[sdiy] Op Amp LED driver / Current Source problem/solution

[rsdio at audiobanshee.com]

I haven't yet figured out why the LED stays on without the 1 MΩ resistor. It doesn't seem that op-amp bias voltages could be that high.

However, just analyzing the working circuit shows that your 1 MΩ and 50 kΩ resistor acts as a voltage divider to keep the (-) input at about 0.57 V. That's enough to make sure the LED stays reverse biased when the op-amp output goes towards 0 V. Of course, when the LED is on, that voltage divider sees an outside current (coming through the LED), so it's not working purely as a voltage divider. I mean, a passive voltage divider is never perfect.

Maybe you need a resistor to ground on the (+) terminal, but I haven't fully analyzed this yet.

Brian


On Nov 26, 2014, at 11:16 AM, Justin Owen <juzowen at gmail.com> wrote:
> 
> Just in case my original post wasn't clear...
> 
> This config: http://www.sdiy.org/juz/unipolar_LED.png
> 
> works just fine with the required input signal between 0 and +5V.
> 
> My concern is a) I was wondering why the LED wouldn't dim completely without that 1M R and b) I've never seen that 1M R added to this config before so I'm worried I've fixed one problem at the risk of creating others (or just done something else dumb).