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Performance and value thats second to none
Since 1886 Bosch has continued to accelerate the progress of automotive technology with precision engineering and the continued search for new innovation.
Bosch Micro Relay with Internal Suppression Resistor
This Bosch 12V, bracket mount SPST relay is rated for 20 amps. It has a suppression resistor across the coil to prevent kickback to the coil-driving circuitry. This eliminates the need for an external coil suppression device.
Specifications:
Coil operating current: 154mA
Operating temperature: -40 C to +100 C
Mechanical life: 10,000,000
Motor load (N/O): 15A (30A in-rush), 200,000 cycles
Resistive load: (N/O): 20A, 300,000 cycles

This Bosch type 12V, 40A relay is popular in automotive sound and security installations. Plastic housing with mounting tab. SPDT, 30/40A.
A relay is nothing more than an electronic switch. Completing a circuit through a coil in the relay causes a reaction that in turn causes a connection between two or more terminals in the relay. In some relays, breaking the connection to the coil causes a connection between two or more different terminals on the same relay. Technical stuff aside, the following will describe how to use a standard Bosch 4 or 5 pin 12 volt relay to accomplish some things you couldn't do (or couldn't do as easily) without a relay.
Here is a basic "Bosch" relay and circuit diagram. These relays are about 1" square, are generally black plastic and sometimes have a mounting tab on them. They are all numbered identically with the exception of pin 87A. Some do not have pin 87A. These relays cost no extra to have that pin so don't worry about that part. If you don't need that pin, you simply do not use it. See the schematics below to determine if your circuit will require that pin for your application.