ULN2003A to drive a large seven segment display

[andrew_baker00]

Hi guys, i have a big led scoreboard project i am currently working on.

For each digit i am using:
1 x CD4543
1 x ULN2003A



I am using a cd4543 BCD to seven segment with seven outputs connected to the seven inputs of a ULN2003A darlington array per attached image. The darlington array has gnd connected to my 18v ground (not the same ground i use for all the other ICs), and the COM is hooked up to that same ground via a switch. The seven outputs of the ULN2003A go to one of the seven segments of the large SSD, and the other end of the segments go to 18v. When i run the system the seven segment display works perfectly for all digits except 8. For 8 the leds are barely on. I have placed a small SSD in between the BCD-To-Seven segment decoder/latch to make sure im getting the correct output from the CD4543. I have also connected a couple other segments from an identical large SSD to rule out that i am lacking of current from my power source.

Why am i loosing intensity from this system when using all the outputs from the ULN2003AD?

Any thoughts?

Andrew

 

[shamikrudra]

give complete circuit for better help
how much current required for your large segments?

 

[liteon]

Hi Andrew.
First of all you need to get the datasheet for your particular SSD because it is important to know the operating voltage and current required to drive them.
The complete schematics of your project posted here will let you get good advise from members here.
And current limiting resistors or resistor network is required at the output of ULN2003 for keeping the control of current flow and equal current distribution from the outputs if some or all of them are turned on simultaneously and also for protecting the Ic itself as the continuous power rating for each Darlington pair is 500mA each and Maximum of 600mA for each under ideal temperature.
So you need to configure the required current to drive your display.

 

[alexan_e]

Actually the total max current is specified in the datasheet
look at page 5, Maximum Collector Current Vs. Duty cycle.

As you will see the maximum current depends on the number of outputs and duty cycle

Alex

 

[bohumilfulin]

I think check out your power supply and ground system (digital gnd, power gnd,...).

I am not sure cause i have no pdf in this computer but check also pdf of uln2003. COM (protection diodes on the output pins) leave unconnected or connect to +18V not gnd.

What is the maximum current consuption of your display? For higher values is important all connector pins, pcb design, wires,... Switch on your display and check all connector pins by your voltmeter.

 

[mishra12]

i am using per segment around 400ma working perfectly till now