Hello everyone
'
I am working on Large 7 segment driver using MAX7219, UDN2981 for HV current source driver and ULN2803 for sink.
Brightness is not obtained using my driver board.
the brightness of Segments is not as when powered directly. for my 2.3" displays its needs 12V through the driver board to get get same brightness as from 7.5V powered directly.
Please forwards your suggestion for improvements and what can be the possible problem in my board.
With a MAX7219, the displays are only lit 1/8th of the time.
Not good if you want large 7-segment (outside/sports) displays to be bright.
Also note that both 8-channel drivers drop/loose about 3volt, current dependent.
You should compensate LED current for the 1:8 matrixing.
Not sure if I see CL resistor value correctly (1k?). Seems rather high.
Common 2.3" displays are usually driven with at least 30mA when matrixed.
Normally CA displays are used, with TPIC6B595 power shift registers.
One chip per digit, no matrixing.
Leo..
Wawa:
With a MAX7219, the displays are only lit 1/8th of the time.
Not good if you want large 7-segment (outside/sports) displays to be bright.
Also note that both 8-channel drivers drop/loose about 3volt, current dependent.
I cross checked my circuit today and it seems that UDN2981 is not able to source enough current for the Segments. when I connected Anode directly to +9V and Cathode through ULN2803 the brightness was acceptable. but when I sourced through UDN2981 and connected Cathode directly to GND of supply the brightness was too low.
My conclusion therefore, the UDN is not able to source enough current.
Also From udn2981 datasheet, i am getting that its currents decreases from 320mA to 120mA practically depending upon the no on channels used of IC.
Should I make my own HV source switching circuit with transistors to achieve enough current?
Wawa:
Did you allow for the 2volt drop of the UDN and 1.6volt of the ULN at 350mA.
You might have to use a 15volt supply to overcome all those losses.
Leo..
Yeah
I am supplying 12V whereas each segment glows fully at 8V when tested open.
Hi all,
I noticed one strange breakthrough today while testing again.
When I supplied direct VCC(5V) to one of UDN2981 then at 10V I was able to get optimum brightness. this proves that my UDN and ULN circuits are proper.( I agree there will be considerable voltage drop of around 2.5v due to ULN & UDN).
These is something I am messing between segment output pins of MAX7219 and Input pins of UDN2981.
may it is impedance mismatch between the two ICs.
PS- I cheked puting pull-down resisters of 6.8K at the input of UDN2981.
raschemmel:
The CL resistor isn't on that chip.
It's on the MAX7219
Yes, but OP is not using the MAX outputs normally.
Because the big display needs a higher voltage than the MAX can deliver, ULN/UDN buffers are used.
The buffers don't have CL resistors, so external resistors must be used.
Leo..
You already have them, on the segments. R2 etc.
They seem to be 1k, and that's ofcourse way too high for 20mA segment current and all those losses.
Did you draw that diagram yourself, or did you get it from some website.
I (and most of us here) would have never recommended the poor setup you're using now.
Leo..
Well, it seems you want some 8V for the LEDs, you need 3.6V for the buffers, leaving 3.4V to drop over the resistor. S0 R = 3.4V / 20 mA = 170 Ohm.
As you're on only 1/8th of the time for high brightness you may want to aim a bit higher than 20 mA, in which case you have to lower that resistor value accordingly. Do note that LED lifetime suffers, so make sure that's still good enough for your application.
