LEDs allow attractive and stylish auto interior lighting: Part 2 - dimming, color mixing

Example 3: Thermal protection
Schematic 3 below shows how thermal protection can easily be applied with just a simple NTC (negative temperature coefficient) resistor. The resistance of the NTC should be selected to be equal to R2 at the desired temperature for the down regulation of the LED current. Note that capacitor C5 may be needed if the wiring is relatively long compared to the NTC resistor. This capacitor is not so important because it controls current only when the LEDs become overheated and the current is regulated down.

Schematic 4 below depicts how changing R2 to a potentiometer (P2) makes for a very simple and cost effective dimming function.

Example 4: Dimming a single LED
Next, Schematic 5 illustrates how to implement a soft ramp turn on, an exclusive function that can be easily applied at low cost. Only a resistor, R3, and a capacitor, C6, need to be added. From the current calculation between R2 and R3, and the size of C6, a time constant can be calculated.

If C4 is large enough, R3 and R6 can be eliminated. If that method is selected, then C4 has to be fairly large with a risk for current leakage that can destroy the precision of the LED current regulation. 50μA implies 50μF to charge 1V in one second, but the voltage is never higher than that set by R2.

Diode D3 discharges C6 when the power is off. This resets the capacitor in order to create a constant ramp up rate every time the supply is switched on.

Schematic 6 below illustrates a slow up and down dimming function can be achieved with just a simple switch or potentiometer (P2)—giving a nice and exclusive look to an interior light for a very low cost. Note that this solution also makes it possible to have an adjustable light-level setting!