Calvin Sambrook wrote:

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.

Thanks. I guess I originally thought that a small box of electronics
would be trivial set against a great big fan motor - except that in
actual fact the fan isn't all that big.

I don't think I can reverse the logic as you suggested, so I guess I'm
going to need to power it separately. Any suggestions other than a small
wall-wart?

Pete