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Old Sun 25 February 2007, 16:20
Mike Richards
Just call me:
Selflatching relay/contactor

Here's a simple circuit showing one way to wire a self-latching relay.

This self-latching circuit requires you to push a momentary switch to turn the relay ON, but then the relay stays ON until you either turn off the power, or until you press the EMERGENCY STOP switch(es).

In this circuit, when you turn on the door switch (SW1), power flows through the AC to DC transformer/bridge rectifier/capacitor. The DC power flows through the relay (R1) when the momentary push button (PB1) is pressed. When power flows through the relay, the relay's contacts are energized. Once the relay's contacts are energized, DC voltage flows through the relay's coil until EITHER the main power is disconnected, or until one of the Normally Closed push button switches (E1, E2) is opened.

In this particular circuit, when relay R1 is ON, the DC voltage that keeps the relay turned on is also available to turn on the other relay (R2). Closing the switch S2 will turn on relay R2. This circuit shows relay R2 controlling a 3-phase line (spindle power).

So, in this circuit, you would turn on the door switch SW1, and then push the start button PB1. Assuming that the emergency stop buttons were closed E1 & E2, relay R1 would turn on. That would allow relay R2 to be turned on if switch SW2 were closed. HOWEVER, pressing either E1 or E2 would immediately open the DC circuit and both relay R1 and relay R2 would be turned off.

Although this is just a simple circuit without much practical use, I hope that it allows those who have never designed a self-latching circuit to see how they work. With a little imagination, this kind of circuit could be used with limit-switches in place of E1 & E2 to cut the power to the stepper drivers. Other variations only require an active imagination.

-Mike Richards
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