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# What is the function of a SPDT switch? Answered

Hi....my question is that what is the actual function of the SPDT (single pole double throw) switch and I would also really appreciate if any body also told that how does it work...Thanks..

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Hi Arif,

Well, a SPDT (single pole double throw) switch is one in which there are three terminals (say A,B,C) ...the middle one is the common terminal and is usually connected to one of the supply lines...Thus, when the switch is flipped to one side, two of the three terminals come in contact (say C,B), and when the switch is flipped to the other side, the other combination--A,C comes in contact with each other....(AS SHOWN IN THE IMAGE--2 ATTACHED...).

In electronics work, many SPDT switches have a middle position in which the electricity is turned off to BOTH circuits. It is an SPDT center off switch. Also, some electronic SPDT switches have a "center on" position. The best example of this type of switch is the "pickup" selector on an electric guitar which can choose the rhythm, treble or both pickups for 3 varieties of sounds...
The most common use for the SPDT switch -- The 3 way light switching circuit (IMAGE--1)....So, how does this work? Let's say that Switch 1 is at the bottom of a stairway and Switch 2 is at the top. Suppose Switch 1 is in a 'down' position (B & C connected) and Switch 2 is in an 'up' position (D & E connected). The light bulb is off. Now someone comes to the bottom of the stairs and flips Switch 1 'up'. If you follow the circuit you can see why the light bulb would now turn on because A & B and D & E are connected. When the person reaches the top of the stairs, Switch 2 is flipped 'down', E & F are now connected and so the light bulb goes off. Another person shows up at the bottom of the stairs and flips Switch 1 'down', connecting B & C thereby turning the light on again. The person reaches the top of the stairs, flips Switch 2 'up' connecting D & E and the light bulb goes off. Notice that in the case of the second person, a downstroke turns the bulb on and an upstroke turns the bulb off. If you have such switches in your house OR if you have purchased household wall switches for this circuit, you now see the reason why they do NOT have the words on and off printed on them...