How to build a switched power supply for accessories
I wrote the following article for another site and thought I would double post here for anyone interested.
Why a relay?
Each pre existing circuit in your bike is fused. If you install a power hungry accessory like heated grips or tank mounted espresso maker it will start blowing fuses if connected to an existing circuit such as the head light circuit and if you up the fuse the original device is no longer protected.
With a high current accessory like heated grips they need to be powered directly from the battery on their own fused circuit. The only issue with a direct battery connection is the connected devices always have power so can run down the battery if left on. A switched power source that is only active when the engine is running is more desirable. This is where a relay comes in. A relay will allow you to take the power draw off the battery where it should be but only be active when the relay is turned on by the ignition.
Several bike accessory sources like Twisted Throttle sell a pre packaged bike accessory relay block for wiring in accessories for about $80. It does the exact same thing as the ~$10 circuit you can build yourself from an automotive relay.
How to build a relay controlled circuit
A relay is basically an electrically operated switch. Buy a generic 4 pin automotive relay. They're cheap and available at most auto parts stores. The switch circuit is very low power, while the switched circuit can handle high current flow. Energize the coil side of the relay from a source that is only on when the engine is running. For example pin 85 to Ground and pin 86 to the tail running light. Power the relay from the POS terminal of the battery (pin 30) via a fuse in an installed inline fuse holder of suitable capacity. You now have a high current power source, that is only energized when the ignition is ON, from pin 87. Optionally you can connect pin 87 to a wiring block to provide more than one connection point if you have multiple accessories.
Note some automotive relays will have five pins with the additional pin labeled 87a. The extra pin functions the same as pin 87 with the exception that it supplies power when the relay is not energized and switches off when the relay is energized, the opposite of pin 87. If you use a relay of this type remember to insulate the unused pin so it does not short out on the frame.
How to make a reliable electrical connection
There are several ways to connect wires together but several of them are problematic and the source of reliability issues. In particular tap connectors that punch through the insulation to tap into a connector are unreliable and should be avoided if possible. The best way to join wires is with solder. Twisting wires together creates another source of potential electrical problems.
To create a good solder joint twist the wires together as a first step. Next heat the twisted wires from the bottom (heat rises) with a hot soldering iron for a minute or two. If you hold the solder against the heating wires (from the top) it should melt into the wires when they get hot enough. Let the solder melt into the hot wires rather than melting the solder directly with the soldering iron. Remember to pre heat the soldering iron for 5 minutes or so before using - they don't heat up instantly.
A clean soldering iron tip will transfer heat much more effectively and just work better. Keep the tip of the soldering iron clean when it is hot, by brushing against a damp sponge or cloth, usually after every solder. Emery paper can be used to sand oxidation off a cold soldering iron tip that has not been used in a while. A clean tip will have a shinny coating of solder. If it is dull it needs cleaning.
A wire stripper like that pictured to the right is cheap (~$5-10) to purchase at a specialty electronics store like The Source and the best choice for removing insulation A dedicated wire stripper makes it easy and quick to remove insulation from wires with a high degree of precision. Knives, razor blades and in particular pliers like wire strippers perform poorly and often cut a portion of the wire as well as the insulation.
An electrical connection can be insulated with either electrical tape or a heat gun and shrink wrap. Do not use duct tape, scotch tape or other kinds of tape as the insulation properties and/or adhesive durability are can often be poor.
Last edited by twowheels; 01-07-2013 at 12:50 AM.