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I'm planning to build a few 8x8 LED matrix using 1,7/2,3V and 20mA LEDs, and for controlling them, I have purchased a PCB mounting a MAX7219 with an included LED matrix that you can easily detach.

Checking the specifications of the PCB, I see it uses a 10k resistor, what makes me doubt if it is a valid one for my matrix.

According to the Arduino page (https://playground.arduino.cc/Main/MAX72XXHardware) I should be using something around 26/27k resistor, but I don't see such a big difference between the mounted led matrix and the one I'm building. Should I change the PCB resistor or is it ok to keep the 10k it already mounts.

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  • \$\begingroup\$ What is a "20mah LED"? \$\endgroup\$
    – Andy aka
    Commented Feb 5, 2018 at 11:23
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    \$\begingroup\$ Mind the units. LEDs will be 20 mA (note capital) not "mah". 'k' for kilo. 'K' for kelvin although it is fairly obvious from your post what you are referring to. Welcome to EE.SE. \$\endgroup\$
    – Transistor
    Commented Feb 5, 2018 at 11:24
  • \$\begingroup\$ Thanks for the corrections, it has been a while since I had to play with some electronics \$\endgroup\$
    – m_jero
    Commented Feb 5, 2018 at 11:35
  • \$\begingroup\$ @Transistor there was a subtlety in my question. I expected the OP to answer and, based on that answer I might be able to understand more about his/her level of learning and therefore decide whether to make an answer. \$\endgroup\$
    – Andy aka
    Commented Feb 5, 2018 at 11:50
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    \$\begingroup\$ @Andyaka: Understood. Your comment popped in while I was typing mine. \$\endgroup\$
    – Transistor
    Commented Feb 5, 2018 at 12:33

2 Answers 2

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From the MAX7219 datasheet page 8:

The MAX7219/MAX7221 allow display brightness to be controlled with an external resistor (RSET) connected between V+ and ISET. The peak current sourced from the segment drivers is nominally 100 times the current entering ISET. This resistor can either be fixed or variable to allow brightness adjustment from the front panel. Its minimum value should be 9.53kΩ, which typically sets the segment current at 40mA. Display brightness can also be controlled digitally by using the intensity register.

Set to suit your display.

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    \$\begingroup\$ The datasheet basically says the same thing the arduino page says. But my biggest doubt is if it is really needed to change the resistor (I would preffer not make any modification in the PCB). According to this post (forum.arduino.cc/index.php?topic=235295.0) some users recognize to use 10k resistors just because the LEDs can afford the load. \$\endgroup\$
    – m_jero
    Commented Feb 5, 2018 at 11:38
  • \$\begingroup\$ You need to check the datasheet for the matrix then. Post a link. See my article lednique.com/datasheet-current for more on this. \$\endgroup\$
    – Transistor
    Commented Feb 5, 2018 at 12:03
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From the data sheet on page 11 it says: -

N = number of segments driven (worst case is 8)

This means that for one character, all 8 segments could be simultaneously driven BUT, not all characters are driven together hence while one character is active, the other 7 will be inactive.

This means that the average current into an LED segment will be one-seventh of the peak current. So, if the peak current is 40 mA then the average current is more likely to be 6 mA.

So, if you have the data sheet for the display you can check to see that the peak handling current of each segment is greater than 40 mA. The 20 mA figue I suspect is the average value and clearly, this is above the 6 mA figure I calculated.

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