[NJARC] Resistor question.

[antqradio at juno.com]

Hi Marvin
Yes, it is a good idea to over specify the resistor's wattage rating. 
The "Radiotron's Designers Handbook" recommends derating to 60% of the
resistor power rating so that there is a reserve margin.  But that should
have been done by the equipment designer, if they did a proper job.

Some comments:
Carbon composition resistors have the resistive material inside the
device with the Bakelite case acting as a thermal insulator.  Modern film
resistors have an insulated form upon which the resistive film in
deposited.  Since the film resistor has more surface area (because the
film is on the outside of the resistor body), it dissipates heat more
readily.  It follows that film resistors are physically smaller then
composition resistors for the same power rating.

The resistor should be further derated for high ambient temperature
conditions.  The hotter the chassis, the higher the derating.

For a sanity check, it should be a simple matter to check the voltage
drop across the resistor, square it and then divide by the resistor's
marked value.  This will give you the actual RMS power dissipated by the
resistor.  Double the calculate value.  If the resistor is in tolerance
and at least this dissipation rating, all should be OK.

I rarely do this unless the resistor shows signs of overheating.  Just
about all of the resistors I have replaced due to out of tolerance value
looked perfect otherwise.

Regards,
Jim

On Tue, 1 Feb 2005 20:34:56 EST Mbeeferman at cs.com writes:
> Win a new Icom IC-756PROIII and help QSL/QTH.net
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> _______________________________________________
> The higher the wattage the better....old radios got buy with as 
> little as 
> needed to keep costs down; I'd go with 1 watt.  Resistors of the 
> time were no 
> better than 5 or 10%.  Unless the circuit is critical, 100 ohms on 
> either side 
> shouldn't make a difference.
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