[sdiy] current sink idea

[Roman Sowa]

It's not honking. I tested wit with rapid changes in control voltage and 
it didn't even overshoot, not to mention ringing

Roman

W dniu 2024-11-30 o 12:51, Mike Bryant pisze:
> As it's not connected directly to GND, I'd put a capacitor across the 
> supply pins of the op-amp.
> And also check it's not honking - the resistor may move the pole of 
> the internal compensation capacitor a little.
>
> ------------------------------------------------------------------------
> *From:* Synth-diy <synth-diy-bounces at synth-diy.org> on behalf of Roman 
> Sowa via Synth-diy <synth-diy at synth-diy.org>
> *Sent:* 30 November 2024 08:32
> *To:* synth-diy at synth-diy.org <synth-diy at synth-diy.org>
> *Subject:* Re: [sdiy] current sink idea
> well, the pictures haven't went through, so here's the schemo in 
> attachment
>
> Roman
>
> W dniu 2024-11-30 o 09:17, Roman Sowa via Synth-diy pisze:
> > Since criticizing circuit ideas is a new trend, here's something I
> > came up with 2 days ago.
> > Special needs require special solutions - I needed to control
> > display's backlit intensity from internal DAC of the microcontroller,
> > which unfortunately is unbuffered and rather high impedance. Obvious
> > simplest solution with emitter follower works, but even high beta
> > transistor loads the DAC too much. I could replace it with Darlington,
> > but then there's even more of DAC range lost and possibly even worse
> > repeatability between units.
> > Next step was well known current source/sink using opamp driving
> > transistor with current sensing resistor. But that's 2 active parts
> > and space is limited. So I made this (hoping ithis shows up in the
> > email body or attachment):
> > Current sink schematics
> > I moved the sensing resistor to opamp's power input thus eliminating
> > the need of external transistor. Quiescent current is 60uA so it
> > doesn't affect performance. It works up to 23mA of LED current, which
> > is all I needed. It can also be controlled by PWM with 1st order
> > very-low pass RC filter, like 1M+100nF adding free fade in/out effects.
> >
> > And here's how it looks:
> > assembled prototype
> >
> > Roman
> > ________________________________________________________
> > This is the Synth-diy mailing list
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>
> ------------------------------------------------------------------------
> *From:* Synth-diy <synth-diy-bounces at synth-diy.org> on behalf of Roman 
> Sowa via Synth-diy <synth-diy at synth-diy.org>
> *Sent:* 30 November 2024 08:32
> *To:* synth-diy at synth-diy.org <synth-diy at synth-diy.org>
> *Subject:* Re: [sdiy] current sink idea
> well, the pictures haven't went through, so here's the schemo in 
> attachment
>
> Roman
>
> W dniu 2024-11-30 o 09:17, Roman Sowa via Synth-diy pisze:
> > Since criticizing circuit ideas is a new trend, here's something I
> > came up with 2 days ago.
> > Special needs require special solutions - I needed to control
> > display's backlit intensity from internal DAC of the microcontroller,
> > which unfortunately is unbuffered and rather high impedance. Obvious
> > simplest solution with emitter follower works, but even high beta
> > transistor loads the DAC too much. I could replace it with Darlington,
> > but then there's even more of DAC range lost and possibly even worse
> > repeatability between units.
> > Next step was well known current source/sink using opamp driving
> > transistor with current sensing resistor. But that's 2 active parts
> > and space is limited. So I made this (hoping ithis shows up in the
> > email body or attachment):
> > Current sink schematics
> > I moved the sensing resistor to opamp's power input thus eliminating
> > the need of external transistor. Quiescent current is 60uA so it
> > doesn't affect performance. It works up to 23mA of LED current, which
> > is all I needed. It can also be controlled by PWM with 1st order
> > very-low pass RC filter, like 1M+100nF adding free fade in/out effects.
> >
> > And here's how it looks:
> > assembled prototype
> >
> > Roman
> > ________________________________________________________
> > This is the Synth-diy mailing list
> > Submit email to: Synth-diy at synth-diy.org
> > View archive at: https://synth-diy.org/pipermail/synth-diy/ 
> <https://synth-diy.org/pipermail/synth-diy/>
> > Check your settings at: 
> https://synth-diy.org/mailman/listinfo/synth-diy 
> <https://synth-diy.org/mailman/listinfo/synth-diy>
> > Selling or trading? Use marketplace at synth-diy.org