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If your load have a constant 250w (+-2.5A) consumption at 110v, you can put a serial resistor, but in that case you need a resistor of 250w!!!
Definitively not a good choice.
The other way it's, make a rectifier to get a DC wave from the 220v line, and next you need to put a circuit that create a PWM wave where the fundamental wave are a 110v AC (maybe 60 HZ).
Note, in that case you must be careful with the load characteristic, if it is one AC motor, no problem, but if it is a electronic load...
As already dito, the perfect way is to change the primary of mains transformer from 110 to 220V, and I know this is not expensive.
I think any other CHEAP solution will carry noise, or something else not very good.
Hi if you want to reduce the voltage you can modulate a 220VAC rectified voltage with a sampled sinewave in a pwm mode such as in motor drive in direct sinusoidal control;you need only one micro with pwm module and for igbt with drivers (high/low side)
It may be worht checking the trasnformer that is alreay fitted to see if it has wto primary windings - these can be converted to 110/220 easily - wire in parallel for 110V and series for 220V.
Just a thought, as it may save you money if it is already suitable
One thing to check is if the transformer in your unit is rated for the power line frequency you are using. If you use a transformer at a lower frequency than it is rated for it will overheat. The suggestion of replacing the transformer with one rated for your power line parameters has the highest probability of working properly.
I had exactly the same problem with a friends amplifier that came from America.
I purcased a 240 to 110 auto transformer and was able to mount it in to the amplifier box . I kept the isolation as nothing touched earth and the transformer feed strait in to the actual transformer of the amplifier I mounted it so that it was screened from the amp section.
You can buy wall plugin converters from Maplin electronics, that use thyrister swithing. they do the mains voltage conversion for you in a neat littl box.
I NOT recommend a thyristor/triac converter. It good for ohmic load,and when the loading go under 50W,sometimes the output go higher as 110V.
If this converter working before a trafo,the secondary rectifier always produced higher DC voltage.
The correct way is autotransformer, or change of original trafo.
the simplest solution would be using a suitable transformer but if you want to use a pwm it might be more complicated as the power you want is a bit higher. As suggested before It might be a good idea to check the original transformer of the unit as it can be modified to your need of voltage.