... f.i. for a vehicle's illumination
which shall light up even at low analogue track voltage. The circuit's dimensioning refers to the known 5V- plug-in bulbs. The resistor's value does not need to be maintained slavishly and the transistor also may be replaced with a similar type. The bridge rectifier may be replaced with a type that can cope with at least 30V. Just the 5V6 Z-diode is relevant as it determines the output voltage.
How many bulbs?
In the state shown on the left two of the bulbs mentioned above can be attached without hesitation. Using a heat sink with the transistor up to 5 or 6 of these bulbs may be attached - depending upon the heat sink's size.A different type of transistor may be able to provide more output power, but this is not very useful here. If more output is required, a solution incorporating a voltage regulator is to be prefered.