Power supply bleeder resistor value and wattage?

I’m assembling a battery eliminator with some spare parts that I have to power filament and plate voltages for home brew experimental regen radios to build. Can any forum member suggest an all around resistance value and wattage for a bleeder resistor for my circuit below?

6 comments

6 Comments

w3hwj

Apr 06

Why do you want a bleeder? To discharge the filter caps when the supply is OFF? The Zener string is already conducting at least 10 mA (see data sheet) and will discharge the caps (eventually). If the desired discharge time is really 0.05 seconds, you don't need a big resistor. 2 Watts should be fine. Rich PS: 250 ohms is way too low. It will draw 400 mA steady state at 100V output. This kind of circuit is intended for radios that draw under 50 mA steady state.

RedOwl

Apr 06

Replying to

Yes, only to discharge the filter caps when the supply is off. Thanks for the tip. I recalculated using a 24K, 3W resistor I found in my parts that will give a 3 mA steady state at 100V output with a 2.4 second delay to discharge the filter caps and disipate about a 1/2 watt in the resistor when the supply is turned off. This should work fine.

LarryDee

Apr 06

I did a quick chatgpt4 question on this. If it's correct, and there is no guarantee that it is even close, then the resistor is huge. I'm not an engineer, but here is the AI answer It's fun to try AI

"To calculate the resistor value and wattage required for your power supply, you will need to know the desired output voltage and current. Assuming you want to limit the output voltage to 120V DC and the current to 1A, you can use Ohm's law to calculate the resistor value:

R = V / I = 120V / 1A = 120 ohms

To calculate the power rating (wattage) of the resistor, you can use the formula:

P = V^2 / R = 120V x 120V / 120 ohms = 120W

Therefore, you will need a resistor with a value of 120 ohms and a power rating of at least 120 watts to limit the output voltage of your power supply to 120V DC and the current to 1A. "

RedOwl

Apr 06

Replying to

I kind of had a feeling that the resistor wattage would be too high. I think I'll scrap the idea of this battery eliminator for building HV type tube regens. Instead I'll investigate building low voltage regens using any of the several 12V space charge tubes that were popular back in the 1950s car radios.

LarryDee

Apr 06

Replying to

Isn't the pull down resistor used to drain the caps so it doesn't pop so much. Can't use it the way it is, without the resistor ? Are the tubes that sensitive to transients ?

RedOwl

Apr 06

Replying to

I was able to determine a bleeder resistor for my circuit from a nice bleeder resistor calculator that I found online written in Excel. Figuring worse case of 100V dc at the output and the filter cap values, the calculator resulted in a discharge time of 0.05 second at 20 watts dissipation if I were to use a couple of 250 ohm, 25 watt resistors I have on hand in series at the power supply output. The resistors would never heat up. I've attached the Excel calculator if you or any other forum member is interested. Be aware that the calculator says to specify filter caps in farads and not microfarads for the calculation.

BleedRes

.xls

Download XLS