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Old 02-19-2015, 08:44 PM   #1
Guano11 OP
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Basic Wiring Question

So I'm replacing the stock tail light on my XR400 with one of these LED tail light assemblies from Baja Designs. It serves as both a running light and a brake light and has 3 wires to connect:



The OEM tail light has a single filament (running light only). The only wires required are these two, shown here disconnected from the original fender's wiring harness:



So the pieces of the electrical puzzle are:

Bike's OEM 2-wire harness:
  • Blue
  • Green
(Don't know which is (+) or (-) .... but does it matter?)

LED light assembly's 3-wires:
  • Blue
  • Black
  • Red

Brake light pressure switch
  • 2 black wires




Here's a pic with all the players congregating in the area under the seat (brake switch is still in packaging):




Below is a sketch showing how I think it would schematic out. Assuming it's correct (big assumption, fellas), the question is what does the other end of the brake switch connect to?

My semi-educated guess is that it goes to the (+) battery lead, same as the red wire -- but I'm asking the experts before I fry the tail light ...or worse, the bike itself.

FWIW, there are no other electrical mods; intent is to run the LEDs right off the stator, with no battery in the circuit. Good idea?



Am I completely off-base? Or at least in the ballpark?
Any tips on what to look out for are greatly appreciated -- thanks Fellas!
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Old 02-19-2015, 09:19 PM   #2
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A digital voltmeter would do you wonders or even a test light. I converted a TT600 long ago to street and don't recall much, however Im guessin the break light switch completes the circuit and actually grounds the second filament thus lighting up the break light.
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Old 02-19-2015, 11:59 PM   #3
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You've got the right idea. Just branch it off the red wire. For that matter, any positive lead will be fine, but using the red wire is a clean way to do it.
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Old 02-21-2015, 12:24 PM   #4
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Quote:
Originally Posted by scapegoat View Post
Im guessin the break light switch completes the circuit and actually grounds the second filament thus lighting up the break light.
Quote:
Originally Posted by slackmeyer View Post
Just branch it off the red wire. For that matter, any positive lead will be fine.

So that's one vote for ground (negative lead) and one vote for positive lead.....

Anybody got a tie-breaker?
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Old 02-21-2015, 12:27 PM   #5
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Is the XRs electrical AC or DC? My no electric start KTM was AC(no battery in system).
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Old 02-21-2015, 02:44 PM   #6
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Ground to ground
Red to running/ low output of tail light
Red through the brake switch to bright output of tail light

You will need to splice the red to make one to two wires.
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Old 02-21-2015, 10:04 PM   #7
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XR400's are AC systems from the factory. Unless your bike has been converted to DC the LED tail light will not work.
Does your bike have a battery?
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Old 02-21-2015, 10:59 PM   #8
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Quote:
Originally Posted by FatBob View Post
XR400's are AC systems from the factory. Unless your bike has been converted to DC the LED tail light will not work.
Does your bike have a battery?
Crap! No battery!

I used the same LED tail light successfully on another XR400; that bike, however, DOES have a full Baja Designs dual sport conversion complete with battery pack.

So with this new development, what are my options?? I'm kinda past the point of no return and certainly prefer NOT to go back to stock fender & light.

Geez.....always something!
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Old 02-22-2015, 05:16 AM   #9
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Quote:
Originally Posted by Guano11 View Post
Crap! No battery!

I used the same LED tail light successfully on another XR400; that bike, however, DOES have a full Baja Designs dual sport conversion complete with battery pack.

So with this new development, what are my options??
How is the brake light on the other XR400 connected?

Option 1: Add a rectifier, regulator, and battery. (This is the standard, safe option.)

Option 2: Add a rectifier, zener diode, and capacitor. (This is the fun, hobbyist option.)

You really need a rectifier to prevent reverse polarity to the LEDs or the power supply built into the taillight. I don't know how sophisticated your taillight is.

You probably need something to prevent high-voltage spikes. A regulator does this well, but for a simple system, a zener diode does it too.

You may want or need something to store just enough energy to keep the voltage steady between voltage spikes from the alternator and regulator. LEDs can turn on and off quickly without any problem but if there's a regulator built into the taillight, the regulator itself may not deal well with frequent power loss and surge.
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Old 02-22-2015, 04:03 PM   #10
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Quote:
Originally Posted by Grinnin View Post
How is the brake light on the other XR400 connected?

Option 1: Add a rectifier, regulator, and battery. (This is the standard, safe option.)

Option 2: Add a rectifier, zener diode, and capacitor. (This is the fun, hobbyist option.)

You really need a rectifier to prevent reverse polarity to the LEDs or the power supply built into the taillight. I don't know how sophisticated your taillight is.

You probably need something to prevent high-voltage spikes. A regulator does this well, but for a simple system, a zener diode does it too.

You may want or need something to store just enough energy to keep the voltage steady between voltage spikes from the alternator and regulator. LEDs can turn on and off quickly without any problem but if there's a regulator built into the taillight, the regulator itself may not deal well with frequent power loss and surge.
Thanks for all of that ^^. I like the zener diode option, assuming it can keep the spikes at bay. I will call BajaDesigns on Monday; they've been pretty helpful thus far -- but I've yet to tap them with an electrical issue. If they try to sell me their rectifier, that's fine as well. Who knows, maybe their rectifier is nothing but a diode bridge anyway...

I'll see if I can summon my HS electrical tech class memory banks and come up with another schematic -- this time with an AC power source instead of a battery.
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Old 02-22-2015, 05:53 PM   #11
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ah.... LEDs are diodes.... they polarity sensitive, but will work in AC.... only half wave though. in any case you would still need a regulator to limit voltage. in order to use full wave output from your generator you would have to wire the rectifier so it receives power from both sides of the stator winding. where I'm going with this is that many of the "AC only" bikes have one stator lead going to ground.... inside the windings. sometimes you can get to it, sometimes not.

there may be another way if you are only running brake & tail lights.

get a rectifier like this 3 amp one....

http://www.mouser.com/ProductDetail/...cwj%2fIBMHk%3d

hook the AC input leads on the rectifier to bike's main power and ground. connect the DC output leads to a regulator chip like this.....

http://www.mouser.com/ProductDetail/...fdLA4QCZmWE%3d

this one can take 1.5 Amp continuous.... should be enough for LED lights (tail lights). the big trick is that everything from the rectifier to the LEDs has to be isolated. none of it can go to frame ground.... including the ground in the tail light housing.... it all has to connect only to the rectifier.

total cost.... $1.17

if you wanted to add a battery, there are small 12V for emergency lighting. then you would want the regulator chip to be a 7813 or 7814 (last 2 digits are the voltage)

edit: if you wanted a 3 amp regulator chip get a MC78T12CT. Mouser doesn't carry them any more but I found them on fleabay for 2 bucks
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Old 02-22-2015, 06:11 PM   #12
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as for a zener to regulate voltage.... old Brit bikes used them. they have to be pretty big & have a heat sink otherwise they fry
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Old 02-22-2015, 08:45 PM   #13
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Quote:
Originally Posted by Beezer View Post
ah.... LEDs are diodes....
Oy! Ya got me!

Quote:
Originally Posted by Beezer View Post
in order to use full wave output from your generator you would have to wire the rectifier so it receives power from both sides of the stator winding. where I'm going with this is that many of the "AC only" bikes have one stator lead going to ground.... inside the windings. sometimes you can get to it, sometimes not.
^^This could be a show-stopper^^

Quote:
Originally Posted by Beezer View Post
there may be another way if you are only running brake & tail lights.
get a rectifier like this 3 amp one....
http://www.mouser.com/ProductDetail/...cwj%2fIBMHk%3d

hook the AC input leads on the rectifier to bike's main power and ground. connect the DC output leads to a regulator chip like this.....
http://www.mouser.com/ProductDetail/...fdLA4QCZmWE%3d

this one can take 1.5 Amp continuous.... should be enough for LED lights (tail lights). the big trick is that everything from the rectifier to the LEDs has to be isolated. none of it can go to frame ground.... including the ground in the tail light housing.... it all has to connect only to the rectifier.
total cost.... $1.17

if you wanted to add a battery, there are small 12V for emergency lighting. then you would want the regulator chip to be a 7813 or 7814 (last 2 digits are the voltage)
edit: if you wanted a 3 amp regulator chip get a MC78T12CT. Mouser doesn't carry them any more but I found them on fleabay for 2 bucks
So these 2 components negate the requirement to dig into the stator leads? I can just tap the power off the connector in the first post?
Been a looong time since I've breadboarded anything, but I'd hate to hardwire everything up only to find nothing worked (wouldn't be the first time....).
Anyway, very appreciative of the education; just wished I'd got it BEFORE starting the project!
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Old 02-24-2015, 04:12 AM   #14
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Quote:
Originally Posted by Beezer View Post
ah.... LEDs are diodes.... they polarity sensitive, but will work in AC....
While LEDs are diode, many LED lights these days include current regulators that may fail with reverse voltage. A dual-intensity light, such as a taillight, almost certainly has a regulator.

While LEDs are diodes, I wouldn't assume that a LED taillight works on AC.
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Old 02-24-2015, 08:30 AM   #15
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About a year ago I bought a 2002 XR250R wit the BD kit on it. It's street legal, in IL, but I wanted a better tail light (than the stock running light assembly or BD rear light on the rear fender). I took the opportunity to figure out the wiring from the BD kit (and created my own wiring schematic).

From what I was able to determine:
The RED wire is Hot for the Running Light(s)
The Black wire is Ground
The Blue wire is Hot for the Brake light.

My kit has Red and Blue wires coming from the hydraulic brake switch(es). If your kit is the same (or similar), I think what I posted, above, should help solve your issue.
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