[sdiy] Signals leaking into the PSU?

[René Schmitz]

On 24.02.2023 04:24, Scott Bernardi via Synth-diy wrote:
> Here's a way to keep a constant current driving a flashing LED using a 
> differential pair.  It only takes one extra resistor and 
> transistor compared to the standard transistor
> I whipped up a page for it:
> https://www.bernacomp.com/elec/og2/constant_current_led.html

+1

I'm using this myself.

Also for smooth signals: If you put a second LED in the collector of the 
other transistor you get a nice cross fading giving a good idea of the 
voltage.

Then put a 1k at the base of the other transistor as well.


Best,

  René


>
> On Thu, Feb 23, 2023 at 3:50 PM Neil Johnson via Synth-diy 
> <synth-diy at synth-diy.org> wrote:
>
>     Hi Florian,
>
>     > The problem is, that there is current draw switched on and off,
>     which puts a sudden load on the supply. The trick is to keep the
>     current constant, but redirect it over a transistor (excerpt from
>     one of my schematics):
>     >
>     > The R19 should be fed by your U1C pin 8. If you have only 12V as
>     supply, you should use a 3k3 or 4k7 resistor for R24.
>
>     Except that's not constant current.  Yes it's better than just
>     switching the LED on or off, but the current still flips between
>     almost 5 mA and around 3.3 mA, so that's a 2.7 mA peak-peak
>     alternating current with sharp edges coming from +5V and going into
>     ground.
>
>     A constant current source good enough for this application is a single
>     transistor, a couple of diodes and a couple of resistors. Across 24V
>     you should be able to string together about 10 LEDs.
>
>     Neil
>     _______________________________________________
>     Synth-diy mailing list
>     Synth-diy at synth-diy.org
>     http://synth-diy.org/mailman/listinfo/synth-diy
>     Selling or trading? Use marketplace at synth-diy.org
>
>
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at synth-diy.org
> http://synth-diy.org/mailman/listinfo/synth-diy
> Selling or trading? Usemarketplace at synth-diy.org


-- 
--
synth at schmitzbits.de
http://schmitzbits.de
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://synth-diy.org/pipermail/synth-diy/attachments/20230224/e6be50ae/attachment.htm>