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FJR Motorcycle Forums
Technical & Mechanical Problems
Headlight relay wiring
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<blockquote data-quote="Fred W" data-source="post: 1327144" data-attributes="member: 3828"><p>Ummm... nope.</p><p></p><p>A 3A fuse will blow in a heart beat if you short the coil. 3 amps is not very big, and it certainly wouldn't let the wiring get hot, which is why the fuse is there, not to protect the relay.</p><p></p><p>But the more important point is, if you install a relay (or any other device) wired directly to the High Beam headlight power without an inline 3 Amp fuse (as suggested), if that relay coil shorts out it will, in the best case, blow the 25A Headlight fuse , leaving you with no high or low beam headlights at all. It will heat up and may burn up the small gauge wire that you probably ran from the headlight high beam to trigger the relay first (because 25A <em>is </em>a considerable amount of current) and light your bike on fire.</p><p></p><p>How hot could that wire get? Well some smart old guy named Ohm said that 12V X 25A = 300 watts. So in the case of a direct short circuit, the wiring will be heated by 300 watts of power before the headlight fuse burns out.</p><p></p><p>That is the reason why you would want to put a small fuse in line with the relay appropriately sized for the smallest wires in the circuit.</p></blockquote><p></p>
[QUOTE="Fred W, post: 1327144, member: 3828"] Ummm... nope. A 3A fuse will blow in a heart beat if you short the coil. 3 amps is not very big, and it certainly wouldn't let the wiring get hot, which is why the fuse is there, not to protect the relay. But the more important point is, if you install a relay (or any other device) wired directly to the High Beam headlight power without an inline 3 Amp fuse (as suggested), if that relay coil shorts out it will, in the best case, blow the 25A Headlight fuse , leaving you with no high or low beam headlights at all. It will heat up and may burn up the small gauge wire that you probably ran from the headlight high beam to trigger the relay first (because 25A [I]is [/I]a considerable amount of current) and light your bike on fire. How hot could that wire get? Well some smart old guy named Ohm said that 12V X 25A = 300 watts. So in the case of a direct short circuit, the wiring will be heated by 300 watts of power before the headlight fuse burns out. That is the reason why you would want to put a small fuse in line with the relay appropriately sized for the smallest wires in the circuit. [/QUOTE]
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FJR Motorcycle Forums
Technical & Mechanical Problems
Headlight relay wiring
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