Circuit Design Methods Part 2

Now for the fun part, checking to see if your circuit will work! First, let’s take a look at the first attempt.

As usual, to open the drawing, you will need to click on it to be able to see  it properly.

Now do a “walk through” of the circuit as drawn.


First thing you will notice is that coming off the supply at the top, we go into two contacts labeled as Safety Contactor. The reason here is that all the Emergency Stop devices are to be wired via a safety relay. This relay will provide an output to the two safety contactors K1 and K2.


Assume the safety circuit is on. Both the normal open contacts will be closed and we will have power through the normal stop onto the start button. By pushing Start, we will energize relay R1 and the retaining contact will close thus holding R1 activated.


With R1 energized, the normal open of R1 will close, and as such provide power to the line for the machine side. If you look at this line, you will note no contactor can be energized as the entire line has normal open contacts.


Now the operator pushes the LOAD button. Power goes through the normal closed contact of Limit 1 and Q1 is energized. As before, the normal open contact of Q1 retains the circuit and your conveyor is now running thus moving the container forward.


Once the container reaches the limit switch, the contacts will change state. That means the conveyor is stopped as the normal closed contact has opened, but at the same time the normal open contact closes on the next section of the circuit.


This now creates a path through the normal closed of Limit Switch 2, and energizes Q2, which as before, has a retaining contact and the second conveyor is now running and moving the container forward. Once the container gets to the end of the second conveyor, it activates Limit Switch 2 and the conveyor is stopped with the normal closed contact now opened.

Again, this closes the normal open contact on the Fill Conveyor section, and Q3 is now energized. This conveyor now runs and the container moves to the end of the line where it activates Limit Switch 3. Now you will notice that apart from the retaining contact in parallel with the normal open of L2, we also have a normal open of relay R3 drawn in to bypass both the limit switches to this coil. The reason here is that we do not want this conveyor to start the unload section until the operator pushes the MOVE button.


So this means the container has stopped on the limit and the conveyor was de-energized. The container is now filled, and once the operator is happy, he/she pushes the MOVE button. This energizes relay R 3, which will start the Fill conveyor as well as the unload Conveyor. This happens with the normal open contact of R 3 closing and energizing Q 3, relay R 4 has also been energized which in turn energizes Q 4.


What you need to observe here is that the retaining contact on both R 3 and R 4 is done different. There is a normal open contact of Limit 3 in series with the retaining contact! This limit has changed state when the container activated it, and will again change state as the container is transferred to the unload conveyor thus stopping the Fill conveyor and de-energizing relays R3 and R4. The Unload conveyor will stay energized as it has a retaining contact Q 4 in parallel with the normal open of R 4. The unload conveyor will be stopped when the container reaches the limit switch L 4 at the end of the line.


That means that for all practical reasons, this circuit should work. What can now be added to it is indication lights, alarms etc. The choice is yours. Have a good look at it and tell me your thoughts.



Please Note: The Author has used this control circuit as an indicative design to show the thought pattern when designing a control circuit. Before using this in a practical application, test the circuit and ensure you observe all safety requirements.

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