[sdiy] current sink idea

[Roman Sowa]

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
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