I'm trying to figure out how many resistors I need for my project, and where in line to put them. I'm using a MAX7219 to drive sixteen 7-segment common-cathode displays for a scoreboard. Yes, I know that the 7219 only actually supports 8 digits, so let me clarify.
The scoreboard has 4 sides. Each side has 4 digits; 2 for each player's score. Since I only need 4 unique digits, the other 12 displays (3 faces x 4 digits) are wired in parallel with the appropriate digit.
Now, assuming I want to have each segment running at 10mA, that makes a maximum current draw for 1 unique digit 280mA (7 segments x 4 displays x 10mA [not including DP since I don't use it]). Total, that means if all the segments on all the displays were lit up at the same time, it would draw 1.12A.
However, something occurred to me. The digits wouldn't actually be on at the same time because of how the MAX7219 works, right? It cycles the digits on/off very fast so they appear to all be on at the same time, but they aren't. Technically only 1 unique digit (in other words, 4 displays) would be on at a time, meaning that if all the displays were on "at the same time", they would actually only draw a maximum of 280mA total. This means that I could just put my resistors on the SEG pins of the MAX7219. This also means that I only need 7 resistors for the whole lot of displays, instead of 7 resistors for each of the displays, right?
Another thing caught my eye though. In the MAX7219 datasheet
, it says that the resistor between the ISET and V+ pins is for controlling the display brightness. Does this mean that with that single resistor between ISET and V+, I don't even need those 7 resistors on the SEG pins?
Also, the datasheet says that a 10K resistor will limit the current on each SEG pin to about 40mA, what value resistor would be required to get that down to 10 or 20mA?
Bonus question: In the datasheet for the MAX7219, it says the following under the absolute maximum section:
DIG 0–DIG 7 Sink Current..............................................500mA
SEG A–G, DP Source Current........................................100mA
Does that mean 500 / 100mA per pin, or in total?