fawadbutt
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Random segments or always the same segments? (I would not be surprised if it´s mainly segments B and C)its work fine but for some time its start dim some segment and in the end its damege
Thanks DanaThe ULN2003A output is pulled up to 14V, not the processor. Per
OP's schematic.
Regards, Dana.
Where do your 14V come from? I see 12V onlyThe ULN2003A output is pulled up to 14V
My error. should be 12V.Where do your 14V come from? I see 12V only
Where does the 500uA cutoff current come from? I see 100uA only.
Maybe I´m looking into wrong informations/datasheets.
Klaus
Yes, it says 100uA max for the ULN2003A.ULN2003A collector cutoff :
ULN2004 versions on right use 2 test conditions; Input open and 6V which does not apply to left '2003Must be me but something does not add up here :
View attachment 187968
View attachment 187969
View attachment 187971
Regards, Dana.
Of course the worst case Vf is 2.35 at 10 mA, 25C, right in the ballpark for red.What is headroom for CA RED at 3.3V? yes tight for poor quality Red displays with high Vf specs.
Continuous Forward Current : 20mA
Average Forward Voltage : 2V
Power Consumption : 36mW
Ref
Allow 0.1V drop for 640 mA digit at 25% d.f. or logic level type Pch RdsOn < 100/640<= 150 mohm
These ought to be rated at ;
Vf =2.1V +/-0.3 @ 20 mA continuous
Peak Forward Current IF(Peak) 100 mA to 180 mA depending on source
For this 0.56" Red from Kingsbrite
View attachment 187962
Most CC limiters using BJT's need 1V headroom or more (Vbe+ VCe(sat).
With some gain and negative FB you can prebias the Vbe current sensor for < 100 mV drop for CC segment drivers.
Thus Vdd - Vfet-Vcc - %tol = 3.3 - 0.1-0.1-0.1 = 3.0V allocated for 2V RED LED segment at 80 mA
Rule of Thumb
Most Vf variations for any given p/n of diode are due to Rs bulk resistance and thermal voltage shift for a given current.
This means you can estimate variance if you know range of Rs which can be screen for high volume users. ( such as my former clients who I sold about 1m pcs in 10 yrs.) screened for 7%Vf vs usual 40% with best quality brightness.
There are much better quality low Vf LEDs but these are >20 yr old technology and what consumers see.Of course the worst case Vf is 2.35 at 10 mA, 25C, right in the ballpark for red.
Regards, Dana.
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