pdf

The 2 W rating is at 125C above the ambient temp. Derating starts at 70C ambient. In choosing a resistor, I would first go to the data sheet. Secondly, I would decide, based on experience, if I am really comfortable with something operating at over 200C in my equipment.

I am not comfortable, so would choose a larger resistor that will operate at lower temperature.

Rich


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