I should have stated right off, actually there are 2 reed switch float=20
units in series, and only one welded the contacts.

The 2 opto's are parallel. The one Turns on/off the pump's ground=20
connection. There are 2 possible power sources for the pump: 1: The=20
manual on/off switch that normally is not used, and hopefully you never=20
forget to turn it off, but are present in the immediate area. 2: The 2nd=20
Opto is an input to the PLC that can automatically turn the pump on/off=20
based on a variable resistance with float automotive fuel gage sending=20
unit. The PLC is programmed to turn on/off pump power well below the=20
overflow condition and the reed switch level.

The automatic fill should turn off based on the level, the backup to=20
that is the reed switch. Agree one needs to have sufficient backup, just=20
how much is a debatable. The current situation has been no problem for 7=20
years, and I   only discovered during the twice a year servicing inspection=
..

I received this morning 2 replacement units, just was a question whether=20
to mount them NC or NO.

Will have to think a bit about it, possibly the PLC could check the reed=20
switch for continuity (either open or closed) when it turns the pump on,=20
and E-mail my cell phone if there is an issue.
On 10/16/13 9:47 AM, Isaac Marino Bavaresco wrote:
> I would use at least two independent redundant circuits for such a
> critical application.
> It is known from residential alarm experience that a common failure mode
> for reed switches is to fail closed.
>
> Perhaps a safe approach is to use one in N.C. and another in N.O.
> configuration "ANDed" together, so if any of them says "stop" the pump
> stops.
> Another solution would be to use a different type of sensor for backup.
>
>
> Isaac
>
>
> Em 16/10/2013 10:22, Carl Denk escreveu:
>> For my standby generator 16 gallon gasoline tank, there is a nearby 175
>> gallon gasoline tank with a 12 VDC transfer pump to keep the 16 gallon
>> full. The pump ground has a 12VDC 1" black cube automotive type SPDT
>> relay that is using the NC contact that opens when the relay is
>> energized. The relay coil is energized with a IRF9530 P-Channel Power
>> MOSFET, which is activated with a H11B1 opto-isolator. The reed switch
>> is a float switch  to ground, NC at normal fuel levels, and NO at high
>> level activates the opto-isolator. There is a suppressor diode across
>> the opto-isolator input and dropping resistor from the 12 VDC power.
>> There is also one section of a PS2501-4 opto-isolator driven by the
>> switch that goes to a PLC that turns on the pump (there is also a manual
>> pump switch).
>>
>> Recently the reed contacts welded together causing a possible unsafe
>> condition of overflowing gasoline. I have new switches ordered, the
>> question is, is the current configuration of the usual alarm circuit,
>> normally powered, and a fault (could be a broken wire) open is detected
>> and shuts thing down. But with welded contacts that's bad. :( It
>> wouldn't be hard to make the reed switch NO, change the relay contact
>> used to NO, and program the PLC for inverted action to make the circuit
>> 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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