# Resistor value on FC-16 / MAX7219

I bought a couple of LED Matrix Modules to practice on. The intention is to use this module to run 64 individual white LEDs. These LEDs will be spread out, and could be as much as 2m apart. I'm not sure if that will make a difference to this question though, but thought I'd mention it for completeness.

I've looked at the datasheet for the 1088AS (8x8 LED Matrix) and it says it has a forward voltage of 2.1V-2.5V and a forward current of 20mA.

The picture shows the information I found for the MAX7219 module. On the module I have, R1 has 103 written on it, signifying a 10K resistor. But this doesn't coincide with the value suggested in the table in the picture. What am I missing?

Also, the matrix has red LEDs. If I want to use white ones (3V 20mA), should I be changing R1 to a different value?

You're not missing anything.

Most of the modules have a 10k resistor, which is way too low. Aside from using a ton of power, the matrices are too bright and the difference in brightness becomes negligible when you try and control it. I usually substitute the R with something like 62k to give a more reasonable output.

You may want to look at this, but I think you understand it already: MAX7219 and LED matrix power requirements – Arduino++

On the other hand, I use these matrix displays as signs, in daylight.

So the 10k is just perfect!

Note that 20 mA is the continuous current rating of the LED, if you look more closely at the datasheet it will generally have other ratings as well - but the MAX7218 is multiplexing them; if the display has eight columns, it is a 1 of 8 multiplex, so double the current for one eighth of the time is a very reasonable arrangement.

Peak Forward Current for a 1/10 Duty Cycle on the 1088AS is in fact 120 mA.

Noted this is a continuation of your previous query.

Thank you Marco and Paul. Looks like there's a difference of opinion, more than likely due to individual usage.

Seeing as it's easier to leave things as they are, I'll test it without as is. I just didn't want to start frying anything.

You won't fry anything.

I don't think it is really a difference of opinion. If you want maximum brightness such as I mentioned, use the modules as they are. If it is for your alarm clock and you do not want the whole room illuminated (and it will! ) during the night, then substitute a higher value as Marco suggests.

Paul__B:
You won't fry anything.

Thank goodness.

I'll be substituting the red 8x8 matrix with 64 individual white LEDs. The brighter the better. But I've read that intensity can also be controlled via software, so I'll look into that to.

If “the brighter the better”, then the default 10k ISET resistor is what you want. Maximum current is about 320 mA.

There are 16 equal steps of PWM brightness, but the top few are not anywhere as noticeable as the bottom few as the eye’s response to light is basically logarithmic.