11 Apr
2008
11 Apr
'08
9:31 a.m.
On 11 Apr 2008 at 2:53, Tony wrote
The right way to do it, of course, is to use an
isolating transformer, but
not everybody has one of those in the workshop.
This is something that anyone who owns a screwdriver and a soldering
iron should have if one intends to go poking around in the "guts" of
powered equipment.
Fortunately, it's easy for us in the US and Canada (and Japan) where
mains power is 120v. Just about anything with a transformer in the
PSU (still pretty common in audiophile gear, as are large
electrolytics) has been constructed with a "universal"
100/120/220/240 transformer, meaning that there's usually a split
primary, so one primary winding can be used as input and other, as
output. (the other windings can be left NC).
Those in 220/240 volt-land can use two transformers of the same
secondary voltage connected back-to-back as an isolation method.
Another option is to scavenge a transformer from a UPS--it's very
common that the same transformer is used to charge the batteries and
as an inverter output, meaning that there are often two sets of line-
voltage windings present.
I have a box with a large scavenged UPS transformer and a Variac for
my workbench--and its own 5 amp fuse. Not only do I get isolation,
but I can adjust the output voltage--and I don't have to rely on
tripping the 20A distribution panel breaker when something goes
wrong.
This is particularly important with some of the old hobbyist gear--
IIRC, the MITS Altair ran a couple of bare PCB traces at line voltage
to the front-panel power switch.
Those with big iron with high-current line-side requirements are
advised to use the "one hand in a rear pocket" technique when poking
around in a PSU.
Cheers,
Chuck