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Storing Signals, Latching Circuits

In practice, you often want to start certain sequences via push buttons that then need to continue when the button is released (e.g. extension of a cylinder). The fact that this button has been pressed must therefore be stored in some way. There are two ways of doing this:

Using bistable magnetic pulse valves. Such valves retain their operating position even when power is removed from the solenoid. It is as if they have "in-built memory".
 
Design using relay circuits with latching. Such circuits are described in the following section.

 

Latching Circuits

The following graphic illustrates the principle of a latching relay circuit. If button S1 (on switch) is activated, relay coil K is supplied with current from circuit 1. The relay closes thus closing the corresponding contact in circuit 2, causing current to flow through that point. If S1 is released, the coil K is still supplied with power via contact K in circuit 2 and the relay remains latched in the activated position. Only activating the normally closed off switch S2 cuts off the current and puts the relay back into its rest position, where it then remains.

 

If the on and off switches are pressed together, current flows through circuit 1, so that the relay switches on. Such a latching circuit is called "dominant set".

By contrast, the following graphic illustrates a "dominant reset" latching circuit. In this case, when both switches are pressed simultaneously, no current flows and the relay remains in its rest position.

 

One disadvantage of latching relay circuits as opposed to magnetic pulse valves is that when the electricity supply fails, valves controlled by latching relays always return to their rest positions (possibly giving rise to unwanted movement of the machine), whereas the status of magnetic pulse valves remains static (and does not cause any movement in the machine).

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