5-Pin SPDT Relay Diagram - Pin 30/85/86/87/87a Wiring & CarPC ACC/IGN Setup

This document is a two-part illustrated tutorial on 5-lead/pin SPDT automotive relays. The first section, titled "5 LEAD RELAY," shows two internal schematic views of the relay: on the left, with the coil unpowered (RELAY 'OFF'), pins 30 and 87a are connected; on the right, with the coil powered (RELAY 'ON'), pins 30 and 87 are connected. Coil terminals are labeled 86 and 85. The text explains that instead of a true 'off' position, the SPDT relay has an alternate electrical contact (87a). The second section shows a typical wiring setup for controlling power to a carPC (carputer) using the vehicle's ACC/IGN circuit: the car IGN or ACC circuit (optionally through a valet switch) feeds pin 86, pin 85 goes to ground, pin 30 connects through a fuse to the car battery positive terminal, and pin 87 feeds the carputer PSU, inverter, or accessories. The battery negative is grounded. The text notes this arrangement is generally used for laptop installs or 'no-write' setups and warns it is not good for a normal carPC because abruptly cutting power without proper shutdown can cause data corruption. Readers learn SPDT relay pin functions (30, 85, 86, 87, 87a) and a practical switched-power wiring example.