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LED flasher mod - choosing correct shunt resistor

19 December 2017

I've been working on a 2003 NSX and found this website very helpful. Can you guys help me choose the correct shunt resistor for my flasher module (38300-SL0-013)?

The LEDs I used are from superbright 1157-R27-T, 1157-A27-T

They draw 105mA and 207mA = 312mA total. I found this post below from another thread. I believe I need more resistance since my LEDs draw less than half of what was used below. I'm thinking at least a 0.3 Ohm is needed for my LEDs.

Can you guys confirm? Thanks.


"So you want to pick a resistor that will give no less than ~75 mV drop across the resistor when both bulbs on a side are good, and less than that when only one bulb is good.

Now we need to know info about your LED. I believe mine used 350 mA each. When you turn, you will be using one in front and one in back. So that's 700 mA for my LEDs. A 0.15 Ohm resistor creates a voltage drop of V = I*R = 0.700 * 0.15 = 0.105 V = 105 mV. That means the module will think that the lights are okay.

If one bulb fails, the current drops to 350 mA, giving a voltage drop of 0.350 * 0.15 = 52.5 mV, which is less than the detection voltage and hyperflashing would be triggered.

One thing to be careful about is the power dissipation. P = I^2 * R. So here, using the LEDs, P = 0.700 * 0.700 * 0.15 = 0.07 Watts. With Hazard lights on, you use all four bulbs, so the power goes up to 1.4^2 * .15 ~= 0.3 W. That means you need a 1/2 Watt resistor. Although, as mcrider pointed out in his very helpful writeup, you should have more of a margin for safety. He chose a 1W resistor. I actually used a 3.75W resistor, because it wasn't much more expensive and it seemed good. I used this one.

If you use incandescent bulbs with this modified flasher, you will have a current of about 9 Amps, so power dissipation of about 12 Watts. Poof goes the resistor and maybe starts a fire and the car burns to the ground. So please be sure to only use LEDs with your modified flasher module, and only the LEDs you used to calculate the appropriate resistor. (At least check the calculation if you change to different bulbs.)

Let me know if something didn't make sense. I chose the resistor that I went with, which was pretty easy to determine with a spreadsheet so I could try different values."