Hello,

My mains supply is 240V.

If I require a step-down 240V to 120V transformer that does not require isolating, could I use a centre-tapped transformer, with 240V across the primaries, and take 120V from the CT, ignoring the secondaries?

I cannot see why not, but haven't heard of anyone doing it!

Many thanks,

Harry

 

Welcome to AAC!

It sounds reasonable, with some some caveats. It needs to be a high voltage side of the transformer, the low voltage side will likely smoke and catch fire.

There is no isolation, this limits the things you can do with it. We get a lot of folks trying to make a power supply with line voltage, which is not allowed here at AAC. For routine use as a line voltage out I don't see a major problem with it though.

With conventional step down there is a current boost to match the voltage reduction. Not so here.

 

Thanks Bill, and thank you for your response!

The device I want to power already has its own isolating transformer - it was just intended for the US market. It can be run from batteries, current draw is around 7mA at 27V so, taking into account the rectifier and leaving some space, a 9V @ 9VA transformer should definitely be robust enough.

 

Using the center-tap point should work but obviously such a step-down transformer would be larger and heavier than a dedicated autotransformer since it has the unused output windings taking up space.

 

And I believe if I remember right there is a higher current in the windings so you may have to derate the output current drawn by the load to preclude overheating the windings. Been a long time so not real sure on exact details. Worth you doing more research on though IMO. Maybe someone else can augment my limited recall on this topic.

 

And I believe if I remember right there is a higher current in the windings so you may have to derate the output current drawn by the load to preclude overheating the windings. Been a long time so not real sure on exact details. Worth you doing more research on though IMO. Maybe someone else can augment my limited recall on this topic.

The input current of the transformer input winding, when used as an 2:1 autotransformer, will be 1/2 the center-tap load current. The center tap load current is out of phase with the input current. Thus, looking at the loop currents, this means the current in the common (bottom) winding connection is also 1/2 the load current.

Thus, for example, if the load current is 2A, the input current is 1A and the bottom current is also 1A. The 2A return current from the load splits, 1A going back to the transformer common connection and 1A going to the mains (to match the 1A from the mains to the top connection).

All this means is that the only wire seeing the 2A load current is the center tap wire from the tap to the load. Both windings see only 1A. Thus I seen no problem in using the transformer at its full rated input load current as an autotransformer.