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

[Phil Macphail]

Have you tried a capacitor across the LED? There is a good chance the op-amp is oscillating. 

Phil

Sent from my iPhone

> On 26 Nov 2014, at 18:43, Justin Owen <juzowen at gmail.com> wrote:
> 
> Yeah - that was one of the first things I tried. Fine if you're feeding it a bipolar +/-V signal (you only get one half) but a signal that only swings between 0 and +5V - you get the same problem where the LED doesn't fully dim.
> 
> To clarify - that 1M to the rail definitely sorts it properly - just not sure where on the 'ugly hack <> elegant solution' scale it is!
> 
> Ta,
> 
> - Justin
> 
> -----Original Message-----
> From: Tim Ressel [timr at circuitabbey.com]
> Received: 26.11.2014 16:55:47
> To: Justin Owen; SDIY List
> Subject: Re: [sdiy] Op Amp LED driver / Current Source problem/solution
> 
> Have you tried replacing the LED with a diode?
> 
> 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: Wednesday, November 26, 2014 8:06 AM
> Subject: [sdiy] Op Amp LED driver / Current Source problem/solution
> 
> 
> Hello,
> 
> I've previously used this: http://www.sdiy.org/juz/bipolar_LED.png as a bipolar LED driver where each LED shows either the positive or negative-going portion of a signal.
> 
> Have a need for a nice linear driver for a varying signal e.g. sine wave LFO going between 0 and +5V and assumed I could use the same circuit and just remove the bottom, negative LED.
> 
> That version doesn't turn the LED fully off when the input signal gets to 0V...
> 
> ...but adding a big old 1M R to the supply rail fixes that:
> 
> http://www.sdiy.org/juz/unipolar_LED.png
> 
> Is that a valid fix or is this potentially going to cause problems elsewhere in the curcuit/make bad things happen?
> 
> Thanks.
> 
> - Justin
> 
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