Brake Lights Not Working/04 Prius

Here are measured voltages. (Though sometimes I wonder how accurate my voltmeter is.):

Undamaged light: CN1: 11.5v. CN2: 11.2v with brake light off; drops to 7.4v with brake light on.

Damaged light: CN1: 11.5v. CN2: 11.1v with brake light off.

I'm embarrassed to admit that it didn't occur to me to measure the current flow.

I don't have answers to any of the technical questions. My electricity education began with Ohm's Law and ended (the same day) with Joule's First Law.

Surely any electrical engineer would immediately recognize the reason for the 120 ohms bridging + and - in CN1?

 

So this is right in line with what I mentioned; looks like 2.46v per LED forward voltage.

I haven't looked at the resistor schematic, but my guess is the added parallel resistance is to prevent the LEDs from glowing from leakage current. With those high-efficiency LEDs, even a few microamps can make them visibly glow or flicker.

 

[Here] is the PDF datasheet on the SuperFlux LEDs. They state that the forward voltage @ 70ma is indeed typically 2.6v. So if anyone wants to attempt repair, the part # is HPWT-RD00. Future Electronics has them in stock for 40 cents each. I recommend you replace all of them at once.

Note that these have a lambertian emission pattern which is typical for car lights, so they have to be oriented correctly.

 

Based upon DeeTrumpet's voltage measurement: 3.8V to the resistor string / 24.8 ohms = 153 mA current flow available to power the six LEDs. Since there are two strings, then 77 mA might flow through each string.

I assume that the voltage measurement is when the car is IG-ON? Might be interesting to see if the measurement is close to 13.8V when the car is READY.

Per the data sheet cited by pEEf, note that 70 mA is the maximum rating at 25 degrees C. However if DeeTrumpet's voltage measurements are accurate, then the LEDs have 10% more current applied than the spec sheet indicates is max. This might help to explain their failure. And as vertex suggested, once one LED fails, that will cause its twin to also fail from overcurrent.

 

Great conclusion Patrick! The supplier must have not met the DOT brightness requirement with a low battery, so decided to go above spec. Most reputable LED manufacturers under-rate their products, but still, exceeding the datasheet max specs is not good engineering practice.

Keep in mind it's also possible that they are not using this exact part. There are lots of competitors that may have similar units with higher specs. Still, I'd bet my lunch money!

A few years ago I built a new LED tail/brake light for my motorcycle. I put a lot of high-flux LEDs in there, and added a circuit that pulses a high current for a short duration when the brake is first applied, the drops down to a level well below the abs max rating. This acts sort of like a strobe which gets instant attention from following drivers, yet isn't obnoxiously bright to stare at when stopped for a longer duration.

I used a microcontroller with a feedback circuit to regulate the current (brightness), so it's always the same regardless of battery voltage and will never stress the LEDs.

I guess building a new circuit for the Prius lights similar to above with proper regulation might not be a bad idea. I'll put it on my list of 1000 things to do when I get the time. While I'm at it, I'll also make them have a taillight function.

I wish I had more time in the day!