Re: 400 VDC from 230V AC
Basically, you can make a voltage doubler of 2 x 1N4007 diodes and 2 x 200uF capacitors with IanP's circuit to get 650VDC maximum from 230VAC mains, and then use a resistor in series with load to reduce output voltage down to 400V on load.
However, since you need a fairily big current (0.75A), so this circuit is really impractical as the series resistor must dissipate .75A x 250V # 190W, that a huge energy waste.
Try to use a step-up transformer before a rectifier and capacitor as our guys suggest, a autotransformer here is really small.
nguyennam