A DELUXE TEST BENCH VARIAC in NUTS AND VOLTS- June,2008

[spindown]

Really enjoyed Robert Reed's article on variacs. I wondered if someone could tell me more about the use of capacitor C12 and how it lowers the voltage from 120 volts ac to 9.7 volts ac, what factors influence the voltage drop, can a conventional cap be used or is an ac rated cap necessary, and how much current is available with this configuration? Thanks Paul

[dyarker]

At first I was just going to say "yes, a conventional cap". Then I realized our ideas of "conventional" may be different.

The cap needs to be non-polar like poly, mylar, paper or ceramic; rated for at least 200V in this case. (These are the types I consider conventional.)

Based on the wording of your question, I suspect you call electrolytics (aluminum, tantalum) conventional. These types are polar, and will not work here.

A series capacitor to drop the voltage wastes less power than a series resistor because the capacitor shifts the phase of the current relative to the voltage across the cap. When the voltage is high the current is low, since power is voltage times current, the power is lower than a it would be for a resistor where voltage and current are in phase. Figuring how much involves math I haven't had to do for years (decades). A search on wikipedia for capacative reactance or impedance will explain it better than I can any way.

Cheers,