In a system consisting of an input and output, latching is a process in which output is turned on by a trigger of an input. The Output is retained long after input is gone. In such case output is said to be in a latched state. To return the output in its previous state is called unlatching.
Latches find their application in almost any area where control is involved
Examples of latched systems include, mechanical latching (toggle switches), digital latching (flip flops), and electrical latching.
Electrical latching is achieved by different methods and circuits. Let us consider working mechanism of a motor control to better understand the phenomenon.
Motor control using latch
Turning a motor ON and OFF can be done using a control circuit. The simplest control circuit is given in image below. By pressing the switch on this circuit the contactor or relay will be energized, allowing the motor to connect to the power supply.
As you can see in the diagram that if we use a momentary button to excite the relay/contactor then as soon as the operator releases the switch the coil will de-energize immediately.
By using a mechanically latched switch you might be able to counter this problem. You can turn the switch to ON position to turn the motor on and You can turn the switch to OFF position to turn the motor off as shown in the image
But this will create problems of its own. For example if there is a power failure and the motor turns off then operator have to turn off the R/S Switch or selector switch every single time the power goes off or the motor will start working as soon as power is restored.
This can cause serious problem in the machine. Both in terms of quality and safety.
Let us now control the motor using the latched mechanism
In latch system the operator or controller sends a signal momentarily (called start signal) that energizes the contactor and our motor start running. Even if you remove the start signal after the energization of the contactor it will continue to run. To stop the motor we have to send another signal (STOP) through a button or controller that will de-energize the contactor.
The circuit and its working is mentioned below
Keep in mind that Here ON button is normally open (NO) and OFF button is normally closed (NC) that is that no current can pass through ON unless pressed . Current can flow through OFF unless pressed that is when the button is not pressed the current can flow through the switch . Pressing the OFF will break the circuit.
The control voltage (that can be AC or DC depending on the coil voltage of the contactor) passes through OFF switch and is held at ON button and at the auxiliary relay of the contactor. When we press the ON button the voltages passes through and reaches the coil of contactor energizing the electromagnet. Once the coil is energized it connects the contacts for power and auxiliary relay. Now the voltage to the coil is being supplied by two paths: ON switch and Auxiliary relay contact. Now even when ON button is released, the coil will still get the power through auxiliary relay contact (normally open), keeping the coil energized. This motor will now keep on running until we press the Stop button. By pressing the STOP button the normally close (NC) contact will open up to break the circuit cutting off the power to the coil bring the contactor to OFF state