One point, along the lines of the distinction between series and shunt regulators, is that the 431 can be converted to a series regulator....
Hello, c4757p, as expected, it works.
Since I was being so dense earlier, I decided to make the most out of this experience.
With only 3 transistors at hand, one happens to be a 2222a. So I disassembled the TL431 regulator circuit and assembled the circuit you depicted. I use a 2k+2k for voltage divider, 100ohm 1W for current limiter between source and circuit.
@ 1ma load it draw 15ma (Vout=5.013 from 20V source)
@ 10ma load it draw 23ma (Vout=5.013)
@ 20ma load it draw 33ma (Vout=5.013)
@ 40ma load it draw 50ma (Vout=5.013)
@ 80ma load it draw 86ma (Vout=5.011)
@120ma load it draw 122ma (Vout=5.009)
@130ma load it draw 131ma (Vout=4.844)
Letting the TL431 controls another transistor neatly turned it into more linear than fixed current draw. With a smaller resistor instead of the 100ohm, I can pull in more current while output stays stable at just over 5V. Looks like it takes around 10ma to drive this circuit, so it won’t save power in this case - but it sure was a nice learning experience.
In hind sight, I was being dense. The LM317 is floating without ground. What comes out of the output pin must equal to what goes into the input pin minus the small amount of reference current. So, input current varies with the load current. Obvious.
The TL431 has a ground, the current is controlled by the limiter; so, whatever the load doesn’t use needs to go somewhere. Ah “shunting to ground” in action. Obvious.
Only a dummy can’t see that. I kept locked into thinking – is there a feature/function hidden in the LM that caused the name difference. No – it is as the name implied for the TL431, shunt regulator - the excess current shunted to ground.
Good thing about a free country is, everyone is free to be a dummy. Bad thing is, I have been exercising that right too often lately.
Thanks for helping! I an having fun with this stuff,
Rick