"Pete Verdon" d wrote in
message ...
Adam Aglionby wrote:

L7812 is an amp in a bucket of water, they run hot , its a linear
regulator which means its having to dump 12V from your 24V as heat,
being a clever integrated regulator its probably entering thermal shut
off land.

That's what I first assumed. But it's not the regulator that's shutting
down, it's the boxed PSU for the fan.

Is there something the regulator might do to the supply when thermal
shutdown happens, causing an upstream device to get upset?


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.

How about re-designing the control stuff to run off 5V. Power it from the
6V but don't use a regulator as there's probably not enough headroom but,
having worked out how much current your control stuff takes and assuming
it's not too great, consider a simple resistor and zener arrangement.
Or do the control at 24V.
More, many more, details needed really - like some idea what the control
circuit looks like.