LEDS on 12v - for cars and trucks
When connecting LEDs to cars and trucks, you have to allow for an increase in voltage during the time when the battery is being charged.
Normally the battery sits at 12.6v, but when charging, it can rise to 13.5v or slightly higher. If you put 3 white LEDs in series, the "head" voltage will be about 1.8v when the battery is 12.6v, but increase to 2.7v when it is charging.
This will increase the current through the LEDs by about 50% and will be noticeable as the brightness will increase considerably. The extra current may also damage the LEDs.
To keep an even brightness, we suggest using strings of LEDs with just two white LEDs and 220R 0.25watt resistors as shown in the following diagram.
Red, green, orange, yellow and blue LEDs have a different characteristic voltage across them when illuminated and so you can have more LEDs in a single string, with a suitable current-limiting resistor. Here is the answer for each color:
25mA is the MAXIMUM for 3mm and 5mm ordinary, high-bright or Super-bright LEDs.