What I have found so far is very incouraging. In the 120VAC mode the
rectifiers in the front end are working in a voltage doubler configuration
which would prohibit what I am trying to do. However in the 240 VAC mode
they work as a more conventional powersupply rectifying the 240 and allowing
it to come to the peak of the AC which is 300VDC. Still havent got this all
traced out yet, but think the rectifier configuration for 240VAC use is a
bridge. If so, the polarity of the input should not matter.

"Wilson" wrote in message
s.com...
I agree with Spudley - its a dangerous thing to do. You are correct that
most offline switchers have this type of direct rectifier front end. While
this might work with a particular supply, you can't make assumptions about
it without detailed info on the supply you are using. Additionally, what
happens if the supply fails and someone replaces it with a different type
or
model? There are no guarantees on internal polarity, or current ratings of
the various input devices.

You should look carefully at the DC ratings of the input line filter
components, (both coils and caps). I bet the "Y" capacitors on the input
lines are going to have a tough time with 300 VDC. They are probably
designed to have no DC bias. Capacitor heating is a concern because its
going to be higher for 300 VDC than for 120 VAC. Constant DC means
constant
leakage, and this results in more internal heat. Add to that the effect of
higher voltage, and you could see capacitor failures or premature end of
life due to voltage stress.

Wilson

"Spudley" wrote in message
...
"Jimmy" wrote in message
m...
It is my understanding that PC powersupplies take 120 AC and run it
through
a voltage doubler rectifying it and giving 300vdc. What I want to do
is feed
300vdc into the powersupply via the line cord with no modification
to the
power supply. As I dont have a schematic could someone please tell
me if
this would work

Where do you get your 300 Volt Direct Current (vdc) supply from?
That's Direct Current, not Alternating Current (AC)
It sure as hell doesn't come out from a wall outlet, as mains supplied
power is AC not DC.

If it is VDC then it could be applied straight across the large Filter
Capacitors. Making sure you have the correct polarity applied.
Positive rail to Positive and Negative supply rail applied to
Negative, else smoke and fire or bodily damage may result.

Sounds like a dangerous project to me.