[HBR] about resistors

[Shoppa, Tim]

That's even easier:

Watts = current through resistor * voltage across resistor

or (given that current through resistor = Voltage across resistor / Resistance)

Watts = (Voltage across resistor squared) / Resistance

e.g. Cathode resistor of 1K ohms with 9 Volts across it is dissipating 0.081 watts. 1/4W will work fine (but more typically in old radios smallest you will see is 1/2W).

or e.g. plate resistor in resistor coupled amplifier: 100 volts across 250K ohms gives a power of (100*100)/250K = 0.04 watts. Again 1/4W or 1/2W will work fine.

As you get to bigger power tubes you can get up into multi-watt values.  e.g. for a 6L6GC operated as spec notes with cathode resistor of 218 ohms and 51 mA current, we get up to 11V * 51mA = 0.56 W. You would use a 1 or 2 Watt rated resistor here.
________________________________________
From: hbr-bounces at mailman.qth.net [hbr-bounces at mailman.qth.net] On Behalf Of Chris Howard w0ep [w0ep at w0ep.us]
Sent: Sunday, May 06, 2012 4:31 PM
To: HBR Receiver List
Subject: Re: [HBR] about resistors

That was very helpful!

I didn't communicate very well, in that I was also
(or mostly) interested in the wattage of the resistors.

If you have similar info for wattage,
that would be really great!


Chris





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