4" 7-segment displays have several LEDs in series per segment. Usually four.Vf (working voltage) of four LEDs can be 8volt or more, depending on LED colour.Easy to drive them with a TPIC shift register.TPIC6B595 (ebay) if you build it yourself.Sparkfun uses the TPIC6C596 on these boards.Leo..
The DP most likely has one LED, while the segments have four.The current limiting resistor has to be calculated for that.Post a link to the display,and post supply voltage and segment resistor values you're now using if you want help with that.Leo..
The third jpg shows five LEDs in series for the segments, and one for the digital point.I'm going to assume you use the RED displays, where the LEDs have a Vf of 2 to 2.4volt and a max current of 20mA.You can safely drop the 220ohm segment resistors to 100ohm if you want more brightness.That will give (12volt supply - 5*~2.2volt LED) / (100ohm + 15ohm) = ~9mA LED current.The DP will have the same brightness with a (12volt - 2.2volt LED) / ~0.009A = ~1088ohm = 1k resistor.Leo..
Nice big displays.Good to see you have it sorted.Karma for posting back.Leo..
A string of five LEDs in a segment uses 9mA (not 9mA per LED).Eight segments (7 + DP) * 6 = (8 * 9mA) * 6 = 432mA (0.432Amp)A 12volt/1Amp supply is more than enough for six displays and the Arduino, even if you decide to use 100ohm resistors for the segments (higher brightness).Leo..