Float switch problem

On 22 Jun 2003 00:23:59 -0700, Andre Malenfant wrote:
> I have a problem installing a float switch in my sump: When the level
> goes too low, the pump stops (till here that's right) but then the
> water continues to return to the sump down to the overflow level. This
> extra water in the sump starts the pump again. The pump oscillates
> like that till not enough water is left.
>
> Someone must have had this problem before. Any thoughts?

Yopu could wire in an NVR switch (commercial or home-made). It's a
switch that if the power supply to the switch goes off, it switches
off, and if the power comes back on, the switch is off, so nothing
happens. They're used a lot in machinery, so that if there's a power
cut (or a trip flips, or fuse blows or whatever) the (presumably
dangerous) machine isn't accidentally left switched on, so it doesn't
start up suddenly when the power returns.

It's relatively easy to do with a double pole relay - one pole
supplies whatever it is (in this case your pump), the other pole
supplies the coil to the relay. You put a momentary on press-button
switch in parallel with the 'other' pole. When you press the button,
that energises the coil, and the relay switches, supplying power to
the pump. You release teh switch but now the 'other' pole is switched
and supplying power to teh coil, so teh relay stays on. If the power
goes off, however, the relay opens and then stays open 'till you hit
the button again.

Of course, this means that a momentary glitch in the power cuts the
supply to the pump, and your tank circulation stops. I'm not sure
I'd want to risk that, but it depends on the particular situation you
have.

The alternative is to build some sort of hysteresis with two floats at
different levels and some slightly clever control.

regards, Ian SMith
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