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driving LED

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auri_z

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i using BC547 for drive my led matrix..

i have same question about how to calculate current flow through LED. my circuit in atachment.

here my though current flow LED.

I(LED)= (Vcc-VCEsat(BC547)-VCEsat(ULN2803)-Vled) / R2.

but my friends tell it's wrong..
please help how to calculate current LED.
 

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Why don't you just connect the LED on the collector side and delete U1?
 
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sorry i forget to mention about my project.

I use column scan for my project so i use ULN2803 for current sink. BC547 use for driving row,
because it's more safe for microcontroller without it my LED can work too.

BC547 use for row driver and ULN for column driver.

value R in picture is to big ( i forget to erase default value from proteus)

is my calculate for current LED right?
because friend tell me to calculate current VBE need to insert to calculate, but i think don't because Transistor in condition saturation.

sorry my english bad.
 
Your friend is correct. The transistor is in an emitter follower configuration so it is Vbe. So it is Vcc - PIC output high -Vbe -Vf Led -Vce of 2803.
 
The transistor might not needed because the max allowed output current from one output of a PIC is 25mA. The max total of all outputs at the same time is 50mA.
 
is PIC output high same with VOH

so it's became VOH - VBE - Vf - VCE ULN2803

if not what is it?

i use transistor for make my PIC more safer i think and if i what to use more current for my LED it's possible. because i use column scan so duty cycle is small.

if formula above use so Rb need to calculate too.
so Rb will became Ic/Hfe ...

it's correct?


sorry my bad english
 
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