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This seems to be the right place to ask my dumb question.
When adding stuff to my bikes, I always run a in line fuse.
OK, I understand that part but why do I need to add a relay?
What does a relay do?
 

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What kind of stuff are you adding to your bike?

Relays are used when you don't want to directly tap into the power of some other item, such as lights. For example, driving/auxiliary lights are powered by the accessory wires, but are 'triggered' through the relay by the headlight power. In essence, the relays is a switch that only flips when it receives power from another trigger source. Your headlight comes on, it switches over and allows power to flow from the accessory line into your driving lights.

If you're using the accessory line, you don't necessarily have to have an in-line fuse. There is a stock 10amp fuse in your fuse box under the left side cover under your seat. If you're doing any kind of custom wiring where you run a line yourself, that's when you want to add an in-line fuse.

That said, adding your own fuse won't really affect the circuit at all. It just provides another break point - which can be a good thing, but it's not necessary if you already have a break point of the same amp rating. Just another thing to fail either through burning out or shorting out.


Give us some more info and we'll see if we can steer you the right way. :)
 

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Relays are high current switching devices. You can use a very tiny switch or a transistor to turn on and off high current devices through the use of relays (solid state of electro-mechanical).
In line fuses are always a good idea. Properly fused with the correct rated fuse you can isolate circuits so that if there is an electrical problem you do not wipe out everything on a given common sourced circuit until the fault is identified and corrected. If you have, let's say three things on a common fuse and it blows you have to trouble shoot each individually to identify the root of the problem. Whereas 1 fuse per item clearly controls and identifies the problem area without taking additional items off line from the common sourced fuse at the same time. Your choice.
There are very nice, however slightly expensive, circuit breaker panels that allows for multiple devices to be sourced in a single, resettable, circuit panel. Or, there are many aftermarket automotive fuse boxes also available.
I've put a lot of do-dads on my bike and have used two separate fuse boxes to accommodate them. One is fused and sourced direct from the battery and the other is switched, through a fuse and, a relay of course! Overkill? Probably, but I'm retired, from that EE field, and being neat about it is personally rewarding.
 

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This seems to be the right place to ask my dumb question.
When adding stuff to my bikes, I always run a in line fuse.
OK, I understand that part but why do I need to add a relay?
What does a relay do?
The short version is that a relay will allow for you to tap into a low amperage line to operate a higher amperage device.

Recently I added an air horn to my 900LT. The stock horn was driven by a 10A circuit. The new air horn draws between 8 & 9 Amps. To prevent the possibility of an overdraw, I used a 30A rated relay to allow for a direct tap off the battery (20A fused). This way the lower amperage feed to the original horn can drive the low draw relay which when engaged will allow the 20A fused current to flow to the compressor.
 
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