What’s a relay? You’ve probably seen a relay race where one runner hands off a baton to another runner. Similarly, an electronic relay hands off control from one circuit to another. A relay is a very simple device consisting of an electromagnet, an armature (a switch that closes when attracted by the electromagnet), and a spring that is connected to the armature. You can see in the diagrams above how the relay works.
In Figure 1 (picture 2), there are two circuits. The first circuit is a battery (3 volts), a switch and an electromagnet. The second circuit is a battery (6 volts), a light bulb and the relay’s armature. While the switch to the electromagnet is off, no current can flow from the 3 volt battery through the electromagnet. So, the armature-switch is off and no current can flow from the 6 volt battery to power the lamp in the second circuit. (picture 2 diagram inspired by http://electronics.howstuffworks.com/relay1.htm)
In Figure 2 the switch to the electromagnet circuit is switched on. When current from the 3 volt battery flows through the electromagnet, the electromagnet creates a magnetic field that attracts the armature to close the circuit to the lamp. Now current can flow from the 6 volt battery to the lamp, and the lamp lights up.
If you look closely at Figure 2, you’ll notice that while the armature-switch is closed allowing the current in the lamp circuit to flow from the 6 volt battery to the lamp, it does not come into contact with the electromagnet so, the 6 volts from the lamp circuit cannot flow into the electromagnet circuit. Thus, the 3 volts in the electromagnet circuit and the 6 volts in the lamp circuit remain separate.
Picture 3 is the electronic schematic diagram of a relay. The curly line represents the electromagnetic coil and the vertical lines represent the metal core the coil is wrapped around. The switch at the top of the diagram represents the armature.
Picture 4 is the electronic schematic of the circuits from figures 1 and 2 (picture 2).
When you press the press switch (S1) you will hear the relay click as current passes from the 3 volt battery block (B1) through the electromagnet. The electromagnet creates a magnetic field that attracts the metal armature and closes the circuit to power the lamp circuit.
When you release the press switch you will hear the relay click again since the current has been cut from the electromagnet. This causes the electromagnetic field to collapse so the armature is no longer attracted by the electromagnet and the spring attached to the armature returns the armature to its rest position. This opens the lamp circuit which cuts the current from the batteries to the lamp and the light goes out.
Often when using electronic relays in circuits it is recommended that you insert a flyback diode to protect sensitive electronics. The magnetic coil in the relay stores electricity like a capacitor and when the voltage is cut from the coil the current can flow back into the circuit in the wrong direction. You can build the circuit in pictures 3 and 4 demonstrate the phenomenon.
1 Battery Holder (2-AA) # 6SC B1
1 Base Grid (11" x 7.7") # 6SC BG
1 Slide Switch # 6SC S1
1 Red LED # 6SC D1
1 Relay # 6SC S3
3 Conductor with 2-snaps # 6SC 02
1 Conductor with 3-snaps # 6SC 03
1 Conductor with 4-snaps # 6SC 04
The following video will demonstrate the relay's magnetic field collapse sending voltage back through the circuit the wrong way. You may have to wait a moment for me to dim the lights a bit to see the led flash, but when I push the button, this engages the electromagnet in the relay and you may be able to hear the relay click on. When I release the button this cuts off the voltage to the relay (you may be able to hear it click off) and the magnetic field collapses and the current flows back into the circuit causing the LED to flash.
This next video demonstrates how the circuit on the left side of the relay (B1 battery block and Slide Switch S1) is used to switch on the circuit on the right side of the relay (another B1 battery block and motor with fan attached).