I have a 110/240 transformer, that's in 110, out 240. Is it safe to connect secondary to 240 mains and use primary for 110 supply? Looks similar to this
Probably will work as long as it is rated for 50Hz operation but perhaps not advisable. If its rated for 60Hz operation then do not connect the 240V side to a 50Hz outlet - it will overheat.
The primary and secondary windings are related by the turns and hence voltage/current ratios of the input and output sides.
So if it is a 240VA unit, on the secondary side it will give 1A at 240V and draw about 2.5A on the primary (110V) side, so connecting the 240V side to a 240V outlet should give you 110V at about 2A (probably less - see below).
That said the primary and secondary windings are optimised to work that way and might not necessarily hold the other way around.
Hi,
Does the transformer have a spec sheet?
Tom.... 
it is and so is mains
Another important thing:
The plug is supposed to be connected to mains, the socket is supposed to deliver power to something else. So please, replace socket and pins and do not make a lead with two plugs to connect this thing to your local mains....
Measure the primary and secondary winding resistances. If the 240V side is higher then its probably safe.
To be safe, connect a 100W bulb between the mains outlet and the 240V side and see. Better still would be to use a variac to slowly increase the voltage...
Goes without saying
But why not use a "proper" 240-110 step down unit instead? If something goes wrong, you can blame the manufacturer! Not so with reversing a 110-240 step up unit.
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They are not cheap
Oh well, I guess you don't have much choice then...so connect a 100W bulb in series with the 240V side and mains. On the secondary side connect an appropriate load (a 110V bulb?) and switch it on. If nothing seems to heat up (the transformer, that is) and everything works, remove the 100W bulb and test again.
Ideally the load should be as per the VA rating of the transformer to be absolutely certain.
BTW do you plan to power 110V rated equipment with this? I am guessing that will be transformer based too (SMPS ones work 90-240V 50/60Hz) - and 110V equipment is usually rated for 60Hz, and using a 60Hz power transformer with 50Hz albeit at 110V runs the risk of overheating it.
Hi
There are "pure" transformers and auto-transformers.
The difference lies in the physical assembly of the coils.
A transformer has two separate windings.
If your transformer is of this type, applying 110V in the 110V coil, you will get 240V in the other coil.
An auto transformer has a single coil with a tap on one part of the turn.
Both models in the frequency that were designed can be used from 110V to 240V or from 240V to 110V.
The difference is that the transformer isolates the 110V circuit from the 240V circuit, but the autotransformer does not isolate the circuits.
And "pure" transformers cost more and weigh more.
See attached schematic.
Check carefully what you have and only use it for the intended purpose .
You also have the safety aspect that using it the wrong way around the plugs and sockets will be sexed wrong and how will you protect circuits on the “secondary” voltage side ( earthing etc)
No one here should advise using such a device in a manner for which it’s not intended or certificated , you’ll feel pretty bad if the OP kills himself .
It is proper transformer with isolated 2 coils
Is this for continuous use of just an hour or so?
intermittent use, was thinking of powering 110v jigsaw, so occasional cuts
I can see no problem in doing what you suggest.
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As per my previous post - At the very least the OP will need to make up a 240v lead which will have exposed male pins on it to plug into his transformer ( as he is using it the wrong way around) - that is a dangerous item to make up.
On the other side of the transformer he will have the exposed prongs of the now 110v output .
Hi,
Can I suggest if/when you do run it 240 to 110 check the input current with no load, and then the input current and output current with a load.
This will give you an indication of power losses, which will be expressed as heat.
Tom...
Verify the tool's rated supply frequency - as with transformers, 60 Hz motors might not work properly with a 50 Hz supply.
Jig saws are cheap , by the time you’ve messed with making up special leads to make it work , you could have bought one .