I've wired up a pair of 10w (each) LED aux lights to my motorcycle (a Kawi KLX250S). I wanted to run them off of the high beam switch. That part was easy.

I replaced the headlight bulb with a low-beam-only HID kit. With this installed, hitting the high beam switch turns off the headlight (while turning on the aux lights). I would like to keep the headlight on.

What I did was run a 12v line off of the relay powering the aux lights (it's a relay with twin 87 outputs), but it's backfeeding and causing the aux lights to be on all the time. I'm told I need a diode to correct this, but having no electrical training (I can solder and follow directions but can't read a schematic to save my life), I don't know how to select the proper one. Any help would be very much appreciated! :)


Connect a diode from the 12V side of the aux lights to the 12V side of the headlight. That way the headlight will still be powered when the aux lights are on. Actually they will be powered just a little less. A diode drops around a Volt in this case, so they will get a little dimmer, but not a lot. This should be more than offset by the aux lights coming on.

You need a diode that is rated for whatever current your headlight draws. I am not familiar enough with motorcycles to know what typical headlight current is, but I expect a few Amps at least. You can figure this out from the light wattage rating. Divide that by 12 to get a rough idea of current in Amps. For example, if the headlight is "100 Watt", then this tells you the current is about 8.3 Amps. There is some slop in this, so in this case you want a diode rated for at least 10 A.

Diodes also have a voltage rating, but in this case that shouldn't be much of a issue because 12 V is rather low. A "20 V" diode should be OK, but I'd feel better about anything rated for 30 V or more. If you can find something called a "Schottky" diode, that will have a little better characteristics but may be hard to find and more expensive if you do. A ordinary diode is good enough.

The diode has to be installed with the right orientation. You want the "anode" end at the aux light and the "cathode" end at the headlight. These are usually not spelled out explicitly like that. Sometimes the cathode end is marked with a band if the diode is the shape of a cylinder. Otherwise, there might be a sort of arrow symbol:

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    \$\begingroup\$ I'd still insist on the Schottky. IIRC a car headlight is something like 60W, and a motorcycle's will be the same. At 1V that's 5W the diode has to dissipate. The Schottky will only have to deal with half of that. \$\endgroup\$ – stevenvh May 19 '12 at 15:28
  • \$\begingroup\$ "Sometimes the cathode end is marked with a band". I was tempted to change "Sometimes into "almost always", but maybe you wouldn't agree, so I'll leave it as it is. \$\endgroup\$ – stevenvh May 19 '12 at 15:29
  • \$\begingroup\$ @steven: For cylindrical packages I've always seen the band. I was figuring the OP might go to a auto parts store and ask for a diode. Something for automotive use probably comes in a metal case with mounting flanges, which probably doesn't have the band. I have see such things with a diode symbol on them. I also agree about the Schottky, but am not so sure how available those are in places the OP will likely look. "Automotive" diodes with mounting flanges will be able to take 5 W. I was worried too much information would confuse this OP. \$\endgroup\$ – Olin Lathrop May 19 '12 at 15:34
  • \$\begingroup\$ Diodes in tabbed packages like TO220 often have no anode/cathode marking at all. You have to check the datasheet to know which is which. \$\endgroup\$ – Federico Russo May 19 '12 at 15:55
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    \$\begingroup\$ Thanks to everybody for the comments!! The headlight is 35 watts (HID). I will look for the Schottky. There's a good electronics store in town (not Radio Shack) that should have one. \$\endgroup\$ – RobG May 19 '12 at 16:46

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