How Relays Work
Relay Contact Information:
Relay contacts in the industrial world are labeled with NO (Normally Open),
NC (Normally Closed), and C (Common). These labels and the function of the
relays cause a great deal of confusion among people, however once you
understand the meaning it is quite simple and literal.
The first thing you need to know is that a relay contact is a switch, nothing
more, nothing less. It does not provide power; it simply opens and closes an
electrical circuit, just like the light switch on a wall.
When the relay is de-energized or turned off there is an electrical
connection between NC and Common, hence normally closed. In the off state there
is not a connection between NO and common, hence normally open.
When the relay is energized or turned on the NO and C make an electrical connection,
and the electrical connection between NC and C is removed.
Here is a sample of connecting a simple 110 VAC light to a relay so that it will
turn on when the relay is energized. Wire the hot 110VAC to the Common (C),
Normally Open (NO) to hot side of the light bulb, Neutral from the light bulb to the
neutral of the 110VAC wiring. Now when the relay is energized the NO will close
connecting to the C and allowing power to flow through the bulb.
The last issue is voltage and current ratings. A typical relay rating will be
24VDC @ 5 Amps, 110VAC @ 10 Amps, 220 VAC @ 5 Amps. This is simply telling you
the max amperage that the relay contacts can handle at the voltage you are using.
For example a 100 Watt light bulb is going to be about 120 VAC @ 1 Amp.
This is 9 Amps below the rating, so the circuit can handle turning on an off the
light bulb with no problem.
More Information from Quasar Electronics Limited (a European vendor)
You will find relay outputs in many of the kits that we sell. A relay is simply
an electrically operated on/off switch. It is important that you observe the relay
voltage and current limitations specified in the kit documentation.
Relay Terminals
The three terminals on our relay outputs are:
C = Common
NO = Normally Open (i.e. switch is open when relay is not energised)
NC = Normally Closed
Look at the kit circuit schematic for help.
Connecting the Device you want to Control
You must provide an external power source to the device you want to control.
No voltage is present at the relay terminals (remember it is just a switch).
You connect the relay in series with the positive (+) power wire of the device
you want to control.
So the positive wire from the power source should be connected to Common. Then
either the NO or NC terminal (as appropriate for your purpose) is connected to the
positive (+) wire going to the device you want to control. The negative (-) wire
does not connect to the relay at all. It goes directly from the power source to
the device negative (-) terminal.
Warning!
Mains power can KILL and must therefore be treated with extreme CAUTION! The
construction, testing and use of kits with mains voltages should only be attempted
by competent persons or under supervision of someone fully experienced in this
field. Kits must be fitted into a suitable fully enclosed box before operating.
We strongly recommend that, where available, you also order the box designed for
the kit so that you can finish your project to both a safe and professional-looking
standard. We accept no responsibility for injury, loss, or damage of any kind as
a result of the purchase, assembly or use of any of our products.
© Quasar Electronics Limited 2004
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