Hi guys,I connected a MAX7219 to a 8x8 LED Matrix. From the datasheet I determined the resistor to be 24.5kOhm (I use 3.3V 20mA Leds). After some tests it turned out, that the LEDs are way darker than the one I just supply with 5V + pre-resistor, no MAX7219.
When measuring the voltage and the current I found, that to the LED 4V and 15mA are supplied. This are rather wrong values.
This is normal. The MAX7219 multiplexes the LEDs. That means they're only switched on for 1/8th of the time.
I don't know how you measured that but you can't connect a multimeter to a pulsed output (which is what your LEDs are receiving).
Is there any chance of increasing the brightness further?
€ Does both statements also hold if only one LED is activated?
I connected a MAX7219 to a 8x8 LED Matrix. From the datasheet I determined the resistor to be 24.5kOhm
After some tests it turned out, that the LEDs are way darker than the one I just supply with 5V + pre-resistor, no MAX7219.
I'm not sure how you calculated that. Generally, the resistor value to use is 10k for an 8 by 8 matrix. Do not alter the "Scan Limit" from 8 however. 40 mA drive is fine for these matrices as of course, it averages to 5 mA as long as you are multiplexing eight ways.
But - just what tests were these?
This Table, taken from http://tronixstuff.com (taken from the Datasheet of the MAX7219) shows how to choose the correct pre-resistor for the IC depending on the LEDs specs. From there I got the value (20ma/3V). I'm still confident that was the proper choice.
What I did was comparing a single LED on my breadboard with the ones in the matrix using different setting (single LED, row, etc) all were constantly darker, what makes perfectly sense according to fungus post.
The LEDs you have almost certainly have a peak current rating of at least 100 mA
so there should be no reason not to select 40 mA - as is done in the commonly available matrix modules.
Just to be sure: You say, 10kOhm would be sufficient, when I do not alter the scan limit? Implying, that the chosen resistor is set to protect a led when setting the scan limit to 1?
Which would strongly depends on the other characteristics of the brightness-function
Quote from: Paul__B on Dec 13, 2013, 04:45 amThe LEDs you have almost certainly have a peak current rating of at least 100 mASays who?