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XT 130W PSU 120V tp 240V conversion, is it possible?

One very interesting thing is to look at the way a 110/220V switched option SMPS was implemented in the 5155 computer.

It also answers an interesting question, which could perhaps be posed in an electronics exam question:

How can you run the 110V primary of a small line fed power transformer from 220V, without using a dropper resistor, series capacitor or a step down transformer ?...might leave a few exam candidates stumped might it not ?

How about doing it with a bridge rectifier and two large value capacitors and creating a virtual mid point rail from the 220V supply ?

This is exactly what IBM (or at least the Zenith contractors) did with the dual voltage IBM5155 supply. I took a little time in the article to explain how they did it:

http://worldphaco.com/uploads/The_IBM_5155_POWER_SUPPLY.pdf

Once these principles are understood, it easy to convert SMPS supplies to dual voltage.
 
Yes, the same mid-rail is employed in the 63.5W 120VAC 5150 PS. When I converted mine to 230VAC I did not know that this point in the circuit would remain stable so used the 325V rail with a suitable voltage divider to supply the driver IC, at the expense of slightly-increased losses. The center rail did end up being 5 or 10 VDC off in the end, perhaps because I kept the original caps.

IMG_0631.JPG

The PS in the 5153 (CGA monitor) on the other hand is already in full wave mode at 120V. By upgrading only the primary cap voltage ratings to 400V I was able to run it on 230VAC. But due to the extra heat generated (in what I understood to be the 'snubber part of the flyback circuit') I decided not to carry on and instead swapped in an industrial 110VDC open frame PS adjusted to the required 115VDC output.
 
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