RELAY LOGIC
Relay Logic is what we use to diagram relay-controlled systems. These systems are hard-wired: you must physically connect the input and output devices to the controller for the control system to work.
The best way to learn relay logic diagrams is to make one yourself!
In this lesson, we will draw a simple relay circuit to explain how relay logic diagrams work.
We will also hard-wire the circuit on our test panel.
The Relay Circuit
We will build a simple relay control system to run a 24 V DC motor. The goal is to create an ON/OFF control system for the motor using the relay as our controller.
The circuit will consist of a relay, START button, STOP button, motor, and standby light.
We will draw the relay logic diagram step-by-step and build the circuit on our test panel.
Our test panel, hooked up to a 24 V power supply.
Let's start with the Relay Logic.
Relay logic diagrams are drawn from left to right and top to bottom, working in rungs. Input devices are placed on the left, and output devices on the right of each rung. It is good practice to put every output device on its own rung.
Typically, the controller is placed on the top rung. Our relay (K1) is what we will use to power the motor and control its ON/OFF condition. So we've placed the relay in the controller position.
The left vertical line, L1, is our “hot supply” line from the power source. It may be an AC voltage (usually 120 V or 240 V) or a DC voltage. In our system it's 24 V DC.
The right vertical line, L2, is our neutral, or ground line.
To wire this rung on the test panel, we hook up the relay coil to the power supply and ground. Connected directly to power, the relay will energize, and stay in that condition. Cutting the power will de-energize the relay.
The relay, hooked up to the power supply.
Let's add the input devices, the START and STOP buttons, to this rung. The START button is a Normally Open switch and the STOP button is a Normally Closed switch. Both are operated by push buttons.
The START button's NO contacts will keep the circuit open until the button is pressed.
Wired this way, the relay will not energize unless the START button is pressed and held.
START and STOP buttons wired in.
It's not very practical for an operator to stand there and hold the button down forever. What we can do is add a sealed-in circuit around the START button. This will ensure that the button only needs to be pressed once for the coil to stay energized. We will create this sealed-in condition using the relay itself!
The relay we are using is a Double Pole Double Throw (DPDT) relay. It has two sets of NO contacts (1 and 3, 8 and 6) and two sets of NC contacts (1 and 4, and 8 and 5).
When the relay is energized, the magnetism of the coil closes the NO contacts and keeps them closed as long as there is voltage across the coil from 2 to 7.
If we want the relay to stay energized after we release the START button, we can use a NO set of the relay's contacts. Let's pick 1 and 3, and wire them across the START button's contacts.
With the seal-in circuit in place, pressing the start button just once will energize the coil long enough for contacts 1 and 3 to close, creating an energizing path around the START button contacts.
With the contacts wired like this, a quick press of the START button will keep the relay energized. Pushing the STOP button will de-energize the relay.
Sealed-in circuit added.
Let's add the motor to the logic diagram. Remember, each component being powered gets its own rung on the ladder.
Our motor is hooked up in series with our second set of NO relay contacts: 8 and 6. When the START button is pressed, these contacts will also close, starting the motor.
With the circuit wired this way, the motor will start when the START button is pressed, and will stay running even after the START button is released.
Motor wired in.
Finally, we place the standby light on the third rung of the relay logic diagram. We want this light to be on when the motor is not running, so we place it in series with a set of normally closed relay contacts. We will choose contacts 5 and 8.
We've pulled in a separate panel with a standby light and wired that up to our circuit. See that light in the bottom left corner? That is L1. It will stay on until we press the START button.
Standby light L1 wired in. The circuit is complete!
And just like that, we've created our first Relay Logic diagram.
And used it to wire a working system!
Review
On each rung of the Relay Logic diagram, the input device is placed on the left and the output device is placed on the right.
When drawing relay logic diagrams, it's good practice to:
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