Hi All -

Mech E with very limited EE background here. I have an application where I have a tank with 2 level switches - a high limit and a low limit. When the high level switch is energized my tank is full and the pump should be off. As the level falls, nothing should happen until the level falls below the low level switch. At this point, the pump should engage and fill the tank up to the high level switch. Then the pump should turn off. How can I achieve this without using any sort of micro-controller? Every time I draw out any circuit using relays or logic gates, I keep getting single level control as opposed to dual level control. I realize that this is probably a simple problem but I would appreciate any help that I can get.

Please let me know if anything is not clear. Thanks!

-Steris

Find a circuit for a pushbutton motor starter on Google. Then substitute one level switch for the start button, and one for the stop button. I'll let you figure out which is which, just in case this college homework.

http://ecatalog.squared.com/pubs/Motor%20Control/0...
figure 2, page 6.

You're asking for hysteresis, which requires memory. Therefore, whether logic gates or relays, there must be at least one latch in the circuit.

TTFN
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Hi - Thanks for the help. I was afraid that this was so simple that it could be confused with college homework :-/ I had originally played with latches but never got it to work completely right. Ideally I want an off the shelf component that I can mount into my electrical panel. Anyway, it appears to me that the motor control circuits are probably my best best. The "magic" part is the holding circuit. I tried searching for "holding circuit" and "holding contactor" on Newark and Digikey without much luck. Are these components ever sold separately from complete motor starters? Should I be searching for a different term?

Thanks for all the help!!

-Steris

Straightforward circuit, you do not need "memory". I would suggest a safety circuit too (not shown) to keep it from overfilling if something sticks!

http://files.engineering.com/getfile.aspx?folder=2...

when the tank empties, S2 closes and energizes the relay. The top relay contacts keep the relay energized for a while. The bottom relay contact turns on the pump motor.

When the tank is full, S1 opens up and the relay opens up, turning off the motor.

If you are talking about big motors, you may need some snubber circuits to protect the relay contacts from sparking.

www.MaguffinMicrowave.com

Maguffin Microwave wireless design consulting

You need pump-up operation



A single dual pole relay where one set of contacts is used for latching action is needed for pump-up action.

As others noted, pump-up action requires a latching action to keep the pump on after the low point switch trips as the level rises above it.

The sketch below shows pump-up control with 2 level switches and a double pole relay. The level switch contacts are closed below the trip setpoint and open when the level rises above the switch trip point. The relay contacts need to be capable of handling the pump load.

This action is called "pump up" and it requires latching action to keep the pump on when the low level trips as the level rises above its trip point.

At 'empty' (below low level), both LS1 and LS2 are 'made' or closed, energizing the relay coil K so the pump turns on. At the same time, the latching contact K is made. As the level rises above the lower level switch point, LS 1 opens, but the motor continues to run because the latching contact K holds the relay coil in.

As the level rises, eventually LS2 trips and opens, dropping out power to the relay coil, stopping the pump motor and opening the latching contact K. At that moment LS1 is open, LS2 is open and the latching contact K is open.

The level doesn't have to fall too far (deadband or hysteresis) before LS2 makes, but the relay's coil stays unenergized until the level falls below LS1, at which time LS1 makes and energizes the relay coil and the cycle repeats.

Sounds like these are what you're looking for:
http://www.liquidlevel.com/pdf/R-DLC-120-230-24.pd...
http://www.ssac.com/catalog/LLC51A01.pdf
http://www.amazon.com/ABB-Liquid-Control-Series-LL...
http://www.capablecontrols.com/liquid.htm

They all seem to be cheap enough that you ought to buy instead of build.

TTFN
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The holding contact, also known as a sealing contact, auxiliary contact or latching contact is a standard part of a magnetic motor starter.
If you are using a standard magnetic motor starter, there is no need for more relays. Go back and re-read ScottyUK's post

Bill
--------------------
"Why not the best?"
Jimmy Carter

Hi All,

Thanks for all of the info! As I was poking around all of the information, I got to thinking about using a latching relay. I was thinking about something like this: http://www.automationdirect.com/static/specs/755re...

My thought was to attach the Low Level to the Latch input and the High Level to the Reset. The pump I'm driving is very small and draws a max of 1 amp under full load so I think it should be fine to drive it off of the relay. Because of the low power, I didn't plan on snubbing it. I'm not worried about bounce because of the latching. Does this make sense? At the very least it will be very cheap to test.

Thanks again!
-Steris

You could make it work, but a standard relay with a spare normally open contact will be adequate. If you use it in the motor starter circuit as I initially suggested then you can use a relay rather than a contactor. The circuit works the same way whether it is switching a big motor or a small one, other than the size of components involved.

Why are you fighting the starter suggestion? The two sensors just need to have a contact that is closed when the water is below the sensor. Replace the start push button with the lower sensor and the stop push button with the upper sensor. Piece of cake.