# DIY almost-variac

Discussion in 'General Electronics Chat' started by RiJoRI, Oct 31, 2008.

1. ### RiJoRI Thread Starter Well-Known Member

Aug 15, 2007
536
29
I remember reading many moons ago about someone using a pair of 120V-12V AC transformers with (an L-Pad? a varistor?) mounted between the two, which would work as a ~0 to ~120VAC variable power supply.

I stuck the idea into the Just-in-case-I-need-it-Someday part of my memory. Now I wonder if this is, in general, A Good Idea, or if it was from Looney-Land. Obviously, a fuse or two would be needed, but is there anything basically wrong with this idea?

The concept, pictorially:
120VAC =[X]=/ 12VAC /==[VR]==/ 0-12VAC /=[X]= 0-120VAC

[X] X'former
[VR] Variable Resisitance

Thanks,
--Rich

2. ### Ron H AAC Fanatic!

Apr 14, 2005
7,012
681
It won't work at any appreciable output current level. The secondary impedance of a transformer is N^2 (N squared) times the primary impedance, where N is the turns ratio. For a 12:120 transformer, your L-pad impedance will be multiplied by 100 as seen at the output of your "variac".

3. ### RiJoRI Thread Starter Well-Known Member

Aug 15, 2007
536
29
Thanks, Ron! It seemed like a good idea, so I guess my BS detector was tripping. I s'pose if it was that easy, there'd be dozens of plans for the circuit. I'm glad I never needed a "semi-variac" ("semi" in that the output voltage would never be greater than the input voltage).

--Rich