Understanding Relays

[Boneman]

I'm not sure why, but most people seem to have an irrational fear of relays. I keep seeing posts and comments to the effect of "how can I wire this up without having to use a relay?" or "can this be done without a relay" like they are some big nasty, life sucking evil device of some kind that one needs a degree in electrical engineering to understand.

They are not scarry things folks and are in fact not that difficult to work with, especially the standard 12 VDC automotive ones!

So I have scowered the web and gathered some basic information about relays and how they work in order to help dispell this myth of relays being these nasty, scarry things. I believe they are just misunderstood and only want to be loved. Please give them a chance!

What is a relay:

In simplest terms, it's a little switch that throws a bigger switch.

A relay is used to isolate one electrical circuit from another. It allows a low current control circuit to make or break an electrically isolated high current circuit path. One example where a relay is useful in car audio is in the power antenna or remote output of a head unit. We already said that the average remote lead can only handle about one half of an amp of current. If a circuit with a large amount of current must be controlled by the remote output lead of a head unit, a relay could be used to buffer the remote output from the head unit. The basic relay consists of a coil and a set of contacts. The most common relay coil is a length of magnet wire wrapped around a metal core. When voltage is applied to the coil, current passes through the wire and creates a magnetic field. This magnetic field pulls the contacts together and holds them there until the current flow in the coil has stopped. The diagram below shows the parts of a simple relay.



Why use a relay?

There are several reasons you should use a relay instead of just wiring your accessories directly to the battery. The most important reason is so that your accessories won't be ON all the time. Sure, you can put a switch in the circuit, but what's to keep a little kid (motorcycles attract kids, you know) from flipping the switch on your parked bike? Or forgetting to flip the switch off yourself? If either of those things happen, you might find your bike's battery dead the next time you try to use it.

The little switch gets "thrown" by your motorcycle's electrical system when you turn the ignition on. The little switch triggers a solenoid (or solid-state equivalent) which flips the big switch ON. The big switch side of the relay is the "power" side that will be used to run accessories you're adding to your bike. The way we'll wire it, the relay will be ON when the bike's ignition is on, and off otherwise. Thus your accessories will also be on only when the ignition is on.

The Famous Bosch Relay: (common 12 VDC Automotive Relay)

The most commonly used relay in car audio and security is the Bosch relay. The picture below is the bottom of a Bosch relay. Take note of the markings (85, 86, 87, 87a, & 30) near the terminals.



Internal Workings of Bosch relay:

The following diagram shows what those external terminals are connected to on the inside of the relay. When there is no difference of potential (voltage) across terminals 85 and 86 (the coil), the relay's movable contact (connected to terminal 30) is held, by spring tension, against the electrical contact which is connected to terminal 87a (the normally closed contact). In other words, when no voltage is applied the the relay coil, terminal 87a is connected to terminal 30. When 12 volts is applied to the relay coil (terminals 85 and 86), the movable contact (connected to terminal 30) is pulled down/in by the electromagnet (coil) so that it physically contacts the electrical contact which is connected to terminal 87. Again, in other words, if battery voltage is applied to the relay coil (terminals 85 and 86) terminal 30 will be connected to terminal 87. The red dashed line shows the path in which electrical current flows from/through terminal 30 to the contact of terminal 87a



Remember that the relay coil has to have a difference of potential between terminals 85 and 86 in order for the coil to pull the armature in/down. This means that you may apply battery voltage to either terminal 85 OR 86 and then ground the OTHER terminal. The positive battery voltage OR the ground connection may be broken to make relay switch terminals (87 to 87a).


Basic Wiring and Applications of a Relay:








Additional Info on Relays:

http://www.r1200gs.info/howto/relay.html

http://www.lunghd.com/Tech_Articles/...elayBasics.htm

[Raid The Revenge]

On my old VF-1000, we had a seperate pinball machine button for starting the bike. We also had a cheapass lever for flicking the highbeams on or off.

What...a...pain...in the ass.

[mpb218]

The reason I avoid relays is I don't want to have to worry about mounting more crap than I need to and want that clean look... and I HAVE a degree in electrical engineering haha. If I came across something I wanted to do, I'd use the heck out of 'em though. Good post.

[SANGER_A2]

Details on how I used one on mine here (A Couple of Electrical Mods) to have my grips etc only on when the bike was on.

A "real-world" example for ya!

[urbanj]

I prefer the definition "controlling a large current circuit with a low current circuit" but thats just me.

It's just better to use them when needed than not use them.

[SovietRobot]

Good write up!

You lose power the longer a wire is, and when you're sending power through an OEM harness, you're restricted by the length and the gauge of the wire

Wiring directly to the battery and using a relay to activate that accessory will give you more, cleaner power, and it won't burn out your bike's harness(horn button for example) by trying to draw more power than it can handle.

For example, wiring HID lights directly to the battery will give you better performance, and on most cars(not sure about bikes), wiring the headlights straight to the battery will give you brighter lights because they are getting the power they need(assuming you use the correct gauge of wire and whatnot).

[MACH3]

ALL I CAN SAY IS WOW! what a sweet writeup i really needed these diagrams to follow your instructions about your horn mod on the webite! would be great if you buy these relay and fuse wiring thingys as 1 loom to fit ready!:

[reiobard]

very nice writeup! thank you for the effort.

[chicagogreg]

Great write up!

[MACH3]

BONEMAN i understand all instructions on your site regarding horn installation! (much helped with this diagram) just one question what do you do with the negative wire going to stock horn?