Industrial Control Basics – Unlatching the Latching Circuit

     When I had the misfortune of getting stuck in my Uncle Jake’s outhouse as a kid, I would allow my hysteria to get the best of me and forget my uncle’s instructions on how to get out.  It was a series of raps and a single kick that would prove to be the magic formula, and once I had calmed myself down enough to employ them I would succeed in working the door’s rusty latch open.  Our relay circuit below has a much less challenging system to effectively unlatch the pattern of electric current.

      Figure 1 shows our latched circuit, where red lines denote the flow of current.

Latched Electric Relay Circuit

Figure 1


     If you recall, the relay in this circuit was latched by pressing Pushbutton 1.  When in the latched state, the magnetic attraction maintained by the wire coil and steel core won’t allow the relay armatures to release from their N.O. contacts.  The relay’s wire coil stays energized via Button 2, the red bulb goes dark while the green bulb remains lit, even though Button 1 is no longer actively depressed.

     Now let’s take a look at Figure 2 to see how to get the circuit back to its unlatched state.

Unlatching An Electric Relay

Figure 2


     With Button 2 depressed the flow of current is interrupted and the relay’s wire coil becomes de-energized.  In this state the coil and steel core are no longer magnetized, causing them to release their grip on the steel armatures.  The spring will now pull them back until one of them makes contact with the N.C. contact.  The red bulb lights again, although Button 2 is not being actively depressed.  At this point the electric relay has become unlatched.  It can be re-latched by depressing Button 1 again.

     Let’s see how we can simplify Figure 2’s representation with a ladder diagram, as shown in Figure 3.

Electric Relay Latching Circuit Ladder Diagram

Figure 3


        We’ve seen how this latching circuit activates and deactivates bulbs.  Next time we’ll see how it controls an electric motor and conveyor belt inside a factory.


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