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

Hugo Holden

Veteran Member
Dec 23, 2015
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:


Once these principles are understood, it easy to convert SMPS supplies to dual voltage.


Veteran Member
Mar 18, 2004
New Zealand
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.


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.