12 Jun
2023
12 Jun
'23
3:48 a.m.
On Sun, Jun 11, 2023 at 9:07 PM Sellam Abraham via cctalk
<cctalk(a)classiccmp.org> wrote:
"While I don’t recall saying that, I may well
have - I know that I was
involved in the engineering question of how safe it would be to use.
The inverter produced 230 VDC, which it turned out could be safely fed to
the 120VAC input without more than a single diode drop reverse voltage
across an electrolytic filter capacitor. This was due to the power supply’s
design having a voltage doubler as the first stage.
I find that curious (read : I don't see how it could work, at least
not reliably.)
The power supply in the Osborne 1 and 1A is an Astec SMPSU. The input
stage is the conventional one. Mains input goes through a filter
circuit to a bridge rectifier. The output of the bridge rectifier goes
to 2 electrolytic capacitors in series. There is a voltage selector
link between one side of the AC input to the bridge rectifier and the
centre tap of the capacitors.
In the Osborne 1, this link is hidden on the PSU board and you have to
dismantle the machine to get to it. On the 1A it's set by a little
plug-in PCB in the mains input module on the back which you turn round
for 115V or 230V mains.
On 115V the link is in place. 2 of the diodes in the bridge and the 2
capactors form a voltage doubler circuit. Each capacitor charges to
about 160V. You get 320V across the pair to feed the chopper.
On 230V, the link is open. The bridge rectifier works as you might
expect, the 2 capacitors get around 320V across them and charge
equally (so about 160V on each). Again 320V to the chopper.
Now if you feed in 230V DC with the link in the 115V position, one
capacitor (only) will charge to the full 230V. That is more than the
working voltage of said capacitor. The other will not charge at all
from the input. Meaning the chopper circuit is fed from the charged
capactior with another one in series. That one is liable to charge up
with a reverse polarity due to the action of the chopper.
Bang!
-tony