[sdiy] Signals leaking into the PSU?

[Oren Leavitt]

Nice idea. Very convenient with dual SMT transistors like the MBT3904 et al.

On 2/23/23 9:24 PM, 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
>
> 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
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