YH’s Lazy Man Constant Current Source

YH’s Lazy Man Constant Current Source

or YHLMCCS

Thanks to YH, here is another nice circuit tweak.

Fancy building a constant current source but don’t like the large parts count? Actually it isn’t that bad as it mostly consist of 2 transistors, 1 resistor and 1 or 2 LED. Or some combination like that. But lazy folks like me still want it simpler…

“If necessity is the mother of invention, laziness is the father of innovation.” —YH

That’s right! So how about a CCS that uses only 2 parts? Inspiration came from National Semiconductor’s datasheet for this ubiquitous regulator… LM317. Do a goole on this datasheet.

Recipe
Very simple! Just one LM317 and a resistor to set the current. No matter what you do, the LM317 will always have ~1.25V between Adj pin and Vout pin. We can take advantage of this feature!

So, if we want 80mA, like for the excellent Simple EL84, then all we need to do is solder a 15ohm resistor (R1 above) between Adj and Vout pin. 1.25V/15ohm = ~80mA. Then use Vin as input and Adj pin as output, hence your 2-terminal YHLMCCS. Voila!

Thanks to YH, the YHLMCCS works great in the Simple EL84! But be careful with your implementation if you want to use it in higher voltage environments. The LM317 doesn’t like more than 40V between Vin and Vout pins. You have been warned.