# How to protect reed switches in dual float switch pump control circuit?

I'm trying to make a dual-float switch control for a pump. I want the pump to come on when the water is at the high level, stay on until water falls to the low level, then turn off and stay off until water returns to the high level.

I am using the circuit described here with some modifications: https://electronics.stackexchange.com/a/74159/99764

The problem I am having is that I keep destroying the upper float switch. I thought adding the diode across the relay coil would prevent this, and the circuit worked twice, then the relay switch stuck closed again.

Sorry for the terrible diagram. Any help? Do I need to put RC protection on the float switches? If so, where? And how do I calculate the values of R and C?

• What loads are you switching with it and is it rated for those? Commented Feb 8, 2016 at 21:05
• This circuit doesn't work as you want to. It just turns on/of at hi level switch. You would need a NC switch for lo level and self sustaining contact for hi level switch. Commented Feb 8, 2016 at 21:26

simulate this circuit – Schematic created using CircuitLab

Figure 1. Reed-switch protection.

Your problem is that the Upper Float Switch is powering the pump until the relay reacts and energises.

Add D2 to prevent that while allowing the relay to hold itself up, once energised.

I assume that the lower float switch is of the normally closed type, as your circuit does not make sense otherwise. A normally closed switch would be drawn like this:

Reed switches are fairly delicate, since the contact area is tiny. Such switches are only rated for a few hundred milliamperes at most. Your circuit actually creates two paths for the current to flow, one through the relay and the other through the switch. This large current trough the reed switch quickly destroys it.

To fix this you can add a diode to the circuit which still allows the latching action but blocks the anomalous current.

simulate this circuit – Schematic created using CircuitLab

You might want to consider a double pole relay instead of the single pole. This would isolate the pump motor load from the switch seal in path.