[sdiy] LED current source needed for 'digital' outputs?

Mattias Rickardsson mr at analogue.org
Tue Mar 4 17:19:18 CET 2014


On 4 March 2014 17:07, Neil Johnson <neil.johnson71 at gmail.com> wrote:
> Richie wrote:
>> I've seen VU meter designs in pro-audio kit where all of the LEDs are
>> connected together in series and driven from the supply rails via a constant
>> current source.  Current is allowed to pass through the LEDs that we want
>> illuminated, and "diverted around" the LEDs that are meant to not be
>> illuminated.  The total current drawn from the power supply is therefore
>> 20mA regardless of whether all the LEDs are lit, all are extinguished, or
>> any combination in between.  That way it doesn't trash the power supply
>> rails with switching transients.
>
> I've done similar with little VU meter driver chips and some
> low-current LEDs, again fixing the current consumption to 20mA
> irrespective of how many LEDs are lit.  For the minimal added cost it
> does help to minimise switching noise off the floor.

With this constant-current approach you can also avoid varying
currents, which are possible causes of disturbances in nearby
circuits. On the other hand it might get impossible to avoid varying
voltages, which can disturb nearby circuits in another fashion. :-)

/mr



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