NPN BJT Emitter Follower and zener for 5 series LEDs powered by 12V to 20V circuit

If I wanted to light 5 series red LEDs with 12V, I can do so if I protect them with 150Ω. Unfortunately “12V” batteries, tend to have a higher than 12V almost all of the time.

NPN BJT emitter follower sets about 10mA through 5 series red LEDs at 12V to 20V
NPN BJT emitter follower sets about 10mA through 5 series red LEDs at 12V to 20V

13V pushes the 5 series LEDs with 150Ω current limiting resistor to it’s limit of almost 20mA of current (shown in a different video than the one below).

A 12V (reverse biased) zener diode with 1k resistor can be added to an NPN BJT (such as 2N3904) wired as an Emitter Follower to limit the voltage across the load to about 12V.

In fact, when I set the supply voltage to 20V, there’s enough current flowing through the zener diode get about 12.6V across it, based on my measurement in the video below. Meaning that about 12V gets set at the Emitter, thanks to the Base-Emitter voltage drop of about 0.6V to 0.7V.

The video below is set to YouTube members only until 6 am CST Feb 1 2024. After then, anyone can watch it.

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