<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
<style type="text/css" style="display:none;"> P {margin-top:0;margin-bottom:0;} </style>
</head>
<body dir="ltr">
<div class="elementToProof" style="font-family: Aptos, Aptos_EmbeddedFont, Aptos_MSFontService, Calibri, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">
As it's not connected directly to GND, I'd put a capacitor across the supply pins of the op-amp.</div>
<div class="elementToProof" style="font-family: Aptos, Aptos_EmbeddedFont, Aptos_MSFontService, Calibri, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">
And also check it's not honking - the resistor may move the pole of the internal compensation capacitor a little.</div>
<div id="appendonsend"></div>
<div style="font-family: Aptos, Aptos_EmbeddedFont, Aptos_MSFontService, Calibri, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">
<br>
</div>
<hr style="display: inline-block; width: 98%;">
<div id="divRplyFwdMsg" dir="ltr"><span style="font-family: Calibri, sans-serif; font-size: 11pt; color: rgb(0, 0, 0);"><b>From:</b> Synth-diy <synth-diy-bounces@synth-diy.org> on behalf of Roman Sowa via Synth-diy <synth-diy@synth-diy.org><br>
<b>Sent:</b> 30 November 2024 08:32<br>
<b>To:</b> synth-diy@synth-diy.org <synth-diy@synth-diy.org><br>
<b>Subject:</b> Re: [sdiy] current sink idea</span>
<div> </div>
</div>
<div style="font-size: 11pt;">well, the pictures haven't went through, so here's the schemo in attachment<br>
<br>
Roman<br>
<br>
W dniu 2024-11-30 o 09:17, Roman Sowa via Synth-diy pisze:<br>
> Since criticizing circuit ideas is a new trend, here's something I<br>
> came up with 2 days ago.<br>
> Special needs require special solutions - I needed to control<br>
> display's backlit intensity from internal DAC of the microcontroller,<br>
> which unfortunately is unbuffered and rather high impedance. Obvious<br>
> simplest solution with emitter follower works, but even high beta<br>
> transistor loads the DAC too much. I could replace it with Darlington,<br>
> but then there's even more of DAC range lost and possibly even worse<br>
> repeatability between units.<br>
> Next step was well known current source/sink using opamp driving<br>
> transistor with current sensing resistor. But that's 2 active parts<br>
> and space is limited. So I made this (hoping ithis shows up in the<br>
> email body or attachment):<br>
> Current sink schematics<br>
> I moved the sensing resistor to opamp's power input thus eliminating<br>
> the need of external transistor. Quiescent current is 60uA so it<br>
> doesn't affect performance. It works up to 23mA of LED current, which<br>
> is all I needed. It can also be controlled by PWM with 1st order<br>
> very-low pass RC filter, like 1M+100nF adding free fade in/out effects.<br>
><br>
> And here's how it looks:<br>
> assembled prototype<br>
><br>
> Roman<br>
> ________________________________________________________<br>
> This is the Synth-diy mailing list<br>
> Submit email to: Synth-diy@synth-diy.org<br>
> View archive at: <a href="https://synth-diy.org/pipermail/synth-diy/" id="OWA9d721686-97d8-c11d-c1c8-b2708b7cb5bb" class="OWAAutoLink" data-auth="NotApplicable">
https://synth-diy.org/pipermail/synth-diy/</a><br>
> Check your settings at: <a href="https://synth-diy.org/mailman/listinfo/synth-diy" id="OWA4a1f9d33-a5de-d4f4-abc9-ec1aad057bb6" class="OWAAutoLink" data-auth="NotApplicable">
https://synth-diy.org/mailman/listinfo/synth-diy</a><br>
> Selling or trading? Use marketplace@synth-diy.org</div>
<div id="appendonsend"></div>
<div style="font-family: Aptos, Aptos_EmbeddedFont, Aptos_MSFontService, Calibri, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">
<br>
</div>
<hr style="display: inline-block; width: 98%;">
<div id="divRplyFwdMsg" dir="ltr"><span style="font-family: Calibri, sans-serif; font-size: 11pt; color: rgb(0, 0, 0);"><b>From:</b> Synth-diy <synth-diy-bounces@synth-diy.org> on behalf of Roman Sowa via Synth-diy <synth-diy@synth-diy.org><br>
<b>Sent:</b> 30 November 2024 08:32<br>
<b>To:</b> synth-diy@synth-diy.org <synth-diy@synth-diy.org><br>
<b>Subject:</b> Re: [sdiy] current sink idea</span>
<div> </div>
</div>
<div style="font-size: 11pt;">well, the pictures haven't went through, so here's the schemo in attachment<br>
<br>
Roman<br>
<br>
W dniu 2024-11-30 o 09:17, Roman Sowa via Synth-diy pisze:<br>
> Since criticizing circuit ideas is a new trend, here's something I<br>
> came up with 2 days ago.<br>
> Special needs require special solutions - I needed to control<br>
> display's backlit intensity from internal DAC of the microcontroller,<br>
> which unfortunately is unbuffered and rather high impedance. Obvious<br>
> simplest solution with emitter follower works, but even high beta<br>
> transistor loads the DAC too much. I could replace it with Darlington,<br>
> but then there's even more of DAC range lost and possibly even worse<br>
> repeatability between units.<br>
> Next step was well known current source/sink using opamp driving<br>
> transistor with current sensing resistor. But that's 2 active parts<br>
> and space is limited. So I made this (hoping ithis shows up in the<br>
> email body or attachment):<br>
> Current sink schematics<br>
> I moved the sensing resistor to opamp's power input thus eliminating<br>
> the need of external transistor. Quiescent current is 60uA so it<br>
> doesn't affect performance. It works up to 23mA of LED current, which<br>
> is all I needed. It can also be controlled by PWM with 1st order<br>
> very-low pass RC filter, like 1M+100nF adding free fade in/out effects.<br>
><br>
> And here's how it looks:<br>
> assembled prototype<br>
><br>
> Roman<br>
> ________________________________________________________<br>
> This is the Synth-diy mailing list<br>
> Submit email to: Synth-diy@synth-diy.org<br>
> View archive at: <a href="https://synth-diy.org/pipermail/synth-diy/" id="OWA428417bd-7fe9-306f-311d-53856e092846" class="OWAAutoLink" data-auth="NotApplicable">
https://synth-diy.org/pipermail/synth-diy/</a><br>
> Check your settings at: <a href="https://synth-diy.org/mailman/listinfo/synth-diy" id="OWAd93d067f-d951-3704-e0ac-0725fd969d50" class="OWAAutoLink" data-auth="NotApplicable">
https://synth-diy.org/mailman/listinfo/synth-diy</a><br>
> Selling or trading? Use marketplace@synth-diy.org</div>
</body>
</html>