Number of LEDs in each Segment =10 Nos (Parallel Connected LEds)
Current drawn by single led = 20 mA (Using current limiting resistor =220 Ohm)
Each segment current = 10x20 = 200 mA
Hence each Complete digit current = 7x200 =1400 mA
4 Digit Current (Max) = 4x1400 =5600 mA = 5.6 Amps
So Power Source capacity should be= 6 AMp/5Volt
Following attached circuit will be used :
Arduino, 74HC595 and ULN2803A & PNP transitors are used.
According to ULN2803 datasheet:
Collector-emitter voltage . . . . . . . . . 50 V
Input voltage (see Note 1) . . . . . . . . 30 V
Continuous collector current ...............500 mA
Output clamp diode current ................500 mA
Before implementing all,I would seek your help.
Are my calculations valid?
is my attached circuit correct?
Shall I use 4 ULNs (in Parallel to reduce load on Pins)?
If you have wired the leds, which make up each segment, in parallel the 220R resistors should be reviewed. Since the displays are being multiplexed, only one digit is ever on at any one time which cuts the load to a quarter.
Putting the leds in series (or grouping them in some other way say 2 chains of 5 with appropriate current limiting resistors) may have been better.
First, you should not connect LEDs in parallel unless each has its own series resistor. This is because LEDS are never perfectly balanced and some may be significantly brighter than others. That means a parallel configuration needs lots of series resistors.
The brightness depends not on the configuration (serial or parallel) but on the current flowing through each led (which you have said is 20mA) and the duty cycle of the LEDs. If you are multiplexing 4 digits, the duty cycle should be 25%.
State the part number of the LED (or link to its data sheet). The forward voltage is useful to know.
If the voltage of the LED chains is higher than 5 volts, you will have to drive the 4 transistors differently (as high side switches).
12v is almost certainly better that 5v in the case of groups of 10 LEDs. Higher voltages are probably even better. It depends on the LEDs you are using. Look at their forward voltage to design the best configuration. Red Leds can have a forward voltage as low as 1.6 volts. For green Leds it can be as high as 4.0 volts. This is why you need the data sheet for the LED.
