Constant currant

102

01:39:27

Constant current down to about 3 volts

R1 = V - (Z1 - 0.07) / IR1  <--- IR1 = desired current...

Example: if you want 1mA on R1

R1 = 9v - (2v - 0.07v) / 0.001 = 7070Ω = 7.07kΩ

R2 = Vb - 0.78 / Ie  <--- Ie = desired emitter current

Example: if you want ~10mA

1.92v - 0.78v / 0.01 = 114Ω

If you have a box of resistors and just want to check IR2 with random resistance:

IR2 = 1.92v - 0.78v / R2

I know it is more complex than that but for my cases it is close enough for anything I do

published 10 years ago

Egoruch

9 years ago

good scheme, but the current stability will greatly depend on the zener diode

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