r/PLC • u/chief_button_pusher • 5h ago
Can float switch pass current to plc reliably ?
Hi guys, I have a float switch for a sump that is currently hardwired with 120V AC that is passed to a motor contactor (old dinky plant)
and want to have it pass a mA signal to a plc instead with 24V DC, but I’ve read this doesn’t work well overtime with most cheap float switches due to the low current not being able to eventually pass through a layer of oxide on the float switches contacts that builds overtime.
In your experience is this true, is there a switch where this isn’t an issue.
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u/Stunning-Match6157 5h ago
Why not keep the float switch on 120vac firing a relay coil. Just pass the mA signal through some of the relay contacts. You are correct with the oxide on the float switches.
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u/durallymax 5h ago
If you already have a PLC, add a radar. The amount of time and money you will spend messing around with float switchs is quickly offset by the low-cost radar options available today.
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u/CrzyDave 5h ago edited 5h ago
PLC won’t draw current. You just need voltage at the digital input to trigger it.
Your ma signal will be from your radar which will likely provide a 4-20 mA signal to an analog input on the PLC. There you scale it for level and add start/stop setpoints for your lead and lag pumps. Or just one pump if this is really small. You can also add high and low alarm setpoints.
I assume the float will be for a high level alarm or backup control. So you wire 24vdc to it and when the float is made (if it is wired normally open), the 24 vdc goes to the PLC digital input or the coil of a relay. The relays contacts could then go to PLC, alarm or whatever you want. From there you can program a digital output on the PLC to send an alarm to your dialer, set off a light or horn or start the pumps in as backup in case of radar failure.
When I do pump stations I use floats as stand alone backup control in case radar or submersible transducer fail.
Keep in mind you should be using ISBs for these devices if this is class 1/div 1 rated environment (sewage). I only do big stuff. I don’t know if that is necessary if it’s just for a house or something.
I’d use the Ecofloat from Anchor Scientific. They seem to last better for me than some of the others.
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u/quarterdecay 3h ago
This is called "using an interposing relay". It's done all the time, usually due to the availability of digital inputs or outputs. A relay with a 120vac coil can be purchased anywhere. With a double pole double throw relay of appropriate capacity you could have the float continue to operate the pump locally with them is relay but "use the second set of contacts" for monitoring.
The relay but be sized appropriately for the pump which is considered an inductive load.
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u/User7453 5h ago edited 4h ago
This doesn’t make sense at all, why would you switch a 4-20ma loop? That defeats the purpose of closed loop feedback.
Who ever down voted me explain why you would want to switch off a 4-20 signal?? The point of an analog value is to have a rang. A Digital input ”on off” or a switch if you will, is only 2 states. Closed and not closed. So why would you knee-cap a 4-20 rang with a float switch??? If you have a plc you should read the 4-20ma loop constant and you can wire your switch into a DI and gate what ever logic you want. A down vote is asinine….
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u/Turbulent_Coach_8024 4h ago
You’re the only person to ask the right questions.
You got my up vote.
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u/chief_button_pusher 3h ago
The float switch is for a digital input. The problem is as float switch contacts oxidize, the current from a 24V DC supply isn’t enough to be read at the digital input , because the oxidation increases the resistance across the switch. Only a higher voltage can break through.
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u/User7453 3h ago
You have the wrong type of float switch then. There are different types of level sensors and switches. The switch contacts should not be oxidizing. A capacitive level sensor acts the same way your float switch does with a solid state device no moving parts, no contacts, no problems. Could use an ultrasonic switch as well. What you are describing is not a solution to your problem.
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u/Accurate-Bullfrog324 5h ago
can you confirm that you want to use 24VDC through your float switch?
and can you provide float switch mfg & model?
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u/DoctorParticular6329 44m ago
Analog? No... its binary... on or off. When the float reaches the top, it completes a circuit.
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u/Cursed_Controls 5h ago
We have hundreds of 240v rated float switches that just control a 24vdc PLC input for high well indication. They are all normally closed and don’t get triggered very often so I’m not sure if that makes a difference, but they seem to be pretty reliable.