Temperature
The LEDs used for the lights can get very hot very quickly. It is important that the temperature of the lights stays within limits that will not interfere with the functionality of any nearby circuits, and will not pose a safety hazard.
Below is a graph displaying The graphs below represent the temperature of revision 3 of the tail light prototypes board over time. All trials were performed with 12V supply.
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A temperature probe was used to measure the temperature. PWM was at approximately 50% duty cycle. No enclosures were used during testing.
The temperature stabilized at around 47 C after approximately 15 minutes.
Next steps would include testing the lights in an enclosure.
Duty cycle was set to 100%.
Red Lights: back
Temperature appears to be unstable throughout the trial, this is because the temperature probe came off of the board. Temperature at the end is somewhat more stable. Temperature stabilized to around 66 degrees.
The power supply drew 0.43 A of current (5.16 W).
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Red Lights: front
Temperature stabilized to around 68 degrees.
The power supply drew 0.43 A of current (5.16 W).
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White lights: back
Temperature stabilized to 79 degrees.
The power supply drew 0.59 A of current (7.08 W).
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White lights: front
Temperature stabilized to about 90 degrees.
The power supply drew 0.59 A of current (7.08 W).
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Voltage Boost
Since a boost converter is used in the driver board, a boost in output voltage is expected.
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Duty Cycle (%) | Voltage Boost (V) | Supply Current (mA) |
|---|---|---|
<1 | -0.503 | 170 |
15.14 | -0.494 | 170 |
44.05 | -0.429 | 175 |
72.24 | 0.077 | 295 |
88.17 | 0.273 | 360 (recommended current is 350 mA) |
Very high (>99) | 0.398 | 410 |
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Burnt out LED
When a single LED was removed, the remaining LEDs were fully illuminated at approximately 3V. Constant current mode was activated on the power supply. Supplied current was 250mA.
Brightness
Red lights:
Brightness was measured in a dark room with a photometer. Potentiometer was set to its maximum. Brightness was measured to be 280 lux from about 30cm away. This works out to be 25.2 candela, which is not bright enough for the tail lights (requires 60 cd)The LEDs were driven directly at 12V, without the driver (they burnt out in an accident, guess who did it). What was the current limit of the LEDs when doing these tests?
This website was used to convert from lux to candela:
https://www.rapidtables.com/calc/light/lux-to-candela-calculator.html
Amber reference lights:
60 lux from 1 foot = 5.6 cd
the ruler that was used was slightly reflective.
Red lights:
12 V * 0.21 A = 2.52 W
850 lux from 1 foot = 78.9 cd
This circuit included the zener diodes used for protection.
the ruler that was used was slightly reflective.
Further Brightness Testing
Further lights testing was conducted by Micah Black.
Things to keep in mind while testing:
The lights and the light meter have different spectral responses - that means that the light meter will not pick up all of the light produced at different wavelengths.
The light meter that was used was purchased here: https://www.amazon.ca/Dr-Meter-Digital-Light-Meter-Illuminance/dp/B005A0ETXY
Spectral Sensitivity of meter:
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Spectral Power of lights (cool white, amber, and red are used for the car):
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So we have to manually apply a correction factor based on the amount of measured light and the frequency that it is at.
Test Setup:
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Test Results:
| View file | ||
|---|---|---|
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Test Summary:
Light Output
Color | Average Cd per LED (at 350mA) |
|---|---|
Red | 54 |
Amber* | 8 |
White | 70 |
*This was measured very low, but it seemed just as bright as the other colors to the eye. The erroneous value can probably be attributed to the spectral power of the led not matching the datasheet, and/or the spectral sensitivity of the meter. This was manually corrected for the red LEDs.
For reference, the values that we measured were slightly higher than what was given in the LEDs datasheet. The LED datasheet gives a minimum value, so the measured values can be somewhat trusted.
The temperature affects the lumen output a fair bit, much more noticeably with Amber lights. If I had read the datasheet fully before jumping in to the tests, then I would have realized why:
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These results guided the decisions for the number of LEDs and the drive current to use for each lights board. The decisions can be found on the Details of Lights Boards page.