Self-Holding Circuit

What Is Self-Holding?

Self-holding is implemented using a relay. A relay consists of a coil and contacts — when current flows through the coil, it becomes an electromagnet and pulls the relay contacts to turn them ON. For a detailed explanation of relays, please refer to About Relays.

Why is self-holding important? Most people studying sequence control will eventually reach ladder diagrams. Self-holding is also used frequently in ladder diagrams — it is very difficult to understand ladder diagrams without understanding self-holding. This page focuses exclusively on self-holding. There is also a video explanation available, so please watch it if you want a more thorough understanding.

How Self-Holding Works Step by Step

Let's walk through exactly how self-holding works, step by step through 16 circuit diagrams. (Click each diagram to enlarge it.)

The circuit shown above is the one we will use. "CR1" is the relay. The same "CR1" coil and contact are used together. The "Push Button" is the switch to turn ON self-holding, and the "Stop" switch is connected as a NC contact (b-contact) to release the self-holding.

Now let's imagine how current flows when power is turned on.

The positive side of the power supply is shown in red. Lines with color are the lines that have voltage applied. In this state, no switches or relay contacts are ON, so no current is flowing. Current only flows when the circuit is connected from the positive side to the negative side. In this state, we now press the push-button switch ON.

Current then flows through the push-button switch contact to the "CR1" coil.

When current flows through the coil, the "CR1" coil turns ON, and the "CR1" contact also operates and turns ON.

Once the "CR1" contact turns ON, current also flows through the "CR1" contact.

At this point, current flows to the "CR1" coil from both the push-button switch and the "CR1" contact simultaneously. Electrically, the current value flowing through the "CR1" coil is determined by the coil's resistance, so adding more contact paths does not increase the current value. You do not need to worry about this point when writing ladder diagrams.

Since the "CR1" contact is ON, the lamp also lights up.

Now, in this state, let's release the push-button switch to OFF.

Even after the push-button switch is released to OFF, the "CR1" contact continues to supply current to the "CR1" coil, so "CR1" maintains its ON state. This is the self-holding state. The full sequence is: "Press the push button once → CR1 turns ON → Releases the button but ON state is maintained → Lamp stays on."

So how do we release the self-holding? Self-holding can be released by cutting off the current to the coil. That is why a "Stop switch" is inserted in the circuit as a NC contact (b-contact).

Since the Stop switch is a NC contact (b-contact), pressing it turns it OFF.

Pressing the Stop switch cuts the circuit to the "CR1" coil, cutting off the current to the coil. The "CR1" coil then turns OFF.

When the "CR1" coil turns OFF, the "CR1" contact also turns OFF.

When the "CR1" contact turns OFF, the current that was flowing through that contact is also cut off.

Once the current through the "CR1" contact is cut, the supply of current to the "CR1" coil through that contact is completely cut off. This is how self-holding is released. To release self-holding, the key is to cut off the current to the self-holding coil.

With self-holding released, the current to the lamp is also cut off and the lamp turns off. This completes the full sequence from self-holding to release of self-holding.

This explanation was given assuming a relay implementation, but self-holding is also used frequently in ladder diagrams. While it is technically possible to write a program without self-holding, writing ladder diagrams in a way that anyone can clearly understand is a fundamental principle. Self-holding can be considered an indispensable technique for ladder diagrams.

Frequently Asked Questions (FAQ)