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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 16th Sep 2021, 3:32 pm   #1
pentoad
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Default What is the resistor wattage of these examples

The brown one is 19mm body length, I think is 1 watt maybe 2watt
The white ones are 30mm body length maybe 2watt
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Old 16th Sep 2021, 3:44 pm   #2
ms660
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Default Re: What is the resistor wattage of these examples

1st photo 1 Watt, 2nd photo 2 Watts a piece I would say.

Lawrence.
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Old 16th Sep 2021, 4:44 pm   #3
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Default Re: What is the resistor wattage of these examples

The first one could be an Allen Bradley hot moulded carbon (composition) or a Radiospares 2 Watt carbon composition type.

The second is an Erie ceramic cased carbon composition type - around 2Watts.
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Old 16th Sep 2021, 5:08 pm   #4
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Default Re: What is the resistor wattage of these examples

Resistor wattage rating is a rather interesting topic. How come that today’s resistors can be so tiny compared with those of 60 years ago?

What design factors determine the wattage rating? Thinking aloud…..

Larger surface area clearly increases rate of thermal dissipation by convection, so it makes sense that higher power devices are larger.
Thermal resistance between resistor element and external surface will vary. The old ‘dogbone’ resistors only had a layer of paint, but the later Erie tubulars had an outer ceramic shell which presumably increased thermal resistance. What of today’s carbon film and metal film resistors? I guess we’re back to just a layer of paint between the resistive element and free air.

So are modern film resistors more tolerant of higher temperature operation than their vintage equivalents? Their smaller size suggests that they are.

Martin
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Old 16th Sep 2021, 5:27 pm   #5
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Default Re: What is the resistor wattage of these examples

I agree 1W for the first resistor on the left, and 2W for the Erie on the right.
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Old 16th Sep 2021, 7:46 pm   #6
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Default Re: What is the resistor wattage of these examples

Interesting that my Erie Hi-stab (30mm long) 1% resistors are labelled on the box as 1w- presumably de-rated to ensure they maintain their 1% performance.
Dave
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Old 16th Sep 2021, 8:54 pm   #7
Lucien Nunes
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Default Re: What is the resistor wattage of these examples

Quote:
So are modern film resistors more tolerant of higher temperature operation than their vintage equivalents? Their smaller size suggests that they are.
Yes, this is the crux of it. Compared to composition resistors, they are able to withstand higher temperatures without drift or damage, and have lower temperature coefficients of resistance, therefore can be run hotter with no detrimental impact on performance. As far as power rating is concerned, the only way in which composition outperforms film is its ability to absorb high peak impulse power, as the dissipation occurs throughout the slug instead of within the very low mass of the spiral element.

Quote:
Erie Hi-stab (30mm long) 1% resistors are labelled on the box as 1w
Erie did quote two different power ratings for different ambient temperatures (IIRC half the rating at 70 deg.C. compared to 30) and the I seem to recall the default rating for the cracked carbon types being at the higher temperature. I have the datasheet somewhere.
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Old 16th Sep 2021, 11:35 pm   #8
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Default Re: What is the resistor wattage of these examples

I remember even in the 60's that Philips rated their carbon films two ways.
1 watt @ 100 C and
1/2 watt @ 70 degrees.

Moulded carbon drift without heat, horrible noise makers.

IRC made VERY small resistors in the 60's too, renowned for the end caps breaking off. They were only about 6mm long and about 1 1/2 mm long and rated at 1/2 watt. I also remember many imprints in my finger tips from the temperature they ran at. They were made on ceramic formers.
As to the origional question, 1 watt and 2 watts as most of us have mentioned.

Joe
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Old 19th Sep 2021, 7:09 pm   #9
pentoad
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Default Re: What is the resistor wattage of these examples

I have just bought some quarter watt 5% Allen Bradley NOS carbon comp, two thirds of them were way out, between 11% and 20% - dissapointing
They are tiny in size compared to the original ones also.
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Old 19th Sep 2021, 7:27 pm   #10
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Default Re: What is the resistor wattage of these examples

I understood that the main difference with modern resistors was that they are rated to withstand a certain power dissipation as long as you can find a way of keeping their surface temperature within the specified limit. Back in the day the manufacturers took that responsibility for you. They rated their components to survive at the specified power as long as they were in something close to 'free air' which wasn't unduly hot. These days keeping the temperature down is your problem, not theirs.

Cheers,

GJ
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