Quick and dirty isolation transformer

Ever get a device that might have a faulty power supply or unsure?

Wells here is a quick and dirty solution if you don’t have a proper Isolation Transformer.
All you need are two filament transformers of the same output voltage. You can rob these from discarded wall warts. Basically we are taking a step down transformer and stepping it back up by using the secondary of the slave transformer to step it back up to mains reference voltage.

Do not put two wall warts in this configuration unless the Output is AC. If you use one or two that are DC output it will not work and you could blow up the filter cap that is inside.

For my build I used two 9v wall warts. The master that goes into the wall outlet is rated for 800mA and the slave that connects to the master transformer is rated for 500mA.
If you have mismatched transformers like I do you always want the higher rated current on the side that plugs into the wall. This way you can get the maximum current on the slave transformer. Even if you use matched transformers say 500mA for both transformers then you might get around 300/400mA. Take note the total output current wise will be whatever the slave transformer can handle. From my findings it doesn’t require it to be in-phase. Be sure to use the same output voltage. If you use a higher output voltage rated then the slave will sag a bit so instead of 120 volts you might get 110 or even 100 volts. Even if they’re matched there might be a slight voltage drop due to hysteresis. If you use a lower voltage rated for the slave you could burn up the secondary coil that is being used as the primary.

Now this is only good for devices that uses low current draw. If you need something bigger then two battery charger transformers would be better since they are rated for much higher amperage.

Just to give a better prospective here is a schematic.
Left side is the Master Transformer and the Right side is the Slave Transformer. Notice there is a center tap for the master transformer. It really isn’t needed but if you run into a center tap you can ground it. However if the slave transformer has a center tap then leave it floating.

Remember when I said it doesn’t matter about it being in-phase? Well I kinda lied a bit. On the output when you go to wire up a wall socket or some sort of plug and you want the proper live and neutral leads you want to toss it on a scope and see what lead has the top of the sine wave.
Keep in mind you do not want the chassis ground mixed with the output’s ground. Then this would defeat the purpose of the isolation.

This setup works well for tube equipment as well if you can’t find a proper transformer. You can use it to power the filament and the plates down the road. You could also toss on a voltage doubler by using two diodes and two capacitors. So even if your total output is 100 volts you could add a voltage doubler to bring it back up but you will loose some current.


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