How-to-guides - Understanding relays

What's a Relay?

A relay is an electromechanical switching device when both power and ground are applied to the ends of a coil, the relay activates, which causes mechanical contact points to complete or open a circuit.

One of the relay’s best features is its ability to use a very small amount of current to switch large amount of current.

This ability helps to make the modern vehicle’s electrical system as efficient as it is. When electrical current flows thought wire, the wire has resistance, which limits the flow.

The longer the wires, more electrical current is lost to this resistance. Devices such as headlights or climate control allows the circuit to be routed in the shortest, most direct route between the battery, or source, and the device, or load.

A much smaller wire is routed to the relay from the controlling switch. This arrangement allows for less total wire length, a smaller gauge wire and more reliable, less expensive switches.

How much can it take?

A Standard Single Pole Double Throw (SPDT) automotive relay’s coil requires approximately between 130 and 170 milliamps (mA) to activate (between 1 and 2 tenths of an amp), and the Normally Closed contacts will switch 30 amps, and the Normally Open contacts 40 amps.

Some manufactures add a “quenching” resistor across the coil of the relay to absorb voltage spikes. These relays with the “quenching” resistor may need up to 170 milliamps (mA) to activate. On the average, most relays require about 150 mA to activate.

How does it work?

In the above images, note the five terminals, or “pins”.  A relay’s operation is really very simple. Ti understand its operation, consider the relay as having two sections, the coil, pins 85 and 86; and the contacts, pins 30,87 and 87a.

When negative Ground is supplied to one end of the coil, and Positive Voltage is supplied at the other end, the coil creates a magnetic field which activated the relay.

This magnetic field attracts the armature, which is attached to pin 30 with a flexible joint, just like pin 30 to pin 87. The terms used to describe the contact points thus: pin 30 switches between 87a and 87, so it is “Common” to both and is usually referred to as COM. In the relay’s normal condition, at rest, pin 30 is connected to pin 87a, making  pin 87a “Normally Closed” or NC Pin 87 is not connected to pin 30 at rest, so its status is “Normally Open” or NO.

Let's take it apart.

This type of relay is defined as “Single Pole Double Throw” or “SPDT. This term means that the single armature terminal (or pole, pin30) can be connected (or thrown) to two other terminals, pins 87a and 87.

The SPDT relay is one of the most useful configurations due to its flexibility-it can be used as a switching device, to isolate circuits, to interrupt circuits and to interrupt and switch at the same time.

Now let's see what we can tear up!

On the following pages there are some relay diagrams which show how to use relays to perform may functions such as trigger reversal, starter interrupt, add dime lights to flashing light output of alarm, using latching relays to change a pulled output to a constant output, and many other uses.

These relay configurations can be very helpful when installing an alarm, remote start, or keyless entry to perform different functions in the vehicle o which they are being installed.

Disclaimer

Mobile Information Labs assumes no responsibility with regards to the accuracy or currency of this information.  Proper installation in every case is and remains the responsibility of the installer. MIL, assumes no liability or responsibility resulting from an improper installation, even in reliance upon this information.

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