# How do I select the Correct Diodes and Resistors for my LEDs?

I've got a 2009 Harley-Davidson XR1200 (that can be seen at the bottom of my bio in my profile).

I've replaced the stock incandescent bulbs with nice looking LEDs; however, the stock bulbs were 2-way and had 3 wires (one for ground and one to each filament).

The new LEDs are simply ON/OFF and have 2 wires. Since LEDs do not draw as much power as the stock filament bulbs, load resistors (that come with the LEDs) are included. These resistors get very hot because they pass a lot of current so the electronics on the motorcycle will not think there is a burnt bulb.

I got to thinking that I could wire a Normally Closed relay just past the Turn Signal Module (TSM) so that the LEDs were ON until the TSM sent the "ON" signal - which would cause the LEDs to blink OFF.

The more I thought about how simple this would be to do, the more I thought that this has to have been done by someone before, so I did some research and found several posts where people have done this.

Different posts use different resistors, then I started discovering where some people were clever enough to place a diode and a resistor in parallel so that the LEDs would dim or get brighter with the blink signals, making them appear to be a 3 wire setup.

So far, the best "How To" I've found is a thread called Converting 2-wire to 3 wire Signals on the R6 Forum. This post talks about using electrical components from Radio Shack, but there is not one very close to me. It would be easier for me to buy the components online, if I knew what to ask for.

So, how would I go about getting diodes and resistors that are the right ratings for my application?

I've found a nice Blue Ox 6 Amp diode pack that is sealed to keep out moisture, which would be very nice on a motorcycle, but I don't know if 6 Amps is over kill or about right. eBay has 17,000+ diodes under \$3, but I don't know how to pick what I need.

Would the resistor in this diode/resistor setup get as hot as the load resistor in the setup I have now?

EDIT:

To answer the comments: The LEDs I have now only turn on when the turn signals are activated.

Q) What type of LEDs are you using?

Their website shows 12V - 13W on this drawing:

• While might not be what you need or safe, they used a 1Amp 1n4001 diode. – Passerby Jan 31 '13 at 20:03
• Form what I can tell, you want your LEDs to blink, rather than be on all the time (I'm guessing they are the turn indicator bulbs as you mentioned the TSM) There are a couple of solutions I can think of , but it would help to know the type of LED you bought (part number/datasheet) and battery voltage (12V?) to advise on resistor values and wiring. FWIW, you can get e.g. dual colour LEDs with three leads. – Oli Glaser Jan 31 '13 at 20:16
• I've edited the question to include LED data. Passerby: How do I read 1N4001? I see numerous similar diodes on eBay, but I don't know how to read these part numbers. Oli: The LEDs are only activated when the blinkers are "ON". I want them to stay DIM and go BRIGHT when blinking. – jp2code Jan 31 '13 at 20:41

For 12-13W and a 12V battery, that equates to roughly 1A, so you could do with at least 2A diodes. This means anything above this is okay, 6A is maybe a little overkill but will work.

Follow the diagram in the thread and you should get dim/fully lit on the turn signal, the 470Ω resistor should be a 0.5W or above.

The 470Ω determines how dim the light is when the blinker is off, and will only pass around 25mA maximum (see Ohm's Law). The diode does pass the fully lit current of around 12W / 12V = 1A, so it ideally needs to be rated for at least double.

• The link in Davos' answer shows many different resistors that fit the 470Ω spec. Would I grab any one? Does it matter? How hot should I expect a 470Ω resistor to get when passing 1A? Would it be a LOT like the load resistors or much less? – jp2code Jan 31 '13 at 22:21
• @jp2code - Any of that value will do for this application (carbon film, metal fim, etc) This resistor won't pass 1A, only a maximum of around 25mA (12V / 470Ohm = ~0.025A - neglecting any drops from the LEDs, so it will actually be lower) so the wattage will be a max of 12V * 0.025A = 300mW, in reality probably more like 150mW, so the half watt resistor will not get very hot. – Oli Glaser Jan 31 '13 at 22:28
• Found some Radio Shack components on eBay. Should Resistor 2W447 and Diode NTE4914 be OK? – jp2code Jan 31 '13 at 22:44
• The resistor you link to is 470kOhm, not 470Ohm - you want something like this The diode needs changing too, this 3A one is okay. – Oli Glaser Jan 31 '13 at 22:47
• Got it all done: youtube.com/watch?v=0XIgbCWpsX8 – jp2code Feb 16 '13 at 20:31