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

Roman Sowa modular at go2.pl
Mon Dec 1 08:42:31 CET 2014


that still needs a bit of offset to make the LED go off at lowest DAC 
output, so it then makes it go to the positive supply when that happens 
- the reverse diode needed too. So basicaly this has the same issueas to 
take care off, and as Neil has pointed out, now you feed up to 5mA of 
LED current straight from DAC output, so it will also have some problems 
go up to the power rail.

BTW, AFAIR initial circuit had 50k current setting resistor (100uA), 
while this one has 1k (5mA).

Roman

W dniu 2014-11-28 17:51, Tim Ressel pisze:
> Hi,
> I am wondering if an inverting circuit work better:
> http://www.sdiy.org/brotheo/LedDriver.png
>
> You probably still need a resistor to null the DAC offset at zero output.
>
> Tim Ressel
> Circuit Abbey
> 503-750-9331
> timr at circuitabbey.com
>
>
>
>
> ________________________________
> From: Justin Owen <juzowen at gmail.com>
> To: SDIY List <synth-diy at dropmix.xs4all.nl>
> Sent: Thursday, November 27, 2014 12:07 PM
> Subject: Re: [sdiy] Op Amp LED driver / Current Source problem/solution
>
>
> 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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