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Repair Of An Auto Transformer Chinese Variac

By on November 5, 2022
how to repair a broke variac









I went to a customers business and he said he was about to ring me. Turns out he had a problem with his Chinese variac.

how to fix variac

Unfortunately some of the pictures are web pictures as I forgot to take some photo’s before pulling it apart. This was not purchased from an Australian E-bay site but from a Spanish one. This had multiple problems.

The first being that the voltage was going backwards when using the dial on the top.

fixing a variac

The second being that the wiring inside was incorrectly and possibly dangerously wired.

As you can see the Neutral is going through the fuse (normally the active brown wire goes via the fuse, not the blue wire.

variac problem repair

I also found under the power wire the active wire was exposed.

wiring of variac

So I opened my similar Australian E-bay purchased variac for reference. The first thing was to replace the power cable which I did and put the active wire via the fuse. I also figured out that the Blue wire going to the transformer belonged on the centre tap of the power switch. The thick black wire belonged with the neutral wire from the power cable. The Active brown wire goes to the via the fuse then to the left contact of the power switch.

how to repair a broken variac

This fixed the voltage going the wrong way on the dial but didn’t fix the voltage on the digital meter. I had to rearrange the wires going to the meter. Turned out that the black wire coming from the meter went to active not the power switch centre tap I tested it via a light bulb and found the light bulb was triggering the test light so I tried with a non resistive load and it worked good.

This article was contributed by Lee Davey from Hobart Tasmania Australia. He has been doing repairs on electronics and PC’s for around 30 years.

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Note: You can read his previous article on LED T-Shirt Controller Repair




  1. Anwar Shiekh

    November 5, 2022 at 9:00 am

    Doesn't one want the fuse in the live? if it is in the neutral, the device stays live.

  2. Parasuraman S

    November 5, 2022 at 10:05 am

    Very new experience shared! Many thanks!

    • lee

      November 7, 2022 at 6:14 am

      Your WELCOME.

  3. Waleed Rishmawi

    November 5, 2022 at 12:09 pm

    Glad you had a reference schematic to go by. Thanks for sharing

  4. Tayo

    November 5, 2022 at 1:05 pm

    Frightening ... actual.

  5. Albert van Bemmelen

    November 5, 2022 at 1:28 pm

    About adjusting the wiring of that 3 wire Volt meter this link of a 3 Volt wires with 2 Amp wires V/A meter I recently bought shows two useful examples on how to connect these meters.

  6. Paris Azis

    November 5, 2022 at 5:54 pm

    Since autotransformers have only one winding,no matter the color or the position of the cable connected to any of the two of its input terminals, the phase will always appear at the output, making this device dangerous due to its own nature.
    The critical point with autotransformers is their proper grounding. Their case,which normally is metallic, must always be securely connected to the earth through the connection plug. This is the most important condition when wor king with them.
    As regards the appearance of the position of the phase on the two of its output terminals, this (at least for the European system, which uses Shuko plugs, which can be connected either way to the wall outlet being symmetrical) renders the "phase" and "neutral" characterisation of the output terminals of the autotransformer completely "plugging depended". Therefore the phase can appear in anyone of them, which, in turn, renders their characterisation meaningless. The operator must always be careful about that. It would be wise to use the variac after an isolation transformer, or, alternatively, buy a transformer based variac, which is much more expensive than its simple version.

  7. Yogesh Panchal

    November 8, 2022 at 6:28 pm

    Good JOB! Lee Davey.


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