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Does a 1/4 watt resistor need any airflow if it is disapating 20mw ?

Hi,

I've made a really small LED light that I'm planning to use and I was thinking about shrink tube wrapping the resistor in with the leads on the LED. The LED is rated for 30ma continuous, but I'm only putting around 12ma through it. Would this be an issue for heat?

Thanks,

SA Dev

Reply to
SA Dev
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You don't say what your supply voltage is, or what ambient temperature
the device is supposed to work in,  but if the resistor's only
dissipating 20mW I'd go out on a limb and say you'd be fine.
Reply to
John Fields

Hi John,

The drop across the resistor is 1.8v and the current is 11.3ma, if I mutliply these I get 20.3mw, right? Ambient temp will be room temp ~75 deg F.

Thanks,

SA Dev

Reply to
SA Development 

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Right.
Reply to
John Fields

Hi,

Thanks everyone, I appreciate the info!!

SA Dev

Reply to
SA Development

If you're running it at ~20V or more, yes. Otherwise, no.

Cheers! Rich

Reply to
Rich Grise

12ma

If the voltage drop across the resistor multiplied by the current through it is close to the 250 mW resistor maximum, it is way too much and there is no safety margin.

Reply to
Lord Garth

Sounds about right and safe.

A question not for you, exactly, but to anyone else would be "What is the C/W specification for a 1/4 watt resistor body in still, relatively dry air?" It would be hard to imagine it being much worse than the Rja=200 or so for a TO-92.

You'd usually just multiply that Rja spec by the watts the resistor is dissipating to get the relative change in equilibrium temperature. If the value were 200, then the change would be about 4 Celsius or less than 8F. That would seem quite a safe change, over 75 F ambient.

I'd suppose that you *could* wrap the resistor in thermal insulation and that if you really tried hard you might cause a problem. But the Rja would have to be in the area of 2500 or so before a 50 Celsius change would be seen. Hard to imagine that big a number for Rja unless you really worked at causing a problem.

Jon

Reply to
Jonathan Kirwan

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