I'm trying to make a cheap variable power supply for a hot wire cutter. As I will be trying different wire thicknesses and lengths I must be able to control both amperage and voltage. To do this I have a light dimmer wired up to an electrical transformer. This lets me vary the power between 35 and 150 watts. My original plan was to follow this with a variable magnetic transformer/ multiple transformers and switches to control the voltage amperage ratio. My problem is the documentation for the electronic transformer says it must not be on the same circuit as inductive loads.

How much of an issue is continuing with my current plan? and is there a way to solve the implied problem? ... perhaps with a diode.

  • \$\begingroup\$ what is an electronic transformer. A circuit schematic would help. \$\endgroup\$
    – Andy aka
    Oct 16, 2013 at 17:45
  • \$\begingroup\$ I expect that your light dimmer will provide sufficient control. You cannot adjust voltage and current independently - if you adjust one, the other will also change, in accordance with Ohm's Law. \$\endgroup\$ Oct 16, 2013 at 18:15
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    \$\begingroup\$ A transformer from a soldering gun may provide enough voltage and current to do the job. Plugged into a variac. \$\endgroup\$ Oct 16, 2013 at 19:30

1 Answer 1


Most common cheap light dimmers use phase controlled dimming, which isn't really compatible with inductive loads such as transformers. It switches the AC on for the load at full power at some point during the sine wave, depending on the dimming level. This works well for resistive loads, but inductive loads don't like to be slammed with voltage suddenly.

You might be successful, but it's also likely that you'll destroy the light dimmer or overheat the transformer, depending on which one is the weaker link.

What you really want is a variac (also known as an autotransformer), then a transformer. For instance,


This will allow you to vary the AC voltage without removing parts of the waveform. It'll handle 300VA, so as long as your end load uses under 300 watts that one will work fine, otherwise you'll need a larger one.

There are other methods, however, of controlling your heating wire. Replacing your dimmer and transformer with a real bench power supply will allow you to control your voltage and current very finely. They aren't as cheap as a dimmer and transformer, or a variac and transformer, but once you've done your testing you can make a cheap supply for your final product and keep the bench power supply around for other products. Just make sure the bench power supply can handle the load you need at the voltage and current.

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    \$\begingroup\$ Inductive loads don't like being disconnected at zero volt crossovers - that's when they have maximum current in them and that's when they can do the most damage. \$\endgroup\$
    – Andy aka
    Oct 16, 2013 at 19:50

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