The following symbols refer to the device shown in the ladder logic diagram S1 =

Question

The following symbols refer to the device shown in the ladder logic diagram S1 = normally open switch S2 = normally closed switch CR = control relay M = Motor L = Indicator Light Assume that initially, the NC switch S2 has been turned OFF. Explain TO THE POINT what happens when, Input S1 is turn ON, and the turn OFF immediately After 2.5 minutes input S2 is turned ON, Explain in not more than 10 lines the function of this ladder logic circuit and a possible application for it. Just an answer alone will not suffice. You must explain your logic at each sequence before deciding the functionality. No credit will be given if you only write the final result, even if it is correct.

Explanation / Answer

As per the question S1 is normally open switch, i.e. it will have a logic 0 when de-energized (open) and logic 1 when energized (closed) so it will act as ON switch and S2 is normally closed switch,i.e. it will have a logic 1 when de-energized (closed) and logic 0 when energized (open) so it will act as OFF switch. In the first rung, the three components S1, S2 and CR are in series (AND configuration where S1 and S2 are inputs and CR is the output). Also, CR is in parallel with S1, i.e. latched to S1 (OR configuration).

For the first part of the question,

when S1 is turned ON it become energized (becomes closed) and its logic value becomes 1 where S2 has no change and it retains its logic value 1.

So, according to AND configuration, when both inputs are 1, the output is 1 which energizes and turns ON the control relay CR as the power flows from S1 to CR through S2.

From the second rung we can see that CR is acting as the input in normally open configuration whereas motor M and indicator light L are outputs. So as soon as CR turns ON it will turn the motor Mand light L ON as the power flws through motor M to light L as seen from the second rung.

Now, after switch S1 is closed immediately (becomes open), at that moment CR, M and L are ON and logic of S1 becomes 0 and S2 remains at logic 1. But CR is latched to S1 (OR configuration), so since CR has logic value 1 at that moment it will keep itself (CR), motor M and light L in ON state even though switch S1 is turned OFF.

For the second part of the question,

Initially the switch S1 is turned OFF so it is in logic 0, inpur CR is turned ON so it is in logic 1 and S2 has no change so it is in logic 1 which runs the CR, M and L at ON state. After 2.5 minutes, switch S2 is turned ON which makes its logic value 0.

So now S1 is OFF (logic 0) and CR is ON (logic 1). Since CR is latched to S1 we will have logic 1 of CR (OR configuration). S2 is at logic 0. Since input combinations in first rung are in AND configuration, so we will have logic 0 as output (1.0=0), i.e. the relay CR is turned OFF which will turn OFF the motor M and light L in the process as they are connected in series with CR and no power will flow through M and L as we can see in rung 2.

For the third part of the question,

the main application is with the light L. This circuit configuration is used for troubleshooting purpose. If the light turns ON when the S1 is turned ON then all the components are OK. The light must remain ON while S1 is turned OFF but S2 is not turned ON. If somehow light L does not turn ON after turning ON the switch S1, then the power is not flowing to L. So we have to check the faulty component then. First we have to check for if CR is running or not. If it is running then switch S1, S2 and CR are alright otherwise check for S1 and S2 if for the combination of S1 ad S2 the relay CR is turning ON and OFF accordingly or not. If CR is alright and L is not glowing, then the motor M must be faulty which prevents the power to flow to L .

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