Description

This article is from the Car Audio FAQ, by Ian D. Bjorhovde (ianbjor@mobileaudio.com) with numerous contributions by others.

5.12 How do I wire a relay in my system? [IDB]

There are two types of relays that are commonly used in 12-volt
automotive applications: Single-pole Double Throw (SPDT) relays, which
have 5 pins, and Single Pole, Single Throw (SPST) relays, which have 4
pins. Depending on the application, you can use either of these; for
remote turn-on leads (see Section 5.11), an SPST relay is fine,
SPDT relays are often used in alarm installations. Make sure that you
get a 12-volt relay - this specifies the voltage required to make the
relay "switch."

The connections on the two types of relays look like this:

                 SPST                       SPDT
         =====================      =====================
                  (87)                       (87)
              +---------+                +---------+
              |   ---   |                |   ---   |
              |         |                |         |
         (86) | |     | | (85)      (86) | | --- | | (87a - center)
              |         |                |         | (85 - right)
              |    |    |                |    |    |
              +---------+                +---------+
                  (30)                       (30)

Pins 85 and 86 connect to the coil which causes the relay to switch.
On both relays, pins 30 and 87 are normally disconnected. When the
relay is activated (switched) pin 30 and 87 are then in contact. The
difference with the SPDT relay is that in the "normal" state, pins 30
and 87a are in contact.

To hook up a relay (either kind) for a remote turn on, make the
following connections:

*Pin 30*
+12 Volts (Battery +)

*Pin 87*
Amplifiers' remote turn-on terminal

*Pin 86*
Head unit remote turn-on lead

*Pin 85*
Ground

*Pin 87a*
No connection (SPDT only)

 

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