RussellMc wrote:
> A 4 channel current source can be made with eg an LM324, 4 x MOSFETS,
> 1 x TL431, 4 x current sense resistors and a few other r's and c's.
> Very cheap.
> LED current flows to ground via MOSFET and  sense resistor.
> Op amp section monitors resistor voltage wrt reference voltage and
> drives MOSG\FET gate to maintain V_R_sense at same voltage as Vref.
> Works well.

I'm sure that will work fine, so fine that there may be simpler ways.  In
this case a little current error or temperature dependency isn't a big deal=
..
As I understand it, these LEDs aren't for illumination but just for
indication.  They shouldn't need to be run at full current.

Bipolar transistors make easy current sources since their collector current
is independent of collector voltage to good enough first approximation.  If
you've got multiple strings, than make a single voltage source of maybe 1.5=
V
once.  Each string gets a single NPN transistor and resistor.  For each
string, 1.5V to base, resistor between emitter and ground, and LED string
between power and collector.  Let's say you want to aim for 10mA nominal.
Assume the B-E drop is 700mV, which leaves 800mV accross the emitter
resistor.  800mV / 10mA =3D 80 Ohms, so the common value of 82 Ohms should =
be
fine.  Let's say it gets hot and the B-E drop goes down to 500mV leaving
1.0V accross the resistor.  That gives you 1.0V / 82 Ohms =3D 12mA, which i=
s
20% brighter (very hard for humans to notice 20% brightness change) but
still well within LED current for long lifetime.

As Russell said, get decent LEDs and they will last longer than your
motorcycle.


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