For my standby generator 16 gallon gasoline tank, there is a nearby 175=20
gallon gasoline tank with a 12 VDC transfer pump to keep the 16 gallon=20
full. The pump ground has a 12VDC 1" black cube automotive type SPDT=20
relay that is using the NC contact that opens when the relay is=20
energized. The relay coil is energized with a IRF9530 P-Channel Power=20
MOSFET, which is activated with a H11B1 opto-isolator. The reed switch=20
is a float switch  to ground, NC at normal fuel levels, and NO at high=20
level activates the opto-isolator. There is a suppressor diode across=20
the opto-isolator input and dropping resistor from the 12 VDC power.=20
There is also one section of a PS2501-4 opto-isolator driven by the=20
switch that goes to a PLC that turns on the pump (there is also a manual=20
pump switch).

Recently the reed contacts welded together causing a possible unsafe=20
condition of overflowing gasoline. I have new switches ordered, the=20
question is, is the current configuration of the usual alarm circuit,=20
normally powered, and a fault (could be a broken wire) open is detected=20
and shuts thing down. But with welded contacts that's bad. :( It=20
wouldn't be hard to make the reed switch NO, change the relay contact=20
used to NO, and program the PLC for inverted action to make the circuit=20
powered is alarm. Think it's about which is better scenario.

Oh the system is 7 years old and working OK until now.

Comments appreciated. :)
--=20
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