Bottlehead Forum

RE: Up and running

75.24.209.235

In Reply to: RE: Up and running posted by awsjr on March 06, 2008 at 11:43:06

Power dissipation across a resistor can be calculated by dividing the square of the voltage drop across it by the resistance. Resistors should be rated for at least 2X the power to be put through them, and PJ prefers 3X as a safety margin. You can use the voltages and resistances listed on the schematic to calculate the power dissipation requirement of the resistor as well as it's voltage rating. However I believe you will find the work has already been done for you on the SR45 schematic on page 4 of the manual. Power ratings for the resistors that are running relatively high dissipation are already spec'd on the schemo. Either it will not be easy or it will not be inexpensive, for example, to locate a 75K resistor rated for 6W and 5% tolerance. It may be far easier to find a reliable 150K 3W that can be paralleled. If you use a Caddock, make sure it can handle 6W of dissipation in that application.
Voltage ratings must take into consideration the actual voltage that the resistor will see through the startup cycle. In the case of the Paramount circuit we use a resistor with 750V rating in one location, that being the 300K. Many resistors are rated for 500V or less and in these cases two 500V rated resistors in series can be used to handle startup voltage peaks that go above 500V.

Edits: 03/10/08

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