Relay Latching Circuit using Push Button

We will go with step by step :

Step 1:-

When we press push button, relay should be on, It means we use Normally Open type push button because when we press this switch supply goes forward.

Step 2:-

When supply comes to relay coil, relay should be ON. Here relay is 24 V DC operated.  These two step we see in following picture:-

Relay Latching Circuit using Push Button

Relay Circuit Example

We connect relay and push button as per figure. When we press push button, supply goes to relay A1 point and relay gets ON and its contact changes, but when we release push button supply cutoffs and relay gets off.

But relay not do not fall in hold. So here we think what we shall do for relay to hold. Now we use Relay NO contact for hold. How we use this contact see in below figure:-

Relay Latching Circuit

We connect +24VDC to COM point of relay and NO point to A1 Point of relay. When we press Push button supply goes to relay, Relay turned ON and contact changes NO point became NC point.

Now +24VDC supply direct connect to A1 and relay turned ON. If we release Push button supply cuts from the push button but the supply is permanently coming from NO point and relay permanently ON or on hold.

Now we want to OFF this relay, how we OFF this relay? Listen! On above example supply permanent coming from NO point that supply do continue ON   Relay. If we disconnect this supply using any element or device relay will be OFF.

How? Here we use NC push button to disconnect the supply. Refer below image :

Relay Hold Circuit using Push button

Example :

Design a Relay circuit such that it activates whenever PLC sends an trip signal. Also in same time, relay has to activate a Hooter which is powered by 230v AC. And provide a Acknowledge/Reset button to stop the hooter.

Note : PLC send a one shot pulse to activate the relay. The realy circuit must hold the signal until it resets by using an Acknowledge/Reset button.

Relay Circuit for Hooter with Reset Button

Note : Here PLC command is shown as NO push button in the above diagram. we can replace NO push button with PLC command.

Sequence of Steps :

  1. PLC given activation signal to relay or NO push button pressed and released.
  2. Relay coil energized as current passed from NO push button to Relay coil A1 to A2
  3. Relay energized, so the Normally open contact changed to Normally closed ( here we are using 2 NO 2 NC type means two number of Normally contacts available in single relay)
  4. As Normally Open contact changed to Normally closed > The First NO contact will be used for Relay Holding / Latching Purpose. The first NO contact is connected to power supply +24V dc and same connected to relay coil also. As PLC signal is a pulse signal, we need to Hold the relay. So after relay energized, the first NO contact will give power to the relay coil as NO contact becomes NC.
  5. The second NO contact is connected to Hooter in series with 230v AC power supply. when relay activated, the 230V AC power will be circulated to the hooter and the hooter activates.
  6. when we press acknowledge/reset button, that means disconnecting the power to the relay coil so relay de-energizes, so again NC contact will be come NO so power to the hooter disconnected.
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