Parts:
Red Lights
Amber Lights
White Lights
Note: white lights have a forward voltage of 2.9V unlike red and amber lights, which have a forward voltage of 2.2V.
Drivers:
Constant current boost converter
https://www.digikey.ca/product-detail/en/texas-instruments/TPS61165DBVR/296-27597-1-ND/2340765
Heat sink:
Calculations:
maximum temperature of Rev3 Lights PCB = 50 C
ambient temperature (generous estimate) = 35 C
power dissipation (red or amber lights) = 2.2V * 6 LEDs * 0.35 A = 4.62 W
power dissipation (white lights) = 2.9V * 6 LEDs * 0.35 A = 6.09 W
Required thermal resistance (red or amber) = (50-35)/4.62 = 3.24 C/W
Required thermal resistance (white) = (50-35)/6.09 = 2.46 C/W
https://www.digikey.com/product-detail/en/ohmite/PA-T2X-38E/PA-T2X-38E-ND/9607314 (two for each LED board will result in an equivalent thermal resistance of <4 C/W)
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Design
Optimizing PCB thermal performance:
https://www.cree.com/led-components/media/documents/XLamp_PCB_Thermal.pdf
Prototype Rev 1:
3 LEDs in series with no additional parts. Lights were sufficiently bright and stable, but became very hot (around 80 C) very quickly.
Prototype Rev 2:
6 LEDs in series driven by a boost converter. PWM signal provided by 555 timer. Potentiometer included in 555 timer circuit to allow experimentation with duty cycles. LEDs and current sense resistors are kept on a separate board to account for the physical constraints of the car.
List of issues:
- Lack of decoupling capacitors
- Incorrect capacitor value for 555 timer (which set the incorrect frequency for the CTRL pin on the driver).
- Pins configured incorrectly
- Lack of ground pour for stability
- Inductor was not placed close enough to the potentiometer (no boost was measured).
Further Testing for Rev 2:
- Measure current at the connectors on startup to determine whether additional capacitors are required at the connectors.
Prototype Rev 3:
- Fixed the mistakes from Rev 2
- Separate LEDs from other components to preserve space
- A zener diode is placed parallel with each LED to prevent entire LED string from burning out
- LED board will have a heat sink
List of issues:
- Too many decoupling capacitors
- LEDs are not bright enough for some lamps - Rsense must be adjusted, or more LEDs must be added for those lamps.
Testing for Rev 3:
- Determine what resistor is required to replace the potentiometer and set the duty cycle of the PWM signal.
- Start looking into light dissipation (with mechanical team)
General Testing for Lights:
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Relevant Regulations
ASC:
WSC:
Electrical
4 Red LEDs
Buck Controller
PWM dimmable via potentiometer
100-500mA drive current
Mechanical
Size constraints (Updated: Feb 19, 2020):
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