Voltage divider calculations

[vetmed]

I need to add some voltage to the heater tap to meet the heater to cathode voltage requirements of a tube I'm working with. Such a setup has an added advantage in that it also acts as a bleeder for the power supply. I know how to make the calculations but have only a vague idea of approximate values of resistors to use. I have 300 volts @ 27 ma current draw and need ~ 60 volts DC on the winding. Suggestions? TIA

Robert Lees

 

[Paul Birkeland]

It's nice to keep the current low (so you don't influence the power supply voltage, and so that you don't need huge resistors).

300K across 300V will draw 1mA, which isn't so bad.

For 60V is 1/5 of 300, so a 60K and 240K will do well as bleeder resistors. 

The 240K will cook over 1/4 of a Watt while operating, so a 1 Watt resistor will be sufficient. 
The 60K will cook very little power, 1/4 Watt is plenty, 1/8 Watt is marginal. 

It is commonplace to bypass the bottom resistor with a small cap.

 

[vetmed]

Thanks Paul, exactly what I needed ;D

Robert Lees