relay logic

Dear all,
i am developing a small project of level control using only NO NC relays.(not PLC).

actually when the lower limit switch is low the motor should go ON,and the motor should be ON untill the high limit switch is high.

so the problem i am facing is when the lower limit switch is going low the motor is on,but as soon as the lower limit is becomes high the motor is switching off.

do we have any relays which can be used as holding relay,or can sonebody tell me how to do the wiring connections to achieve the holding.as this is very easy in ladder logic.

thanks in advance

Ladder logic and hard wiring relay logic are nearly identical. Ladder logic was designed that way, if you can do it one way, you should be able to do it the other way.

You want to "seal in" the motor circuit. Similar to your basic Start/Stop motor circuit wiring. But instead of a Start and Stop pushbutton, you have a low limit switch and a high limit switch

High limit switch is normally closed. Low limit switch is normally opened. When the low limit switch is made, this acts like the start signal. WHen the high limit switch is made, this acts like the stop signal.

thanks dear,
the holding problem will be solved by keeping a normally opened low limit switch,but when the water level is slow, than how to trigger the motor to start.

as of now i have kept NO low lmt swtch attached to NC relay.
but as the level rises,low lmt switch becomes NC and thus NC relay becomes NO and the circuit opens and the motor stops.

so i want to hond the signal when the low limit swtch is NO untill the high limit switch is activated.

any sort of wiring for this,or do i need extra relays for that...any solution plz..

thanks.

You don't need any additional hardware.

You only need an auxiliary NO contact from your motor starter. Then just wire up the limit switches as the start/stop buttons in your basic motor start/stop circuit.

Adapt the circuit as needed. Once the low limit switch energizes the motor circuit, the motor remains running until the high limit switch opens the circuit.

This is where 'Combinatorial Logic' differs from "Sequential Logic'

Combinatorial Logic sets the outputs based on what the inputs states are RIGHT NOW.

Sequential Logic sets the outputs based on what the input states are RIGHT NOW PLUS some 'memory' of what the states were before.

A 'Latch' circuit is is simple 'memory' circuit. Once a set of circumstances exist that turn an output on then the 'memory' that the output WAS ON becomes part of the equation.

Let's say that the low level switch CLOSES when the level becomes low but OPENS when the level is above the low point.

And let's also say that the high level switch is CLOSED until the level is at the high point where it OPENS.

So now the tank function is - 'If the Low Level switch CLOSES then turn ON the motor. Keep it ON even though the low level switch may OPEN. Keep it on until the high level switch OPENS. Then turn OFF the motor.

Let's see if we can implement that in ladder logic.

LOW LEVEL    HIGH LEVEL          MOTOR

---] [-----+---] [----------------( )

           |

MOTOR ON   |

---] [-----+

When the Low Level switch closes and the high level is still closed then the motor turns on. The motor stays on even though the low level opens as the level passes it. The motor stays on until the level rises to the high level switch. At this point the motor turns off. The motor can't turn on until the level falls back down to the low level switch. The 'memory' that the motor 'was on' makes this sequential logic.

This is normally a valid method because we don't want the level to overflow. If the wire to the high level switch is broken, or the high level switch itself is broken, then the motor cannot turn on. This may not be best in ALL situations (think nuclear reactor)

I hope this helps.

And here is another picture. X1 is your low level switch X2 is your high level switch.

Edit:You guys type faster then I do.

thank u all,especially tharon..now it is cleared

one more thing can i use the 12v relays,so that can use 230vac to 12v dc smps.will there be any disadvantage for not using the 24vdc..i want this just to reduce the cost.

thanks again.

this is how i do it.please ignore the rough drawing.

basharath said:

one more thing can i use the 12v relays,so that can use 230vac to 12v dc smps.will there be any disadvantage for not using the 24vdc..i want this just to reduce the cost.

Click to expand...

As long as the 12v DC PS is able to to supply the current to operate the 12v relay coils, there is no particular problem in doing that.

The issue with using a lower control voltage is that current needed to drive the relay coils goes up to compensate, which is why some control panel manufacturers prefer to use 24v DC, or 110v AC, as it means that the wiring sizes are smaller, than if you used 12v DC.