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

[Justin Owen]

FWIW - the original, bipolar implementation of that circuit <http://www.sdiy.org/juz/bipolar_LED.png> always worked well for me as is - although too would be interested to hear if there is a case for adding a cap and whether the non-inverting input R is necessary.

In hindsight I think the original unipolar circuit I posted was working as intended - the LED was doing exactly as it was being told. The problem was that that's not what I was expecting to see as visual feedback (i.e. LED dims to nothing at the bottom of the waveform cycle).

So it would seem the 1M R is just for the benefit of visual feedback on the unipolar version and not needed on the AC version because that definitely swings across 0V which definitely dims the LED to nothing.

HTH.

 

 

-----Original Message-----
From: Michael Zacherl [sdiy-mz01 at blauwurf.info]
Received: 27.11.2014 19:07:54
To: SDIY List
Subject: Re: [sdiy] Op Amp LED driver / Current Source problem/solution

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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