variac help

Discussion in 'The Projects Forum' started by 3rror, Oct 29, 2010.

  1. 3rror

    Thread Starter New Member

    Oct 29, 2010
    Hi guys/gals,
    I’m trying to build up a variac that was given to me. My grandfather was big into electronics and he had one he used for soldering and charging batteries quit a bit. I have a few questions about the build up, however. Let me give some details on the parts I have now and what I’m trying to achieve here.

    The variable transformer I have is a 5 amp type 21. I robbed an old C.T. transformer out of a battery charger for the isolation transformer and have a 1000V 35A full wave bridge rectifier. I also have a 1000uf 50V cap for smoothing.

    I planed on ordering a transformer that wasn’t a center tap, but found this one and have some questions about the wiring. The battery charger I robbed had a input switch for 6V 6A, 12V 2A, and 12V 6A.

    On the primary side there’s a black, yellow, brown and red wire. If I put 120V on the black/red wires, I get 27V across each end of the transformer. With 120V on the black/brown 23V, black/yellow 16V. I’d assume that from the middle tap to each end would give me half the volts listed above. Does this mean this is a 120v to 24v transformer at around 8 amps? There’s a number on it (MS496 00) but I can’t find any info on it. I guess it’s sufficient for what I want to do, just not sure about the wiring on it.

    It also had a half wave rectifier, but I want to use a full wave along with a 1000uf 50v cap. Will this change the output voltage? High output isn’t really a concern as I can adjust it with the variac.

    I would like to have 2 120v outlets…one regulated and one that’s not. A DC output with a ground is in the plan also. I”ll post the schematic I had before I came across this transformer. I know how to wire it with a regular transformer, but this center tap throws me off, as I’m no electronic expert.

    Forget the LED on the DC's in the wrong place as it should be right after the switch and be a AC light.

    Can someone help me, or point me in the right direction here?

    Thanks guys for any help!

    Last edited: Oct 29, 2010
  2. SgtWookie


    Jul 17, 2007
    Well, a Variac is an autotransformer with multiple taps on the secondary, as you seem to be aware; it provides for adjusting the AC output voltage, but there is no isolation from the mains provided.

    In order to make a Variac safe, you need a 1:1 isolation transformer for the input. The battery charger transformer is a step-down type, which won't work - unless you want the Variac output to be quite limited. A 1:1 isolation transformer for the power rating of the Variac will likely be pretty expensive.
  3. DonQ

    Active Member

    May 6, 2009
    Just to be clear... Even an isolation transformer does not make messing around with line voltages safe. Take the proper precautions. A couple of the best I know is "keep one hand in your pocket" (this virtually eliminates the possible current path that goes from one arm, straight across your heart to the other arm), and "don't lean on anything" (it may be grounded).

    What appears to be a 'half-wave rectifier' on a center-tapped transformer may actually be a full wave rectifier since this only needs 2 diodes instead of 4 on a non-center tap transformer. Each half of the center tap are 180 out of phase with each other so while one is conducting, the other is 'resting', and vice-versa. Current only passes through one diode at a time instead of 2 like on a 4 diode bridge, meaning less losses. Lots of times that's why a center-tap is used. So don't let that fool you.

    And yes, when you rectify and capacitor filter an AC signal you will get a different voltage than what the AC meter told you. The capacitors store the peak voltage, the AC meter tells you the RMS voltage (about 70% of the peak for a sine wave). Then, depending on your load, the capacitors discharge between the peaks, so you get a slightly pulsating voltage, the amount of pulsing depends on your load and your capacitors value. This will appear on a voltmeter as a slight reduction in the average voltage as the load increases, but if you feed it into a voltage sensitive circuit, it will see the minimum voltage between the peaks, and that will be less than the average voltage you saw on the meter.