Introduction to Industrial Automation

Figure 2.10  Two internal side views of a general purpose relay.

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Hardware Components for Automation and Process Control 

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relay will operate in one direction when power is applied with one polarity, and will reset when the 

polarity is reversed, thus overcoming the holding effect of the permanent magnet. On a dual coil 

device, when polarized voltage is applied to the reset coil, the contacts will return to their rest state. 

Latching relays are used in situations where the energized status is held for a long period or when the 

relay condition should be held invariant during a breakdown or power supply interruption.