Problems with float switch circuit

Thread Starter

RampagingWombat

Joined Jul 28, 2021
4
Hi Folks,

I have an incredibly simple circuit that is regularly giving me problems.

I have a small irrigation system, at the base of this is a small sump which needs periodically emptying into the main water holding tanks.

To do this I have installed a small 12v pump, switched by a float switch. The circuit is extremely simple (see diagram attached) comprising a 12v supply, connected to a float switch (NO) then feed to 12v pump. When the tank fills the switch changes to NC and the pump activates, until the water level drops and the switch returns to the NO position.

I installed a Cynergy RSF40 https://uk.rs-online.com/web/p/float-switches/0339730/ float switch This worked initially, but then I found that it would not turn off once the initial contact was made. I tried replacing the unit with the same model, but found that after a short period of time the same thing would happen (in all cases the valve is free from obstructions within the sump).

Testing with a continuity tester, found that both appeared to be working fine.

I then tried an alternative, a top mounted float switch RFS50 https://uk.rs-online.com/web/p/float-switches/0845960/ and this seemed to work, but I have just found that the pump is still running even when the tank is dry (and the switch should be off).

I wish to retain the low voltage (12v) system as it is an area that is potentially exposed to the public and wildlife.

Testing the circuit without the float switch finds the system runs correctly.

Any thoughts on why I might be having this recurring problem? Is the switch arcing across somehow?

Thanks for any tips or advice

Mark
 

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Yaakov

Joined Jan 27, 2019
3,579
So, you are using a switch which has a very low current rating:

1627511821161.png

You are almost certainly drawing more current than it can manage, and it is getting welded on. You need to use a relay. Have the float switch operate the relay (contactor) and the contactor operate the motor.
 

Thread Starter

RampagingWombat

Joined Jul 28, 2021
4
Thank you, i did wonder if it might arcing and causing the current to stay on.

Have a relay so can try putting that in the system to break the supply. Thanks M.

So, you are using a switch which has a very low current rating:

View attachment 244663

You are almost certainly drawing more current than it can manage, and it is getting welded on. You need to use a relay. Have the float switch operate the relay (contactor) and the contactor operate the motor.
 

Alec_t

Joined Sep 17, 2013
12,249
The switch ratings are for a resistive load only. A relay is an inductive load. You may get away with it, but it's not ideal. Use a snubber across the relay coil.
 

Yaakov

Joined Jan 27, 2019
3,579
The switch ratings are for a resistive load only. A relay is an inductive load. You may get away with it, but it's not ideal. Use a snubber across the relay coil.
I really should have mentioned that. I do think with a snubber it will work but it surely won't handle a pump.
 

Yaakov

Joined Jan 27, 2019
3,579
Hi Yaakov,

Thank you for your advice

Would you be able to recommend a suitable snubber for this particular circuit please

Thanks again

M
Actually calculating a snubber for your circuit requires more information. The purpose of an RC snubber is to absorb the energy that would cause an arc when the circuit breaks. This means you need to know the inductance of the relay coil not one of the things we know (at this point).

There is a lot of information on snubber design online, maybe you could look at that and see what you do and do not understand, and we could go from there. You want an RC snubber, which goes across the switch.
 

Thread Starter

RampagingWombat

Joined Jul 28, 2021
4
Thank you, will go off and have a read. M

Actually calculating a snubber for your circuit requires more information. The purpose of an RC snubber is to absorb the energy that would cause an arc when the circuit breaks. This means you need to know the inductance of the relay coil not one of the things we know (at this point).

There is a lot of information on snubber design online, maybe you could look at that and see what you do and do not understand, and we could go from there. You want an RC snubber, which goes across the switch.
 

Tonyr1084

Joined Sep 24, 2015
6,352
Just came across this thread. The 12 volt motor - I'm assuming it's an AC motor??? If not - there must be more to your circuit than you're showing us. Rectification and maybe capacitance involved as well? Just guessing here, but I think we need more information to better understand and solve your problem. Or - that is - to help YOU solve your problem.
 
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