"Franc Zabkar" wrote

On Thu, 17 Feb 2005 21:35:49 +1030, Matthew Smith wrote:

I have an irrigation system using 24V Hunter solenoid valves
(hydraulic
assist). Solenoids have DC resistance of 24.5 Ohm when healthy.

The solenoids are controlled by a microcontroller driving BUZ11
mosfets;
+24V is fed to the solenoids, the mosfets ground a return wire to
turn
them on.

I am totally mystified as to why this pair of solenoids would both
suddenly go high resistance at the same time, leaving everything else
working fine.

The combined heat dissipation is about 47W. Could ventilation be an
issue, especially in an enclosed box on a hot day?

How do you come up with that? 24V across 24.5 Ohms is only ~1A,
therefore 25W. I still say the problem is using DC where AC is called
for, resulting in cooking the coil. Hunter valves use AC.