"Pete Verdon" d wrote in
message ...
Calvin Sambrook wrote:

Yes. My guess is that the 7812 is shutting down because it's too hot and
that's causing the relay to turn the fan off (which is what you'd expect)
and the SMPS has a minimum load so it shuts down too.

The relay is wired with the fan on common, 6v on NC and 24v on NO. So the
fan switches between the two speeds, but is never off.

How about re-designing the control stuff to run off 5V.

Not an option unfortunately - it's a pre-built kit rather than my own
design. Building it from scratch is probably not *quite* beyond me if done
with a microcontroller, but it's a lot more work than I'm prepared to do.

More, many more, details needed really - like some idea what the control
circuit looks like.

I think it's this one:
http://www.quasarelectronics.com/314...elay-timer.htm

Cheers,

Pete

Well that claims to take 55mA tops which, at 12V, is 0.6W. So your 7812,
which is also dropping 12V * 55mA, is dissipating 0.6W as heat. Your SMPS
is seeing an extra load of 1.2W

From your description of the way you've wired it I have to say it sounds
like you're overloading the SMPS, it must be really near the edge in normal
operation.
A quick test would be to power it from an external power supply and make
sure it all works OK.

It's probably not easy but if you could arrange your sensors so that the
timer was triggered when the low speed was required rather than the high
(and swap the 6V/24V at the relay obviously) then you'd only be adding 0.72W
when the fan was running at full power which is when it matters.