I was given a little grief (failed) when I got my car inspected in the evening tho (for the annual state inspection) because when I use my turn signals, the rear upper LED in my brake system on the opposite side will flash VERY dimly (I guess some sort of feedback loop or something also).
Personally I resisted using resistors....I wanted to minimize electrical draw and resistors don't help in that regard.
Andrew
Why didn't you just used diodes in the circuit then your feedback to the 3rd brake light wouldn't happen? Diodes only allow current to flow in 1 direction.
Also I'm confused on why resistors don't help minimizing current draw? That's exactly what they do; they draw current pushed by voltage and dissipate it through heat...so anything downstream of them doesn't have full current flow. That's exactly how your regular halogen tail lights work. They are resistors, that when get hot enough, release energy in light and heat...
I have LED brake bulbs installed now but I am getting the flashing glow plug idiot light because it's detecting low amperage. ..
Just for anyone's info if they're wondering.
Lets take a 3157 bulb because I just did the calculation for my truck which takes these for the front markers, and I went to switchback LED's. If anyone knows Dodge trucks, knows they are the most picky regarding lamp out warnings, LED's, HID projectors, etc etc.
A 3157 halogen lamp is rated at approx 2.1 A at 12.8V.
R=V/I , therefor 12.8V/2.1A = 6.09Ohm
P (Watts) = I^2 * R, therefor power dissipate = P = 2.1^2 * 4 =26.9 W approximately. This is given off in heat/light
Now lets say that we switch to an LED bulb. I'll use the switchback draw for example. @ 12V, there is a current draw of 250mA (.25A), which when you calculate for power above, dissipates about 3W. The reason for the low current draw is because of
how LED's emit light. Lots of info on that.
.25A of current draw is much less than the 2.1 A of current draw, which is why we get the hyperflash when there are no inline resistors. Vehicle thinks there is a bulb out.
So what do we need to do? We need to make up the difference in current, so that the vehicle thinks the circuit is identical of one that has a halogen bulb.
The LED is requiring .25A, and the halogen bulb requires 2.1A
2.1A-.25A = 1.85A.
So, we need something inline with our circuit that is going to "mimic" or "draw" that extra 1.85A so the vehicle thinks its a halogen.
And that's exactly what resistors are made for.
R=V/I
R=12V/1.85A
R=6.5 Ohm
So we need a 6.5 ohm resistor in this circuit to make it think that there is a halogen bulb in there. 6 Ohm resistors are extremely common for this reason (the gold ones you see everywhere)
Asleep yet? We're almost done
Now, we know that resistors dissipate their power in the form of heat (in the case of a halogen, heat and light). How much exactly?
Let's go back to our P=I^2*R equation
P=2A^2*6 ohm = 24W.
So our resistor needs to be rated to handle this power. That's why there is a Watt rating on the resistor. Common ones are 25W and 50W. What's the physical difference? Size. To be safe because people will not always mount these where there is direct airflow, it's safe practice to design for double the power that is calculated. That would be around 48W. Which is why most choose the 50W, 6 ohm gold colored resistor.
So what else could you do if you don't want to wire in a resistor? Theoretically ( and some have done this) you could wire a regular halogen "dummy" bulb in-line with the circuit and tuck it away somewhere (not touching anything critical). No error to vehicle and LED works as it should.
Not sure if this will help the OP or anyone for that matter, just figured I throw out the information behind why hyperflash happens and how to get to the fix.
