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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 20th Jun 2009, 11:04 am   #1
TNC
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Default Resistors

I have checked previous component listings and I don't think I am repeating issues from any recent posts.

My question/comment relates to assessing the power handling capabilities of older resistors and asking whether there a "rule of thumb" which can be used to do this?

An example is a resistor in a TV I am restoring. It's a 39kohm in the Line Scan section: it is 1 1/2" long and approx. 7/16" in diameter (it's a body/spot not ring-marked component). Obviously I could disconnect and measure the current in line but it would be useful to be able to assess the likely power handling capacity visually by reference to its dimensions.

An additional concern is sourcing replacement resistors with >5w handling; any suggestions here would be most helpful.

Thanks, Trevor.
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Old 20th Jun 2009, 11:14 am   #2
brianc
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Default Re: Resistors

Hi Trevor
It sounds like a 1W jobby. The other, somewhat larger resistors found in the older TVs would be 2W. It was fairly unusual for anything over 2W to be colour coded - they were normally wirewound. The very small ones are 1/4W and the next size up 1/2W. I always use modern 1W components as replacements. To my mind, the tiny modern 1/2W resistors look totally out of place in this environment.
Hope this helps.
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Old 20th Jun 2009, 11:19 am   #3
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Default Re: Resistors

You could measure the voltage across the resistor and then use Volts squared over Resistance (E^2/R) to determine the power.

I avoid the problem of working out the original power rating by using 2W flameproof resistors when replacing resistors in valve equipment. They have a high voltage rating too.
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Old 20th Jun 2009, 11:24 am   #4
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Default Re: Resistors

If you replace any vintage resistor with something modern the same physical size, you'll be OK. The temperature rise is governed by the size and the power dissipation.

So if you use something the same size, the temperature rise will be the same (because of course the circuit determines the power dissipation), and if this temperature rise was OK for the original resistor, it will certainly be OK for the modern one because they are generally still happy at much higher temperatures than the old types.
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Old 20th Jun 2009, 11:29 am   #5
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Default Re: Resistors

Quote:
An additional concern is sourcing replacement resistors with >5w handling; any suggestions here would be most helpful.
Such resistors will generally be wirewound. RS Components and Farnell have a good selection.
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Old 20th Jun 2009, 11:36 am   #6
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Default Re: Resistors

Thanks chaps - most helpful.

Using that formula with a measured voltage drop of around 100v, I am amazed to discover that it's only 0.256 watts - and it's such a monster component!
So as you say, 2 watts is going to cope with almost everything.

Thanks again, Trevor.
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