The “Bosch style” relay is also known as a 5-pin, 5-prong, SPDT, or changeover relay.
As compared to a standard 4-pin SPST relay (with terminals 85, 86, 30, and 87), the 5-pin Bosch relay has an extra terminal 87a, which is powered only when the relay is not energized. That is, when the control circuit connecting to terminals 85 and 86 is open, high-power current is allowed to flow to terminal 87a.
In many applications, the relay is energized via a simple ON/OFF control switch. When the control switch is open and the relay is not energized, only the N.C. terminal 87a of the Bosch relay has power. In contrast, when the control switch is closed, the relay is energized across terminals 85 & 86 and power is fed to the N.O. terminal 87 for both 4-pin and 5-pin relays.
4-pin relay acts as a high-power ON/OFF switch
5-pin Bosch relay acts as a high-power ON/ON switch
Why Called Bosch?
Bosch is an engineering and technology company that produced the commonplace 5-pin relay for the automotive industry over many years. Although the relay division at Bosch was acquired by Tyco Electronics in 2006, the 5-pin relay is still referred to as a Bosch-style relay.
While the “Bosch” tag is reserved for 5-pin relay variants, some distributors may erroneously mark 4-pin relays as Bosch-type. If you’re shopping specifically for Bosch relays, it’s important to check for the number of terminals.