LED Foglight Project Options

[Spoke]

The design of an early foglight LED board has begun. mepstein sent me a couple of foglight fixtures to use to design the board. 2 main issues are fitment and heat dissipation. If things work out ok the design should be able to be re-laid out for the later square foglight.

The board will have 8 Cree XP-G3 high power surface mount LEDs with 350ma current each. These will burn about 1W of power with a total power dissipation expected to be less than 10W. Hopefully these will be almost as bright as a headlight.

Each LED will have its own focusing lens. The lenses are Carclo 10192 elliptical 20mm lenses. The light pattern for the 10192 lens is shown below.



This is the spectral dispersion for the lens.

[orthobiz]

Why do these need a converter when your other taillight/sidemarker lights don't?

Paul

[Spoke]

Why do these need a converter when your other taillight/sidemarker lights don't?

Paul

There's a couple of reasons for using a converter on these lights. First the current is a lot higher for each LED. 350ma per LED for this foglight vs 20ma per LED for brake/turnsignals. Thus the current limiting resistor for brakes doesn't burn that much power. Total power burned by all the current limiting resistors on the brakes is less than 1W and it is evenly distributed across the board.

The brakes/turnsignals have 56 LEDs configured as 16 strings of 4 LEDs in series. 4 red or amber LEDs only need about 2V each or 8V total to turn on. In the foglight there are 8 LEDs total in one series string. White LEDs need about 3+ volts to turn on. Thus the total voltage of the string is over 24V and the need for a dc-dc converter.

With resistors providing current limit on the brakes/turnsignals, there is some brightness difference between battery voltages of 12V to 14V although it is not so noticeable.

For the foglights the light output must be constant for all operating voltages. This converter will turn on when the applied voltage is about 9V and the LED current will remain constant over the entire battery voltage range. This way there will be no flickering or dimming if the battery voltage is high or low.

In the 356 front light there were 19 LEDs in series and the voltage needed to drive that stack was over 50V.

[Morrie]

Ideally you would have a converter with no resistors running in constant current mode rather than constant voltage. Or at least that’s how the growers are doing it.

Why do these need a converter when your other taillight/sidemarker lights don't?

Paul

There's a couple of reasons for using a converter on these lights. First the current is a lot higher for each LED. 350ma per LED for this foglight vs 20ma per LED for brake/turnsignals. Thus the current limiting resistor for brakes doesn't burn that much power. Total power burned by all the current limiting resistors on the brakes is less than 1W and it is evenly distributed across the board.

The brakes/turnsignals have 56 LEDs configured as 16 strings of 4 LEDs in series. 4 red or amber LEDs only need about 2V each or 8V total to turn on. In the foglight there are 8 LEDs total in one series string. White LEDs need about 3+ volts to turn on. Thus the total voltage of the string is over 24V and the need for a dc-dc converter.

With resistors providing current limit on the brakes/turnsignals, there is some brightness difference between battery voltages of 12V to 14V although it is not so noticeable.

For the foglights the light output must be constant for all operating voltages. This converter will turn on when the applied voltage is about 9V and the LED current will remain constant over the entire battery voltage range. This way there will be no flickering or dimming if the battery voltage is high or low.

In the 356 front light there were 19 LEDs in series and the voltage needed to drive that stack was over 50V.

[Spoke]

Ideally you would have a converter with no resistors running in constant current mode rather than constant voltage. Or at least that’s how the growers are doing it.

(IMG:style_emoticons/default/agree.gif)

This is a boost converter running constant current of 350ma. There are a couple of control loops. One for the FET/Inductor current, one for the LED current and a 3rd constant voltage output in case of open LEDs to protect the components from over voltage.