Schematic showing the coil and switch inside the relay, and then the pins of the relay86/85

Electrical device used in heavy duty automotive circuits. Normally, switching circuits carrying more than 3-4 amps is problematic. In order to minimise these problems, we use relays for switching. 

There are two circuits in a relay:

  1. A light duty switching circuit, terminals 86/85, which controls a different heavy duty circuit.
    For example, in a headlamp circuit, the light duty switching circuit is controlled by the headlamp switch. This is connected to 86/85 on the relay, and only requires about 150 mA to operate.
  2. This in turn controls the heavy duty supply, 30/87, that serves the headlamps.
    The main advantage of this arrangement is that the heavier wires that serve the headlamps do not have to be run into the cab and up to the driver's dashboard. Only the light duty switching wires have to go here.This is the only wire that has to be brought all the way back up to the headlamp switch.
    The heavy duty circuit, 30/87, is where wires can go directly from the live source, to the relay (terminal 30), out of the relay (terminal 87) and off to feed the load-in this case, the headlamps.

Note: Because 86/85 is a coil of wire, every time we open this circuit, it can produce a significant voltage spike. If there are any electronics in this circuit, they will be damaged by this spike, so we must take measures to reduce or eliminate the spike. We do this by fitting a clamping diode across 86/85. Check out the glossary entry for 'clamping diode' for more information.

Relay contacts and Mick Ryan holding them.

Electromagnetic relay.

» Course Glossary


Please note, if you want to contact the main Training Centre in Baldoyle, you need to ring 01-8167400.

If you want to contact the CP Fitters' Section, you can Contact Us by email, admin@cpfitters.info, or by telephone: 00 353 1 8167543 This number gets you through to the CP Fitters' Section, Baldoyle Satellite Unit.

Our Address is : Construction Plant Fitters' Section, Unit 116, Grange Way, Baldoyle Industrial Estate, Baldoyle, Dublin D13 CA48.
We operate in a satellite unit which is about 300 metres away from the main training centre. You can see the location of this unit here, in Google Maps. You can also Check here for directions to the Baldoyle Training Centre.

Find Us on Find us on YoutubeFollow cpfitters on Twitter

Is Féidir Linn! You can view all the menus on this site as Gaeilge by going to the homepage, top right hand corner, and dropping down the language selection menu. By default it reads English (en), so just select Gaeilge (ge) instead.