24 Dec
2010
24 Dec
'10
10:39 p.m.
I know it's obvious but a Variac is AC out.
It's capable of 2.2KVA! You
would be relying on the old PSU's transformer and rectifier circuits to
generate the reform voltage.
DC from a bench PSU across an isolated capacitor via a limiting resistor =
is
probably a better option.=20
I would agree. While I have a Variac and find it useful for testing PSUs
at high/low input voltage and things like that, I don't find it much use
for repairing linear or switch-mode PSUs.
As many switchmode PSU's are automatic dual voltage. Usually 110v to =
240v
They might be OK to run from the Variac. Otherwise probably not.=20
Not always!.
Older SMPUs geenrally had a manual voltage selecrtor switch. Often the
input circuit was configured as a bridge rectifier for 230V mains
charging a pair of smoothing capacitors in series (with suitable
bleeder/balanace resistors, which often also provided the startup
supply), and as a votlage doubler fro 115V mains. In either case you got
abot 350V DC across the capacitors.
A lot, if not all of the automatic voltage-seelcting supplies do much the
same thing with an automatic circuit in place of the selector swtich.
They will not nevessarily work correctly fore any input voltage from
(say) 100V to 260V. I was readign a specification for such a supply the
other day which speicifically stated that for input voltages in the range
140V to 190V (or something), the PSU would not operate, but it would not
be damaged. I beleive some PSUs are acutally damaged under such
conditions. Certainly in either case running it up on a vVriac will do
nothing useful.
-tony