What is a latching circuit?

Answer 1

In electric control of a motor you can have two momentary push buttons. (Start And Stop) A latch circuit is used to keep the motor running when the start button is pressed so you don't have to hold the button down.

The Stop button is a normally closed PB switch in series with parallel combination of a Normally Open Start switch and a normally open Contact. This switch contact combination is in series with a double pole relay.

One contact of the relay is in parrallel with the Start switch and the remaining contact is used to start/stop the motor contactor(basically a large relay that can handle high amps)

---STOP-------START--------COIL M---
.................CONTACT M

Imagine the above circuit a stop PB in series with a parallel combination of a Start PB and a Relay contact M , finally inseries with a relay coil M

With power comming in from the left through a normally closed Stop Push Button(PB) . Current cannot flow to the relay until the START PB is pushed which completes the circuit to relay coil M. The relay energizes and closes the contact in parallel with the START PB (Contact M) . This keeps the circuit
energized when you remove your finger from the START PB.
The second contact on the relay controls the motor.

The circuit will remain energized until the circuit is interrupted by pressing the normally closed pushbutton. Pressing the STOP button breaks the circuit to the relay coil M . The relay contacts open breaking the latch across the START PB and stopping the motor

Answer 2

pl refer http://en.wikipedia.org/wiki/Latch_%28electronics%29

hope this hlps