Relays, How to connect

Thread Starter


Joined Apr 22, 2007
I have a need to install a power relay in a circut with a ext. fuse box at the end. I have several Bosch relays of the correct amp rating but i'm confused with what to do with the switch lead on the 4 post relay, same with the 5 post. what swithch (Type) would I connect it to?-JSP2242

Tube Tech

Joined Jan 11, 2007
Pin numbers are not standardized among relay manufacturers. 5 pin tells us nothing.

Bosch relays don't have a 4 or 5 pin. Google Bosch relays under images tb, you find the images below

Typical relay:

5 and 6 in the images of the typical relay perform the same function as 30 on the Bosch relay. It's the common contact.

The typical relay is a DPDT switch. Pin 5 normally connects to pin 1, NC, normally closed. When the coil is energized, that contact opens and common connects to pin 3, the NO, normally open contact.

So: on this typical relay, A and B are the coil. 5 and 6 are the common contacts for the 2 switches. 1 and 2 are normally closed, they are connected to common when the relay is relaxed. 3 and 4 are normally open, connected the common contact when the relay is energized.

You could just read the tutorial: