Relays

Today, I finally got around to working on the wiring of my car. Some while ago, I had ripped out all of the aftermarket wiring done on my car (done by me 3 years ago) because I wanted to clean up the wiring work and make a few changes. Exactly how all the added systems and controllers are wired together on my car is something I may discuss some other time (if there is any interest in that) but here's what 2 out of 4 pages of dog283-made wiring diagram for it all looks like;

Anyway... The topic for today is Relays. Those magical little black things that are a substantial part of most modern automotive electrical system. I use a lot of them in my car. Not counting the original wiper motor relay that the car still uses, 22 will be going in to this current build. Which, is quite a lot for a car that runs carbs and has no audio system or A/C.

I have them in 3 clumps, 2 in the engine bay, and one in the trunk and a few scattered around the car.

Most car enthusiasts with a fair bit of working knowledge know what a relay is, but when I chat with people about the technical aspects, not too many really understand how they work or what exactly they do. Most just know that they are 'needed' so today let's have a little closer look at what exactly a relay does and maybe you can get an idea why I use lots of them.

First, what is a relay. it is basically a switch controlled by a small electromagnet. There are two types of relay switch; Normally Open (N/O) and Normally Closed (N/C). Quite a number of relays made have both types of switches built in. When the relay is energized the internal switch will move either to connect the switch in an N/O relay or to disconnect the switch in a N/C relay.

To better understand this, let's refer to the diagram printed on the relay itself. Most relays have standardized the naming convention of the terminals.

30, 87 and 87A are the switched parts of the relay. The Potter & Brumfield relay pictured has both N/O and N/C switches. 30 and 87A is the N/C connection, meaning that the default of the relay has 30 and 87 connected together, and when the relay is energized the connection between 30 and 87A will be opened, disconnecting the two. This is useful for having the relay switch things off.

30 and 87 is the N/O switch. When the relay is energized the connection will be closed, connecting the two. This is useful for having the relay turn things on. The relay can of course be used to divert power. Having 30 connected to 87A when the relay is off then transferring the connection to 87 when the relay is energized.

85 and 86 control the electromagnet. Either one can be the trigger. 85 connects to a positive 12 volt source and 86 is grounded. Only one of these will always be connected. Connecting the other one will trigger the electromagnet to energize the relay. So, for example, in a ground triggered relay 85 is connected to a positive 12 volt source, when the 86 terminal is grounded the relay will energize activating the internal switch. Or in a positive triggered relay 86 will be grounded, and when 85 gets a positive 12 volt current the relay will be energized.

The most common use for a relay is to switch things on and off. This is useful because the trigger switch can be extremely low voltage. It only needs 150 milliamps to trigger the electromagnet, while the relay switch itself can hold 30 or 40 amps. This is nice for switching on high load accessories like fog lamps and auxiliary fans because the high current wires from the battery can be routed through the relay to the accessory without having to pass through the switch making the wiring of the car safer.

Relays also be used to give accessories direct connection to the battery. By connecting the lights or horn directly to the battery they get a stronger more constant power supply so their performance is better and more consistent. I use 1 relay for each of my headlamps, brake lamps and signal lamps in the car so they are nice and bright.

Relays can also be used to invert a signal. In my MSD setup for example, the high speed timing retard of the Digital 6 box needs a positive 12v signal to activate. The MSD rpm activated switch however gives a ground. So for that setup, I use a ground triggered relay so when the rpm activated switch gives a ground to the relay (85 connected to postive, 86 switched ground), it sends a positive 12v signal to the MSD box (30 connected to postive being connected to 87 which is connected to the MSD box.)

What else can be done with relays is only limited to the users imagination. I've seen a setup where a relay triggered by the dome light dimmer switch was used so that when the car was parked the radio would keep playing when the engine was shut off until a few seconds after the door was shut. The two step rev limiter of car used for the launch control is activated by the hand brake on warning light. When the hand brake light is grounded is activates a relay that sends a positive 12 volt signal to the MSD box to activate the 2 step rev limit while the handbrake lever is up and release the 2 step limit when the hand brake lever is released. Basically any 'event' that has a switch going off can be used to automatically trigger another switched event by simply using a relay to connect the two. Using two relays in series can then be used to have two criteria for a switched event happening. You just have to be creative with it.