Level Control - Tank with Two Level Switches

I think Wikipedia has a good section on PLCs and logic diagrams.

I personally find squared paper and a pencil is the best start point.

Del

This can be simply done by an analogue circuit, But it seams you want a digital one,

Just convert signals from S1 and S2 in digital form by a snap trigger and feed it to an "AND" gate that can control an SPDT electromagnetic relay, connect your valve with the relay in right manner,

Sorry I don't have luxury of making Logic diagram on my home PC.as its national holiday today.

I'm going to try some ASCII art here, so I hope this works for you.

----S1-----S2-------------(R1)---

|---R1--|

Ugly, I know. S1 closes on a falling level. S2 closes on a falling level. R1 uses two normally open contacts. One is used to seal in the R1 coil when S1 closes. The other will open the valve.

When the tank is below S1, R1 is energized. The R1 contacts will close. This will open the valve and will also form a path around S1 for when the liquid level rises above S1. The valve will stay open until the level rises above S2. This will deenergize R1 and close the valve. This will also unseal the path around S1. The valve will stay closed until the level drops below S1.

RAJ KUMAR, Not sure if this is what you are looking for but here goes anyway:

In addition you should check for the condition:

S1 On and S2 Off. If this case arises then something is wrong with your switches so you need to do something.

I would also be inclined to have a max duration that the valve can be opened at any time, for example if it takes 3minutes to fill the tank normally (whatever normally is) then the valve should stay open for a max of say 3minutes 30sec's to prevent overfilling of the tank.

Kind Regards

Mr. W.A Snow

Hi Raj

Use this circuit diagram to start and stop the starter of your motor.

Hope I am able to clear your requirement. Do revert with your views.

regards,

Ashish

Dear Ashish

Problem is that doing this will cause the level to fluctuate at high level only i want to start the motor when level is below lower switch.

May be i need a motor contact to bypass lower switch when motor is start.

Dear Raj,

The circuit I had provided will start the motor at low level and stop the motor at high level. Do look into the circuit. Hope it will resolve all your problems.

regards,

ashish

This circuit can be wired easily without the use of a PLC. S1 should use Normally Open (N.O.) contacts and S2 should use Normally Closed contacts (N.C.) then all you have to do is wire the two switches in series with the hot wire for the valve.

Good Luck!

Not sure that would work, wouldn't the flow stop when the level reached S1? thus the tank would never fill?

You missed the latch!

The other circuits are correct though.

Sorry for the careless mistake.

Try this instead:

Using the names normally open and normally closed when dealing with level switches is ambiguous. Do they close on rise, or do they close on falling level? If when the tank is empty, you are saying that S1 is open, then what opens the valve? S1 will need to close on a falling level so that the valve can open. If this is the case, then as soon as the water level rises above S1 it will open and the valve will close. The tank will not rise above the level of S1.

Picture it as a 3 wire motor starter circuit. S1 would be the Start button and S2 would be the Stop button. You need a seal in contact around S1 to maintain power to the valve until S2 is reached and the circuit will open. The circuit will remain off until the level drops below both S1 and S2.

There is no PLC required, however you do need one 2-pole relay.

The terms N.O. and N.C. should always refer to the condition that the contact would take un-operated. As water operates the switches, although the lower switch is usually seen open it is nonetheless a N.C. switch, since it is the absence of water that rather than the water that it is detecting.

If, for clarity, a diagram wishes to show a N.C. switch in its usual position, as is the case with the lower switch, then the diagram should be noted with the condition in which it is depicted.

Kind regards

Chas

Why not use continuous level device? With 4-20mA signal for inlet valve control.

Hello Rajkumar above is the circuit schematic and the logic diagram, I believe it should work for you,

Regards

The logic diagrahm are u referring to a PLC LADDER DIAGRAHM or what

Can it be drawn using AND/OR gates?

This Boolean gate diagram is equivalent to the ladder diagram drawn in comment #12.

The diagrams are drawn assuming that the switches pass current when not submerged, so the circuit will be true when the tank is completely empty. If your switches function the opposite way (only pass current when submerged) then 'NOT gates' can be added in line with the switches to make the circuit work the same way. In either case a control relay is needed.

Sequence of Operation:

1. Assuming the switches pass current when not submerged, the switchs S1 and S2 provide a path to the outputs when no water is in the tank. This energizes V1 allowing fluid into the tank and energizes R1 for circuit control.
2. Fluid in the tank fills to the level of S1, then S1 stops passing current. (The circuit output is still true because the contact R1-1 still satisfies the OR gate.)
3. Fluid continues to fill the tank untill level reaches S2, then S2 stops passing current, and the AND gate is no longer satisfied. This de-energizes the outputs.
4. When R1 is de-energized the contact R1-1 is no longer satisfied so the OR gate will not be satisfied again untill the fluid level drops below the level of S1, and the cycle repeats itself.

Sorry eelectrician but that circuit is abit cock eyed! Who would put a relay in that circuit?

If you have the logic then you would use a simple latch, Lower switch sets the latch and gives an output and the upper switch resets it, clearing the output. The relay, now it is spare, can be used to drive the valve motor as the logic is unlikely to have the neccesary power.

regards

chas

In the original question Raj Kumar lists the items that he has to work with - two level switches one at bottom(S1) and other at top(S2). such that when level is below S1 valve opens. Since Raj made no mention of a circuit board, so the most likely assumption is that he is doing this with hard-wiring.

Raj also states that he/she is a novice, and based on his statement, "Can it be drawn using AND/OR gates?" I don't think that he can read ladder logic, so I redrew the circuit using logic gates for his/her convenience.

But your statement is correct. If for some reason Ray chooses to do the logic with a circuit board (which would require him to purchase a circuit board, chips, a power supply, and a relay), then he would have to use the relay as the output, and power the solenoid through the relay's contacts.

Hi RK,

Here is a common way for 2 point level control.

Both S1 and S2 level relays are activated while NOT immersed in liquid (if S1 and/or S2 are working in contrary then use NC terminals instead of NO simply). I assumed with this sketch that your pump required 3 ph. power.

If liquid level is under S1-level then pump is activated until liquid level is reached the S2-level. An auxillary open contact of PR relay -contactor - is used to lock the PR itself.

regards

Wow! What a lot of homework help....