7segmentdisplay
I have a simple question, but cannot find the answer anywhere and the datasheets are not very clear. Each 7-segment display have maximum current per segment. If this current is 40mA, single digit display can draw 40mA * 7segments = 280mA. OK…
Here is the question – if a datasheet of a 4 digit display states maximum current per segment 40mA, does a 4 digit display draw 280mA * 4digits = 1.112A (a lot!) ? Or does the value mean "maximum current per segment of all digits shining simultaneously" ?
+1 - you need a common-cathode display; and you don't need the apostrophes around the 7 or 0, for example you just use:
lc.setDigit(0,0,0,false);
Have written up a fair bit about the MAX7219 here.
The two schematics are two versions of the display, common cathode at the top, common anode at the bottom. I'll assume you have the common cathode version.
You connect the segments A..G, DP via 8 series resistors to 8 I/O pins of the microcontroller. Driving a pin high will light that LED on the selected digit. To select any of the 4 digits you make the corresponding common cathode low via an NPN transistor, which you again drive from an I/O pin via a resistor.
If your supply voltage is 5 V and you're using red LEDs then you can use 150 Ω resistors instead of 330. Also decrease the transistor's base resistor values to 2.2 kΩ, and use for instance BC337s for the transistors.
To drive the full display you first make pin 12 low by driving its transistor with a high level, and set the I/Os for the segments of that digit. Some time later you switch pin 12 and the segments off, and switch 9 on, and again the segments for the second digit. And so on. If you go from 1 digit to another in less than 2.5 ms, then the whole display cycles at 10 ms, or 100 Hz, which is enough to avoid noticeable flicker.
You can use the Maxim driver, like the MAX7219, but it's Damn Expensive™: 12.80 dollar in 1s at Digikey. The good thing about it is that it takes care of the multiplexing for you, so you just have to load it with the segment data for the 4 digits. It also has software brightness control.
I checked the PIC16F690 datasheet, and unlike other microcontrollers its I/Os don't seem to be able to source 20 mA (which is disappointing). So you'll need transistors on port 2 as well:
R1 was one of the resistors on port 2. So you insert Q1 and R2 between them, and repeat that for each of the 8 segments. Attention, Q2 is a PNP! Any general purpose PNP transistor will do.
Best Answer
You kind of answered your own question: the datasheet states max per segment.
Is your 4-digit display direct-drive or multiplexed? If it is multplexed the max total current will be for one digit only. If it isn't, then it's max current per segment * #segments * #digits, which can be a lot for a big display as you can see.
Multiplexing the digits helps reduce the current since you light up only one digit at a time. You can use this technique even for a non-multiplexed display.
You can also use PWM techniques to manage the LED drive to achieve good brightness with less power.
The tradeoff of multiplexing and PWM is in increased electromagnetic noise due to the switching activity, which you need to manage at the system level.
Finally, you need to consider not driving at max current. You can get very good brightness at even 1/4 the max or so, especially if the LEDs are high-efficiency.