Relay Control

Operating a Relay

Let's build a control circuit using relays. This is the foundation of ladder circuits and ladder diagrams, so make sure you understand it thoroughly.

From the explanations so far, relays may seem like they only operate from push buttons or sensors, without being useful for actual control. First, let's connect a switch and a coil.

Pressing the push button operates the relay; releasing it returns the relay to its original state. This circuit alone is meaningless. Next, let's make it so that pressing the button operates the relay, and even after releasing the button, the relay continues to operate.

Self-Holding

The coil's contact has been connected in parallel with the push-button switch. When the push button is pressed, the coil operates, and the coil's contact also operates. Even after releasing the push button, the coil's contact continues to supply current to the coil. This is called "self-holding."

Releasing Self-Holding

One important point about self-holding: once latched, it cannot be released automatically. A contact (stop switch) must be added to the circuit to release it.

As shown in the diagram above, a NC contact (b-contact) switch has been inserted in series. Cutting the power supply to the coil releases the self-holding. Turning off the main power supply has the same effect. In this circuit, pressing the push button operates the coil (self-holding), and pressing the "Stop" switch releases the self-holding and returns the coil. Since a coil has multiple contacts, connecting a green lamp to the NO contact and a red lamp to the NC contact allows displaying operating/stopped status with lamps.

Building a Simple Relay Circuit (Step Control)

Relay control basically applies self-holding in sequence, then releases all self-holding at the end. Here is a simple example circuit.

This circuit operates on DC power. Explained from top to bottom: the top two coils (CR10 and CR11) are for sensor signals. When a sensor activates, the corresponding relay operates. Control circuits should be designed simply so that anyone can understand them at a glance.

The operation flow is as follows: pressing the push button turns ON CR1 (auto run) via self-holding. If the photoelectric sensor responds and auto run is active, CR2 turns ON and the cylinder operates. When the cylinder reaches the extended end, CR11 (cylinder sensor) turns ON, turning on CR3. When CR3 turns ON, the self-holding of CR2 is released and the cylinder returns. This type of control — turning coils ON one by one in sequence — is called "step control" (sequential control).

Also, changing the connection position of the self-holding conditions changes the behavior. In the circuit above, pressing the Stop button while the cylinder is in operation will cause an immediate stop. However, by changing the position of CR2's parallel contact, you can implement "cycle stop" — the system stops only after the cylinder completes its current operation.

Precautions for Relay Circuits

One important point in relay control is the response time of contacts. It takes approximately 20 ms (0.02 seconds) from when power is supplied to the coil until the contacts connect. Even more important: the NC contact (b-contact) turns OFF faster than the NO contact (a-contact) turns ON.

If a circuit is designed assuming simultaneous operation, chattering (rapid ON/OFF switching) may occur. This happens because the coil's power is cut before the NO contact (a-contact) turns ON after the relay operates. Always verify the precise sequence of operations when designing circuits.

Once you understand relay circuits, it's time to move on to ladder diagrams. A detailed explanation page for self-holding is also available. If you want a deeper understanding of self-holding, please check that page first.

Frequently Asked Questions (FAQ)