Hi All

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. SMD equivalent to 1N4007 from mosfet drain to +24V to protect
from reverse EMF.

There are two sets of solenoids, both fed by standard irrigation cable.

I find that two of the solenoids - one set, both in the same valve box -
have suddenly gone high resistance. The four solenoids on the other
cable are still fine. There is no problem with the controller - still
delivering 24V down both lines quite happily.

The 24V power supply is actually derived from a large dot matrix printer
and holds a steady 23.79V.

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.

Ideas anyone?

Cheers

Matthew Smith
South Australia
http://www.mss.cx