Hello!
This is no complaint I am just curious about the reason why LM317 seems to not be able to sink current.
I wish to describe how I discovered this:
I was building a primitive opamp-tester which only focused on if the amps could trip or not so I made a tripping point (and virtual ground for the plus-inputs) of 3,5V with the LM317, then I used a simple switch which set the high and low input voltages just above and below the tripping point, the opamp was supplied with 9V as Vcc so in my world this means in general something like 7,5V as maximum swing.
My tiny system was set to go to low output level (red) at powering up and to high outpul level (green) while pressing the switch.
Using a flawless opamp (TL074 which was fresh out of storage) and turning on the voltage, all LEDs turned red.
So far so good.
Now, pressing the switch made all leds go green (high) which they should but when I released the switch they stayed green.
Measureing the voltage at the output of the LM317 revealed that it was almost as high as the input.
I wondered about this and decided to test if a Zener diode of slighter higher voltage than the tripping point (think I chose 3,9V) parallell over the output would fix it for me.
And yes, it did.
But why?
All I can see is that the sinking current when the outputs went high went through the Zener instead of the LM317.
Conclusion (if my LM317 is not faulty), LM317 cannot sink current (my total sinking-current/LED-current wasn't more than a couple of milliamps).
Nice forum you got here, I actually know very little of electronics but I am willing to learn.
Best regards, Roger
