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ok,it work well with a Rb=2.2K and i will have aprx 1ma at the base ,so it is perfect
finally i use 5V of supply voltage,so my max duty cyle is 60% to have 3V across my motor
thanks for your help
---------- Post added at 18:41 ---------- Previous post was at 17:41 ----------
and i red this about darling,what is the meaning of this?if i understand it,i red that the transistor won´t be satured ever,so my circuit won´t work well?
The output transistor is not allowed to saturate (i.e. its base-collector junction must remain reverse-biased) because the first transistor, when saturated, establishes full (100%) parallel negative feedback between the collector and the base of the second transistor[3]. Since collector-emitter voltage is equal to the sum of its own base-emitter voltage and the collector-emitter voltage of the first transistor, both positive quantities in normal operation, it always exceeds the base-emitter voltage. (In symbols, \mathrm{V_{CE2} = V_{CE1} + V_{BE2} > V_{BE2}} \Rightarrow \mathrm{V_{C2} > V_{B2}} always.) Thus the "saturation" voltage of a Darlington transistor is one VBE (about 0.65 V in silicon) higher than a single transistor saturation voltage, which is typically 0.1 - 0.2 V in silicon. For equal collector currents, this drawback translates to an increase in the dissipated power for the Darlington transistor over a single transistor. The increased low output level can cause troubles when TTL logic circuits are driven.
darlington transistor
google " darlington pair"
it means the all over hfe is the multiplication of both transistors
u are complicating things.... its just the first transistors collector to emitter current will feed the base of the second transistor , and the second transistor vce will be 0.1-0.2 volts.in ref to grd first base volt=1-1.2v ,
sec base volt=0.5-0.7volts.
first collector volt =0.1-0.2v+0.5-0.7 sec base volt.
did i mention this course ...https://www.electronics-tutorials.ws/transistor/tran_4.html
read it all the pages
instead of the 555 i will have compactrio if you remember,and i got the enxt results:
for duty cycle 50% the voltage across the motor is 1.5V
for duty cycle 75% the voltage across the motor is 2.1V
for duty cycle 100% the voltage across the motor is 2.5V
why i can´t get 3V when the duty cycle is 100%?? what is hapening?
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