Q: How do I choose a resistor when building an anodizer?

September 17, 2012

Daniel asks:

I just built an anodizer as you described, but used an old Variac I got from eBay. I used a light bulb as a resistor across the capacitor, and wanted to replace it with a resistor. You specify a 100 ohm 200 watt resistor, but since you can use different light bulbs, I imagine I could use a resistor with different ohm rating as well. What would be the difference if I used a different ohm rating and what range would be acceptable?

Well, Daniel,

If you have a variac (instead of an unstable and load-determined dimmer) then the only function of the resistor/bulb in parallel with the capacitor is to drain it, to reduce the voltage.
It is only needed when you turn the voltage down. My schematic shows the resistor with a series switch (use a momentary, normally open).
The resistance does not matter much, as it is dependent on the power produced and the time it takes are really the only things of interest.
Given up to 120v, you need to be able to handle the power of whatever the resistance is: (P=V2/R)
So a 120 ohm resistor needs to have a 120 watt rating at 120 volts.
But a 240 ohm resistor needs only 60 watts at 120v. Or 30 watts at 60 volts.
Also, as a momentary device you can use a lower power rating than if it were on continuously. The voltage (thus power) drops quickly, and the time to drain grows with the resistance (V2=V1 x e-t/RC) .

So my design suggests some middle-of-the-road options for resistors that are easy to find at electronics salvage places. And light bulbs are great because their resistance increases as the voltage does, so they drain faster than a fixed resistor. But they break.