At 07:27 PM 6/3/01 +0100, you wrote:

>If it's an auto-setting PSU it works fine, but if it has a manual switch
>it's likely to go 'BOOMMMMMM!!!!!'. Usually the part which blows
>(loudly!) is the main electrolytic, they often have two low voltage ones
>and switch them in parallel for 110V and series for 230v.

No, they are always in series. It's a clever circuit, requiring only a
SPST switch to do the changeover. Behold the beauty:-

                     x----------x
0--------------------|~        +|---------------x-----------------> + 300V
                     |          |               |
Input                | Bridge   |              --- +
0----------x---------|~        -|-----x        --- C1
           |         x----------x     |         |
           |                          |         x---------x
           |                          |         |         |
           |                          |        --- +      |
           |                          |        --- C2     |
           |                          |         |         |
           |                          x---------x-------- | --------> 0v
           |                    /                         |
           x-------------------o  o-----------------------x
                              open => 240V in
                            closed => 120V in
C1, C2, ~200VDC rating.

The topology of 200W+ PC supplies is quite different from the "universal"
flyback topology, which is useful mostly for low power. This way the
converter always works on about the same voltage.

Best regards,


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