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Transistor as a current limiter


Jasen Betts

Jan 1, 1970
Speaking of which, any thoughts on this issue:

I would plan to design the power supply to provide about 1.5V
of headroom beyond what is needed by the LEDs at peak current
drive. That should provide a good 1.2V for the current mirror
side of things and 0.3V for the switch on the other end. (If
it can be done with less headroom, so much the better.)

The issue is this:

To have a controlled current sink (or source) capable of
supporting ... let's say at least 8 sinks/sources ... and
where each sink/source can be individually enabled or
disabled without affecting any of the others. Only BJTs as
active devices. Only discretes, no ICs, no opamps, etc.

My imagination fails me for a reasonable solution here. How
about yours?

all NPN emitter marked 'e'

I_set I_LED
| |
+-----. |
\ | /
\| | |/
/| |\
e e
+ +
| |
/ \
\ 22R / 22R
/ \
| |
Q1 / /
+V |/ enable |/
--[1K]-| ---[1K]--|
|\ |\
e e
| |

scale the 22R as apropriate for your application 200mV drop is plenty
scale the 1K sufficient to ensure reasonabe saturation.

Q1 can probably be replaced by the apropriate resistor
(measure or calculate)