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