[Kevin Darrah] wanted to make a simple 3.3V regulator without using an integrated circuit. He wound up using two common NPN transistors and 4 1K resistors. The circuit isn’t going to beat out a cheap linear regulator IC, but for the low component count, it is actually pretty good.
In all fairness, though, [Kevin] may have two transistors, but he’s only using one of them as a proper transistor. That one is a conventional pass regulator like you might find in any regulator circuit. The other transistor only has two connections. The design reverse biases the base-emitter junction which results in a roughly 8V breakdown voltage. Essentially, this transistor is being used as a poor-quality Zener diode.
At normal temperatures, the pass transistor will drop about 0.7V so dividing the 8V across the “Zener transistor” by two gives 4V for the other device’s base. Drop the 0.7V and you wind up with 3.3V. [Kevin] does some quick tests and the performance really isn’t as bad as you might expect.
If you want to read about issues with Zeners, we posted about it. Note, though, that by that post’s definition, the Zener here is used as a reference, not a regulator. We also covered a video on Zeners.