Help with ICL power supply fault (Farnell SMPS) - test-drb@ccmp.vtda.org

11 Apr 2008

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