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

Michael Zacherl sdiy-mz01 at blauwurf.info
Thu Nov 27 20:04:38 CET 2014


So returning to Justin's original post, referring to http://www.sdiy.org/juz/bipolar_LED.png
it would make sense to have the 1M resistor there as well?
And the cap anyway.

On 27.Nov 2014, at 9:51 , Neil Johnson <neil.johnson71 at gmail.com> wrote:

> Why does the resistor "fix" the problem?  You need to consider the
> op-amp input offset voltage (Vio) - think of it as a little voltage
> source in series with the Vin+ pin.  Now, depending on the sign of the
> offset voltage you wil either find that even with the control input at
> 0V you still have some total control voltage due to Vio.
> 
> If you have a very efficient LED then even a few microamps could be
> enough to produce a very small glow (I have some blue LEDs that need
> microamps to glow nicely).
> 
> Your extra resistor introduces an external bias current that pulls the
> Vin- pin up a little bit, so that when your control input is at 0V the
> op-amp is trying to pull the Vin- pin down, but it can't due to the
> reverse biased LED, which gets turned off very hard.  So no glow, but
> not good for the LED.
> 
> The simpliest solution is a 1N4148 anti-parallel to the LED to protect
> it, and keep the 1M resistor to bias up the Vin- pin.



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