Power transistors have gained popularity for high current switching, but lack the flexibility of the “make and break” functions of the relay.
A typical 12-volt relay requires a coil current of .150 Amps to energize. The relay contacts can switch currents up to 30 Amps, creating a power gain as high as 200 to 1.
The classic automotive relay is the Bosch E5000. There are many variations of this design, but thankfully most manufacturers follow a standard nomenclature.
Terminals 85 and 86 form the coil contacts. As current is passed through the coil, a magnetic field is produced, which actuates the switch contacts, causing terminals 30 and 87 to close. In order to pass current through the coil, 12 Volts must be applied to one side of the coil while Ground is present on the other. Most relay manufacturers recommend that terminal 85 should be connected to Ground, and 86 should receive 12V, but in fact the coil is non-polarized and either connection works. The only time polarity must be observed is when a “quenching diode” has been installed across the coil terminals.
Terminals 30, 87A and 87 form the contacts. When the relay is at rest, terminals 30 (Common) and 87A (Normally Closed) are connected. After the relay coil is energized, terminal 30 breaks its connection to 87A, and makes a connection to terminal 87 (Normally Open). Knowing this action of the contacts allows you to apply the relay in various configurations to achieve the desired results.
Common Types of Automotive Relays are identified by poles (the number of isolated common contacts) and throws (the number of isolated connections that the wiper arm contacts). The classic relay is a single-pole (one common contact) double-throw (one normally-open and one normally-closed contact), or SPDT.
Positive Trigger to a Negative Output
Converting a Negative Trigger to a Positive Output
Using Relays To Reverse Polarity