This LED diode test is suggested by the Texas Instruments optoelectronics manual. The transformer current can be between 50 mA and 300 mA and the voltage between 6 and 9 V.
This circuit with 1W of output was found in a Mullard manual from 1960. The consumption is low and...
This circuit was found in an old documentation. It converts DC voltages and its maximum current...
In the figure we have the way to implement a 5 to 45 V zener diode with 50 mA of maximum current...