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| Created | January 20, 2013 |
| Last modified | January 20, 2013 |
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This circuit comes from Wiki. It demonstrates a constant current (~1mA) at Vc even when Vs is allowed to vary from 4.8 to 5.2 V or the load changes from 240 ohm to 1K ohm. The diode voltage, Vd, is selected at 2V. Vbe = 0.8V to assure Q1 is turned on. IR2 = 1mA (goal). Vs = 5V nominal. Id = 15mA to light LED. Ib (calculated) = .667mA. K = 1.5 given.
Calc. Ic = (Vs-.2)/Load = 20mA . . . . . . Calc Ib = 0.667mA = (Ic/Hfe), since Hfe = 30
R2 = (Vd - Vbe)/IR2. . . . . . R1 = (Vs - Vd)/(Id + K*Ib)
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