A relay consists of two major parts:

The Solenoid Assembly (indicated by "S") is usually referred to as the "Coil". The solenoid is constructed of the armature, (generally an iron based slug) and the windings, a continuous wire wrapped around the armature. When a direct current (DC) voltage is applied to the windings (through connections "S1" and "S2"), a magnetic field is created with the ends of the armature becoming the "poles" of an electro-magnet. This magnetic field will attract the arm of the wiper extended over the solenoid causing it to move.

The Contact Assembly. Consisting of a "Wiper" which pivots on "fulcrum" (indicated by "F") and its associated contacts. Without any electrical current flowing through the solenoid, the Wiper will rest against one of its contacts called the Normally Closed (N/C) Contact creating an electrical connection between them. Note there is a gap between the Wiper and its second contact called the Normally Open (N/O) contact.

When a DC current is applied to the coil through connection S1 and S2, the magnetic field formed will pull the section of the Wiper overhanging the Armature (as indicated in the diagram) down towards the Armature. Since the wiper is mounted on a pivot point or Fulcrum, the entire wiper will swing counter-clockwise breaking the connection between the N/C contact and making a connection to the N/O contact. (Remember that "Normally" refers to the state of the relay without the application of electrical current to the Solenoid.)

When the DC current is removed from the coil, the magnetic field collapses, the wiper will swing clockwise returning to its original position. This will break the connection between the N/O contact and reestablish the connection to the N/C contact. Often the is a small spring used to insure the wiper return to its original position.

• As there is no electrical connection between the controlling current applied to the coil and the controlled current connected to the Contact Assembly, there is a very high degree of electrical isolation between the two.

• A smaller current can be used to control a larger one therefore lighter duty switches can be used to operate the coil than would be required to operate the large current directly.

• The relay can be located some distance away from the switch that supplies current to the coil so it can be located near the large current device. Beside the obvious "remote control" advantages, this also avoids running heavy gauge wires long distances (with associated losses) to the control switch.

• Relays have been with us a long time and are very rugged and reliable. They are well suited for the hostile enviroment of the automobile.

The Solenoid or Coil:

• Nominal Coil Voltage is the typical operating voltage needed to operate the relay. In other words, this is the typical voltage that would be applied to the solenoid to cause the N/O contacts to close.

• Maximum Continuous Coil Voltage is the maximum voltage that can be applied continuously before the coil windings will be destroyed by overheating.

• Pull In Voltage is the MINIMUM voltage required to close the N/O contacts.

• Drop Out Voltage is the voltage at which the coil no longer produces enough of a magnetic field to keep the N/O contacts close. This is usually 10% or more LOWER than the Pull In Voltage.

• Coil Resistance, rated in Ohms, is the resistance to the controlling current. To determine the current flow through the coil, divide the applied DC voltage by the coil resistance.
  • Example: 13.8 VDC / 150 Ohms = 0.092 Amps or 92 Milliamps (92ma)

• Contacts are rated as to the number of "Poles" or sets of Wiper/Contact pairs they include and the "Throw" which is if the relay has only one contact (Single) or two contacts (Double or a N/O and N/C pair).
  • SPST - Single Pole, Single Throw. One Wiper with only a N/O or N/C contact. These should be marked as N/O or N/C. If not specified, assume it to be normally open.

  • SPDT - Single Pole, Double Throw. One Wiper with both a N/O and a N/C contact.

  • DPDT - Double Pole, Double Throw. Two wipers each with a N/O and N/C contact (as shown in the diagrams).

  • 3PDT - Three Pole, Double Throw. Three Wipers each with N/O and N/C contacts.

  • 4PDT - Four Pole, Double Throw. Four Wipers each with a N/O and N/C contacts.

• Maximum Contact Voltage is the largest voltage than can be applied across the air gap between a wiper and an open contact before the voltage will arc or jump across the gap.

• Maximum Contact Current is the largest current in amps than can be applied across a closed wiper and its contact before the wiper/contact point will melt.
  Note: The current rating of the contacts can be "increased" by wiring them in parallel. That is two 10amp contacts in parallel will give a 20amp rating through the two contacts (10amps through each).

Here is an electrical diagram of the relay shown above wired in a typical application. The Plus symbol [+] indicates the positive voltage (12 or 13.8 VDC) and the Minus symbol [-] indicates ground (0 VDC). (The squigglly line to the right of the solenoid body is the symbol sometimes used as an alternative in some schematics.)
The dashed line indicated the magnetic field's effect on the wiper HOWEVER note that schematics ALWAYS show the position of the wiper WITHOUT the application of current to the coil. That is, the relay is shown DE-ENERGIZED.
When current is applied by closing the switch, the wiper will move from the N/C contacts to the N/O contacts, making a connection between contact terminal 1 and wiper terminal 2 and between contact terminal 4 and wiper terminal 5. Note that terminals 3 and 6 are not used in this application.

Assume voltage was hooked to one side of a driving light and the other side hooked to terminal 1. If terminal 2 was hooked to ground, when the relay is energized by closing the switch, the ground would then be supplied by the closing of contact 1 and wiper 2 -- lighting the light. A second driving lamp could be hooked to terminals 4 and 5 with the same result.

Note, however, that neither the relay nor the switch carries the full current of the driving lamp. That is carried by the relay contact assembly. Also note that the relay could be mounted near the driving lamps with light gauge wire run to a dashboard switch.

A practical modification to the driving lamp application is to hook the [+] terminal of the solenoid to the high beam circuit rather than to 12VDC. If the switch is closed, the driving lamps will come on automatically whenever the high beams are lit. This would add very little current to the original high beam circuit since only the coil is being powered not the driving lamps themselves.

When the button is pushed, current is supplied to the solenoid energizing the relay. Note that a set of N/O contact are wired across the push button. Since both set of N/O contacts will close when the relay energizes, they will short across the switch so the relay will stay energized or "latched" when the button is released. The second set of contacts can then be used to switch power to some device. NOTE: Wiring the second set of contacts to a coil primary wire and hiding the switch makes for a very inexpensive theft deterrent as there will be no ignition until the button is pushed.

To add an "off" function, wire a Normally Closed (N/C) push-button switch between the solenoid and ground. Once the relay has latched, pressing the second N/C button will remove the ground from the solenoid and de-energize or unlatch the relay.

Since several N/O and N/C push button switches could be wired in parallel, this circuit could be used to control interior lighting with several "On" and "Off" buttons located around the car for each passenger.