A transistor ensures that the output of the source is a constant current of approximately 25 mA . Therefore, LEDs can be connected without a series resistor. They shine from achieving the minimum voltage is always the same brightness, regardless of the applied voltage.
Maximum Number of LEDs
The number of LEDs that can be connected to a circuit board in the maximum number depends on the level of the supply voltage and the color of the LEDs.
Example of 18 V power supply: 4 white or blue LEDs or 8 different colored LEDs.
Supply voltage: Analog AC or DC voltage or digital voltage
Minimum voltage: 3 V (yellow LEDs) or 4 V (white LEDs)
Maximum voltage: 20 V AC or 24 V DC
Output current of about 25 mA
Board dimensions: approx 18 x 12 mm
Manual: http://www.tams-online.de/htmls/produkte/lks1/lks1_man_ger.pdf