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I'd be grateful for some advice about protecting the reed switches in this pump-down setup I am installing. enter image description here

I naïvely thought that the relay alone, which is rated as drawing only 5.3mA, would be adequate to protect the reed switch contacts from welding shut. I now know that this is not the case and that some sort of additional protection is required.

Research led to this post that recommended using a TVS (specifically a P6KE300CA) across the coil. However, other posts put snubber circuitry across the switches themselves and I'm not confident about what configuration will produce the most reliable results. Advice and suggestions would be welcome.

To give some more details of the application, it is a condensate pump for a gas-fired ducted heating system. The unit was installed with a pump used for split air conditioners. With a very small reservoir, it ran almost continually and was been very unreliable as, I believe, is often the case with these things.

There is plenty of space for a bigger tank to store the litre or two of water produced in day, so I sought to design a latching circuit that would fully pump out the tank only when full and operate just once or twice a day. The float switches used are on Amazon and were sold as having a contact rating of 1A at 220V AC so I didn't think powering a relay would be a problem.

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  • \$\begingroup\$ What is the purpose of the lower float switch? it does not look to me like it does anything, other than potentially cause the current to the pump to flow through the switches in the case that both the upper and lower float switch would be activated, is this intentional? and if so why?. Some more details on the application would be helpful. \$\endgroup\$
    – user173292
    Jul 11, 2020 at 7:44
  • \$\begingroup\$ Do the reed relays in the float switches have a specification? I doubt that any ordinary reed switch will withstand 240 VAC switching, let alone into an inductive load like a relay coil. Only if they are rated for that voltage will a snubber or TVS work to keep it to something like that voltage, hopefully no more than 2x. Might be better to go slightly more sophisticated and drive a triac with the float switch rather than the relay directly, much much lower voltage on the reed contacts. \$\endgroup\$
    – Neil_UK
    Jul 11, 2020 at 8:06
  • \$\begingroup\$ have you chosen a reed switch that is suited to switchiong 240V ? \$\endgroup\$ Jul 11, 2020 at 9:25
  • \$\begingroup\$ Note that when you use the CircuitLab button on the editor toolbar and "Save and Insert" on the editor an editable schematic is saved in your post. That makes it easy for us to copy and edit in our answers. You don't need a CircuitLab account, no screengrabs, no image uploads, no background grid. Unfortunately you haven't done this so we can't copy and edit your schematic into our answers. \$\endgroup\$
    – Transistor
    Jul 11, 2020 at 10:11
  • \$\begingroup\$ Thank you for your responses, and apologies for not using CircuitLab which I can see is a much better way to present circuits. You have certainly demonstrated the flaws in my thinking about the problem. \$\endgroup\$
    – Andrew
    Jul 12, 2020 at 8:12

3 Answers 3

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enter image description here

Figure 1. The original circuit. When both switches turn on the pump is powered directly through the reed switches.

As @vu2nan has pointed out the likely problem is switch on rather than switch off.

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 2. The solution is to separate the control circuit from the "power" circuit. For this you need a 2-pole relay.

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  • \$\begingroup\$ That's indeed the right circuit for automatic pumping down! \$\endgroup\$
    – vu2nan
    Jul 11, 2020 at 12:17
  • \$\begingroup\$ However, the original circuit fails safe: the pump will always switch on even if the relay fails. \$\endgroup\$
    – P2000
    Jul 12, 2020 at 2:28
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There is an error in the schematic that requires correction. The lower and upper float switches, being simultaneously on, would carry the high pump motor current till the relay switches on, and get damaged.

In an AC circuit, reed switch protection could be a resistor and capacitor (in series) wired across it. It could also be a VDR across it or across the relay coil.

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  • \$\begingroup\$ This looks like a great solution to me - the relay I have is a 2 pole model, so I can even use it. (I couldn't find any 2 throw float switches to implement @Jasen's solution.) The remaining question I have is whether float switches like ones I used really cope with 240V or whether that's a sales fantasy. If I do use them, should I put any circuitry across them or the relay for protection? \$\endgroup\$
    – Andrew
    Jul 12, 2020 at 8:11
  • \$\begingroup\$ Hi Andrew, Yes, Transistor's circuit is the right one and you should use that. To protect the reed switches from switching transients, a 0.1μF 600V ~ metallized polyester film capacitor, in series with a 100 Ω 0.5W resistor, may be connected across each reed switch or across the relay coil. \$\endgroup\$
    – vu2nan
    Jul 12, 2020 at 10:26
  • \$\begingroup\$ Thanks for the additional information. Given the current situation it may be nearly Spring before I can get new float switches but I'm looking forward to trying it out. \$\endgroup\$
    – Andrew
    Jul 13, 2020 at 8:21
  • \$\begingroup\$ Anytime, Andrew! \$\endgroup\$
    – vu2nan
    Jul 13, 2020 at 12:44
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As others have said trhe problem is that the reed switches are passing the motor start current. if you can use a dual-throw reed switch in the upper position this can be prevented.

enter image description here

AC relays are slow enough that the interruption while the upper switch changes will not cause the relay to disconnect.

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