Pete Verdon wrote:
I have a Vent-Axia HR25 fan ( http://www.vent-axia.com/range/hr25.html
). It has a separate switch-mode power supply with both 6v and 24v
outputs (for the two fan speeds). Rather than use the pull-cord switch,
I've put together an electronic timer/relay module with inputs from a
couple of sensors to switch between the speeds as required.

The control module requires 12v. Since I don't want to have another
supply stepping down mains, I've arranged it to be powered from the
fan's supply, via a voltage regulator chip (item L7812CV on this page:
http://www.quasarelectronics.com/ele...age-Regulators
). This is fed from the 24v supply.

When I built and tested it, this seemed to work fine. However, now that
the fan is in place and I'm trying the system out for a bit longer
before finally mounting the electronics, I have found a problem. After
some period of time (between ten minutes and half an hour - I haven't
pinned it down exactly) the fan stops. At first I thought it was a
problem with my electronics, but it turns out to be the fan power supply
cutting out (no voltage across its outputs). After being turned off for
a while, it will work again, until it once more cuts out.

Does anyone have any advice on how to sort this? I am no electronicist;
all the non-standard stuff here was put together from kits and a basic
reading of component datasheets.

Pete

I'd be suspecting your timer/relay module is taking just a bit too
much power from the 24V rail and the SMPS is falling over after a
while because it's getting to hot.
The fan needs 1Amp at 24V at it's "Boost" setting. 'Boost' usually
implies only intermittent operation of the fan at this max power,
which means the SMPS can be built cheaper, down to maybe a tiny '6W'
continuous rating.
What I'm getting at, is your control gear may be taking a couple of
watts and the 7812 dropper is losing maybe a few watts. Add on a fan
2W load and you could be over the top for the SMPS static power
consumption.