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Simple yet Brilliant:
How a three-way-switch works.
Most of us are familiar with three-way-switches.
If you have a light that can be turned on or off from two different locations (i.e. a stairway light that can be turned on with one switch at the top of the stairs and turned off with another switch at the bottom and vice versa), you are using three-way switches.
How do they work?
Let’s start with the operation of a standard (two-way) switch, which is easy enough to figure out:
.
??? <-switch (in on position)
Hot ????????????????????????????
??? ? <-light bulb (on)
?
Neutral ???????????????????????????
Electricity flows from the hot wire, thru the switch, into the lightbulb, and out thru the neutral wire.
If two two-way switches were on the above circuit they’d both need to be in the “on” position for the bulb to be lit.
This would enable either switch to independently turn the light off.
…but…
Neither would be able to independently turn the light on.
A three-way switch allows both the “on” and the “off” of either switch to operate independently.
Here’s how:
.
?????
a? ? ?b
? ? ?c
?????
This is a three-way switch, with the switch in the “up” position.
“a”, “b” and “c” are terminals that wire is attached to via screws.
With the switch in the “up” position, an electric path exists between terminals “a” and “b”, while “c” isn’t connected to anything.
.
?????
a? ? ?b
? ? ?c
?????
This is a three-way switch, with the switch in the “down” position.
With the switch in the “down” position, an electric path exists between terminals “a” and “c”, while “b” isn’t connected to anything.
Now to redo the original diagram with three-way switches:
.
?????s1 ?????s2
Hot ????a? ? ?b?????????c? ? ?
? ? ?c?????????b? ? ?a????????
????? ????? ? <-light bulb
?
Neutral ??????????????????????????????????
1) In the above sketch will the bulb be lit?
Answer: yes, because
– The electricity flows into s1-a, out of s1-c, into s2-b, out of s2-a and into the light bulb. The bulb will be lit.
.
?????s1 ?????s2
Hot ????a? ? ?b?????????c? ? ?
? ? ?c?????????b? ? ?a????????
????? ????? ? <-light bulb
?
Neutral ??????????????????????????????????
2) We flipped switch s1. Will the bulb be lit?
Answer: no, because
– The electricity flows into s1-a, out of s1-b, into s2-c, but it stops right there since s2-c isn’t connected. The bulb will be unlit.
.
?????s1 ?????s2
Hot ????a? ? ?b?????????c? ? ?
? ? ?c?????????b? ? ?a????????
????? ????? ? <-light bulb
?
Neutral ??????????????????????????????????
3) We flipped both switch s1 and switch s2. Will the bulb be lit?
Answer: yes, because
– The electricity flows into s1-a, out of s1-b, into s2-c, out of s2-a and into the light bulb. The bulb will be lit.
Try this yourself with different switch up / switch down combinations, and you will see that in all cases flipping either switch will reverse the on-off state the bulb is in.
Sometimes the wiring is done differently (I won’t get into how and why) but the three-way-switch functionality remains the same.
(lekovod the festival of lights. gong!)