multi-level switch interface with AB compact logic

[wbug2000 0]

Hi, I want to use a multi-level switch in a water tank to control a feed pump (on/off). Each reed has a power rating of 15w. How do I interface the switch to an AB plc? Do I need additional electronics? I am thinking about a 3 level switch - the top reed for hi alarm, middle reed for pump control, and bottom for lo alarm. Is this okay or should I go with a continuous level sensor? Thank you very much.

[TConnolly 13]

The swtich loading of either 120AC or 24VDC inputs is only a few milliamps, so 15W reeds won't be a problem. Just wire them directly into your discrete inputs. If you are using 120AC make sure the switch is voltage rated for it. Whether you need three our four control points and whether a continuous sensor is required is a matter of your application. If your pump will be cycling off and on a lot you may want to consider using four points, one for low alarm, low level turn pump on, high level turn pump off, and high alarm to control the tank level between two points instead of trying to hold it at a single point. This builds some hysterisis into the system so your pump is not cycling quite so much. But it still depends on the application, if the tank level does not change much over time then a single point will be just fine. Even if you do use a single point then you should debounce the swtich, ie, it should be on for a few seconds before the pump is activated, and then the pump should run for several seconds after the level reaches the switch, this keeps tank turbulence from causing your pump to switch on/off rapidly. If your application demands that you hold the tank right at a specific level then you'll probably need a continuous sensor and a VFD drive for the pump. Edited 29 May 2008 by Alaric

[wbug2000 0]

Thanks Alaric. I will go for it.