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The relay can have more than one set of contacts, which can be any combination of normally open and normally closed contacts. For instance, in ill. 1 (below) notice that the relay can have all of its contacts normally open ( ill. 1a), all normally closed ( ill. 1b), or a combination of two or more sets of normally open and two or more sets of normally closed contacts ( ill. 1c). The fourth combination (ill. 1d) is called the double-pole, double-throw (DPDT) switch. The word pole refers to the number of sets of contacts and the word throw refers to the number of positions or combinations (open or closed) the contacts can have. The main feature of the DPDT contacts is that a set of normally open and a set of normally closed contacts are used together. They have a terminal marked common (C), which is electrically common to both sets of contacts. AMAZON multi-meters discounts AMAZON oscilloscope discounts |
Above: ill. 1: Examples of various combinations of relay contacts. a) Four-pole control relay; b) DP DT control relay; c) Five-pole control relay, all NO contacts; d) Five-pole control relay, all NC contacts.
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