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Wiring Fundamentals: Relays

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Relays

04.01

00:00 - Relays form a critical part of our wiring harness construction.
00:03 They're the devices that we control to send power to the parts of the EFI system that require it, when they require it.
00:10 We'll look at them in detail now, as a clear understanding of their function is critical to understanding the power supply section of our wiring harness design process.
00:19 Relays are simply electrically controlled switches.
00:22 They allow us to use a very small current to switch on or off a part of the EFI system which draws a much larger current.
00:29 This is extremely useful as it allows us to use much smaller switches and wiring to control the high power parts of our EFI system.
00:37 Relays work by having us power an electromagnet coil within the relay which is sized to only draw a small current.
00:43 This electromagnet then flicks the switch, that is turns on the high power part of the relay which supplies the larger current to the part of the EFI system that requires it.
00:53 The most common automotive relay is known as a normally open relay, and has four pins, 85, 86, 30 and 87.
01:02 These numbers might initially seem a little odd but they are part of an automotive standard and almost all relays use them, so it is best to get familiar with them.
01:10 86 is the electromagnet coil positive.
01:13 This is the low current pin you connect to 12 volts to turn the relay on.
01:18 85 is the electromagnet coil negative.
01:21 This is the low current pin that you connect to ground to turn the relay on.
01:25 30 is the high current input.
01:28 This pin is usually connected to the battery positive terminal.
01:32 87 is the high current output.
01:34 This pin is connected to the EFI system components we're wanting to power.
01:38 When the relay is switched on using the electromagnet coil, this pin is connected within the relay to the high current input, pin 30.
01:46 This allows the high current to pass from pin 30 to pin 87 and then on to the EFI system components that require it.
01:54 With most relays it's not important which way around you connect the electromagnet coil pins, 85 and 86, as long as one is powered and one is grounded.
02:03 There are some relays out there though that use an internal device called a diode to help protect against the inductive voltage spikes the electromagnet coil generates.
02:12 On these relays you must power pin 86 and ground pin 85.
02:17 For this reason it's best to just adopt the standard of always powering pin 86 and grounding pin 85 to turn the relay on, as you'll never have a problem if you do it this way.
02:27 If you come across a relay that has a pin labelled as 87A instead of 87, it will be what's known as an normally closed relay.
02:35 The high power side of these relays works opposite to a normally open relay.
02:40 Pins 30 and 87A are connected internally by default.
02:44 And when you power the electromagnet coil it breaks the connection between them.
02:48 Occasionally you will find a relay that has both an 87 and an 87A pin.
02:54 This really is known as a change over relay.
02:56 Pins 30 and 87A are connected by default and when you power the electromagnet, it disconnects pin 30 from 87A and connects it to pin 87.
03:07 Because relays form such a critical part of our wiring harness design process, we'll look at a quick demonstration to show how one works.
03:14 Here we have a pari of 55 watt driving lights.
03:16 Which when powered from a 12 volt battery, draw a current of approximately eight amps.
03:21 The switch we want to use to turn the lights on and off is only rated for a current of one amp.
03:27 If we were to put the switch directly in line with the bulbs, when we turn them on it would likely overheat and be damaged.
03:33 Instead we use a relay to control the high current of the bulbs and we use the switch to control the electromagnet coil of the relay.