(4) Variable AC Power Supply With a Twist:





Introduction: (4) Variable AC Power Supply With a Twist:

About: Been To:1) All 50 states 2) Guam 3) Adak 4) Hawai'i, Mo Bettah. The only good thing about snow is that it goes away...Would that it never comes.

One day I needed a 90 volt AC power source for a project I had in mind. I thought about it for a few days thinking of all kinds of things that I had no parts for. Then one day I was in Home Depot looking for PVC pipe for a plumbing job I needed to do in the house. As was my habit, I walked down the electrical isle and spotted the dimmer controls they had and thought maybe I could use a dimmer switch.

As it turns out, I have found that to get the variable voltage part out of it you need to plug in a window fan as a ballast to stabelize the dimmer circuitry. If you plug in some electronic thing and try to lower the voltage you will get wild voltage swings and damage the equipment. A running fan will stabilize the dimmer and you will get accurite voltage measurements.

Dims lights, controls fan speeds, and provides variable voltages. My computer runs cooler on 115v.
In Hawaii, the voltages are often as much as 128 volts so I plug the dimmer into a surge protector, a fan into the dimmer and turn it on low, measure the voltage to 115v, and plug in my computer.
The computer seems to like it because it runs cooler and so doed the computer power supply.

Step 1: And Parts Is Parts:

See the photos below for the parts that were used.
Its pretty simple really.

Step 2: Assembly:

The knockout can be hit with the claw of a hammer to bend it in and then grabbed with pliers and twisted around to break it off.
The restraining nut has got to be tight so the wires don't get twisted around inside. If they do, there will eventually be a short circuit.

Step 3: Electric Wireing:

Follow the photos closely.................

Step 4: Dimmer & Outlet:

When the wireing is done tuck as much as you can in the box under the dimmer and the outlet so nothing is between the outlet and dimmer.

Test the outlet to see that it is wired properly by plugging it into the wall and plugging a serge protector into it. If the protected light comes on, its right. If not, re -wire it the right way. It has to be done right.



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    Yeah except the thing only clips the sine wave to lower the voltage. On induction motors it's death but if you need a temperature control for a soldering iron it works good.

    This thing works best with incandescent lamps and motors that have Brushes...induction motors tend to just make noise and burn out.

    I made this exact same thing to control the speed of my corded drill for when I needed a constant slower speed for use with larger bits so I could concentrate on the straightness and appropriate pressure for the drill bit size instead of trying to hold the trigger halfway in. Works great! Cool to see it here

    I made this setup 4-5 years ago for a Dremel that was single speed. Since then I've been using it for a plastic welder to control how hot the tip gets and I've not had any issues. =)

    I'm trying to do a build involving 5 dimmers. 4 of the dimmers each have their own outlet, but I'm sending my power through the fifth dimmer to maintain ratios (as a master dimmer). When I wire it this way, however, I get some strange flickering phenomena that would be hard to explain.

    Any suggestions?

    1 reply

    If you have it wired the way I think you do the voltage may be to low to operate the other dimmers. granz answer below may be more to the point though. Try using an auto transformer as the master and it may actually work the way you want.

    Unfortunately, this is not a variable AC supply. A dimmer is a phase-angle power reducing circuit, which essentially chops the sine wave to deliver less effective power to the load. If you measure the voltage with a common meter it will likely read a lower voltage because the meter is performing RMS calculations for you, but this isn't the whole story. Look at the waveform on a scope. If you want a nice variable amplitude sine wave with the least amount of hassle then use an auto transformer.

    1 reply

    That would explain why it cooks the fan.

    Does anyone know how many watts a standard wall dimmer can handle?
    I'm trying to build a PSU with transformers in series, but they're gonna draw much more power than a light bulb.

    2 replies

    I think this one was 400 watts but don't quote me it has been a while since I've seen the package. A quick trip or phone call to the hardward store should answer your question.

    Here in the U.K usually 250 watts although some can handle 400 watts.

    I think you will find this does not reduce the voltage at all. What it does is turn it on and off so that to something like a lamp or fan the AVERAGE voltage will be reduced.

    This is important because if you try to connect something which is only meant for say 50 volts and not the full mains voltage it will be damaged by this.

    I like you variable AC power supply. It's a simple wiring job, but a schematic of the connections would be good for double checking one's work. Thanks

    This is a great Instructable.

    You did a good job illustrating it.

    However, the box above seems to be VERY unhappy about it...

    (Look at the center yellow box in the first picture.)

    I was building one of these and needed to double check my plan for wiring it up, quick check on Google and here was the answer. Thanks for the sanity check.

    Your computer has a switching mode power supply, which will mean it doesn't matter what input voltage is supplied (the range is probably from 100 - 240 volts).

    2 replies

    Yes I know. Switching power supply.

    I have noticed the power supply runs cooler.

    My point to all this was not so much for use on the computer as this type of supply creats a lot of RF noise.
    W hat I did this project for was to supply old school equipment with low voltage. Restoring old AM Radios for instance. Sometimes the need arises to reform an old capacitor in a radio like this. The need for low voltage is there.

    I work projects using old style neon bulbs that fire at 90v +/-. My supply needs to to be at 90v then. I really don't like to use voltage dividers.

    Also I'm thinking of putting in a circuit breaker for the line supply.