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

[Ben Riggs]

Also,

Using the non-inverting input sources/sinks the current for the LED from the 0v. If it's your dgnd, probably not an issue. If your agnd, maybe best to use the inverting config.

Depends if you care if the 0v is clean or not.

Ben.

Sent from my iPhone

> On 27 Nov 2014, at 2:07 am, Needham, Alan <Alan.Needham at centrica.com> wrote:
> 
> There is no negative feedback path for negative inputs without that diode???
> Circuit becomes a comparator with zero hysteresis - asking for instability when the inputs match.
> Can you confirm that the op amp has +/- supplies here.
> 
> 
> -----Original Message-----
> From: synth-diy-bounces at dropmix.xs4all.nl [mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of Justin Owen
> Sent: 26 November 2014 17:43
> To: SDIY List
> Subject: Re: [sdiy] Op Amp LED driver / Current Source problem/solution
> 
> 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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