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

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

Hi,

Thanks everyone, I appreciate the info!!

SA Dev

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

Cheers! Rich

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.

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