John Stumbles wrote:
On Wed, 06 Jan 2010 16:54:59 +0000, Ian Bell wrote:

Pretty much but I was trying to avoid having a bleed resistor
permanently across the supply. The PSU has a lot of stored charge and a
bleed resistor that takes only a nominal 1% of the load current would
take about 5 minutes to discharge it. That in itself is not a problem as
the PSU is in an enclosure but when I am testing it, it is a pain to
have to wait that long.

You seem to be changing the design criteria. As I understood it the design
brief (if you like) was that the connections on the outside of the box had
to assume safe potentials in the time it took a British Standard Idiot to
unplug the connector from the power amp and plug his finger in there
instead. Now you seem to be saying you want the potentials within the
PSU box to drop to safe levels in some arbitrarily short time so you can
stick your finger on conductors inside the PSU box without harm. That's a
different problem. I think what I was proposing addresses the original
problem, but if you want to make it safe for yourself to prod around in
the PSU arbitrarily I suspect that's going to be a non-trivial problem to
solve.


I have not changed the brief. Originally it was simply to delay the
switch on of HT until the heaters had warmed up; that and nothing more.
It then occurred to me that when the relay dropped out a contact could
be used to discharge the PSU caps more rapidly than a bleed resistor. It
also occurred to me that I could include the relay coil in a loop in the
dc power cable so if it were disconnected the HT would be disconnected too.

The delay of the HT until heaters warm up is the requirement. The other
'features' came out as possibilities in using a relay to meet the
requirement.

Hope that clarifies things.

Cheers

ian