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Isolation transformer and 
series light bulb


An isolation transformer can be made of two similar or equal power transformers with their highest voltage secondaries connected together. On the second transformer's primary side you then get back the line voltage, minus a few volts due to losses. The 1M8 resistor is to bleed away any static charge on the output side.

The series light bulb provides current limiting when powering newly repaired devices. It can be deactivated using a bypass switch.

The "plug orientation wrong indicator" LED lights if the plug is connected to the socket the wrong way, that means if the intended neutral wire (N) becomes hot. If it lights, the plug orientation should be changed.

For the switches you can use small 3A 250V things; however, if you use more powerful transformers think of increasing switch (and fuse) ratings.

Some people say that the lamp should be put into the live wire. I posted a message to and got more different answers than I expected ;-)

One statement concerning another schematic which had the lamp connected in the live wire:

"Note that the lamp is in the live side of the mains supply, not the neutral.
This is for safety, so that the neutral side (and the chassis in AC/DC sets)
stays close to earth potential no matter how much voltage is dropped across the lamp."

This is true for devices where the neutral wire is directly connected to the chassis, as in some old television sets. In this case you can hurt yourself if you touch the chassis with the lamp switched on. However, if you plug the set into the socket the other way round, you'd surely get the full 230 V from the chassis, so it's better to avoid touching the chassis anyway, and checking whether it is live or not.

I decided to put the lamp into the neutral wire to avoid the risk of electric shock if anybody touches the empty lamp socket by accident. And so far I have survived ;-)

Please note that building this circuit occurs on your own risk! If you are not sure what you do here, leave it. I do not assume responsibility for any damage you or any equipment may suffer from building or using this device. Please respect the common security guidelines when working with electric equipment. A description of these can be found at the FAQ.