[sdiy] current sink idea

[Mike Bryant]

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
> Submit email to: Synth-diy at synth-diy.org
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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/
> Check your settings at: https://synth-diy.org/mailman/listinfo/synth-diy
> Selling or trading? Use marketplace at synth-diy.org
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