On 24 Oct, 17:07, Pete Verdon
d 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...htm#Voltage-Re...
). 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

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. Add a bigger T0 220 heatsink, or bit of bent alloy or
insulating wahers to metal case.

Cheers
Adam