On Thu, 12 Feb 2004 18:22:23 +0000, Woody wrote:


"db" wrote in message
...
I am anxious not to burn out the relay that switches power to other
equipment by loading it with a valve power amp. Can anyone recommend the
source of a mains contactor that will switch on
and
off a few amperes whilst drawing only minimal current? UK based so 230v
working required.




Look in the surplus market and get a solid-state relay. They usually work
of 6-48Vdc and will switch 30A or more. They use triacs as the active
component and switch at the xero crossing of the mains so they are safe to
use to feed a transformer.

There is a problem with SSRs. They do not actually isolate the current, so
you still have to treat the output from them as "live" even when they are
switched off. They give a false sense of security. An ordinary "plug-in" 8
or 11 pin 10A rated relay would probably be fine in this case as many
modern designs of that rating have the required 3mm contact gap to provide
isolation. Don't worry about zero-voltage switching as this is irrelevent
for simple switching of a load like that. The ordinary power switch
doesn't use it! Use a suitably rated fuse (probably 6A max) of course, not
a 13A as this is beyond the current carrying capabilities of the relay!
Use 2 poles of the relay and switch neutral as well. That way you can
always just unplug the relay for true double-pole isolation.

--
Mick
http://www.nascom.info for Nascom & Gemini information
Also at http://www.mixtel.co.uk where the collection started.
Currently deserting M$ for linux... :-)