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<p>Nice idea. Very convenient with dual SMT transistors like the
MBT3904 et al.<br>
</p>
<div class="moz-cite-prefix">On 2/23/23 9:24 PM, Scott Bernardi via
Synth-diy wrote:<br>
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<blockquote type="cite"
cite="mid:CAM-tPNMjwqfCUOcYDKbp2uWWDcfQ70uhpOOdv965YveEtoxvkA@mail.gmail.com">
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<div dir="ltr">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
<div>I whipped up a page for it:</div>
<div><a
href="https://www.bernacomp.com/elec/og2/constant_current_led.html"
moz-do-not-send="true" class="moz-txt-link-freetext">https://www.bernacomp.com/elec/og2/constant_current_led.html</a><br>
</div>
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<br>
<div class="gmail_quote">
<div dir="ltr" class="gmail_attr">On Thu, Feb 23, 2023 at 3:50
PM Neil Johnson via Synth-diy <<a
href="mailto:synth-diy@synth-diy.org" moz-do-not-send="true"
class="moz-txt-link-freetext">synth-diy@synth-diy.org</a>>
wrote:<br>
</div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px
0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">Hi
Florian,<br>
<br>
> 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):<br>
><br>
> 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.<br>
<br>
Except that's not constant current. Yes it's better than just<br>
switching the LED on or off, but the current still flips
between<br>
almost 5 mA and around 3.3 mA, so that's a 2.7 mA peak-peak<br>
alternating current with sharp edges coming from +5V and going
into<br>
ground.<br>
<br>
A constant current source good enough for this application is
a single<br>
transistor, a couple of diodes and a couple of resistors.
Across 24V<br>
you should be able to string together about 10 LEDs.<br>
<br>
Neil<br>
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