hi,
i recently built a 8x8 matrix controlled with an arduino and a MAX7219

it works all fine, but when i try to light more than 50 leds (20mA, 2.2V) all go blank and the MAX7219 is unresponsive. it does not matter which LEDs are lit up. when this happens, the arduino is still running, i.e. generates output on the serial port.

when i measure the current flowing through V+ I can only light up ~20 LEDs with a current reading of 50mA. I'm not sure why the multimeter should change the behavior.

however, interpolating the 50mA for 20 LEDs to 64 LEDs, gives about 160mA, which is close to the PD I calculated using the formula in the spec sheet (166mA).

does anyone have an idea what could be wrong?
thanks in advance.

ps: why can't i include an URL to my blog (on blogstop) ? apparently it's rated as spam.

What power supply are you using for this setup?

>why can't i include an URL to my blog

Apparently the site has been blacklisted for having too many spam sites; I hate to override this, because spam really is a problem for us. You might consider linking with an URL shortener.

> What power supply are you using for this setup?
the 5+ from the arduino, I tested with power sourced from USB and from a 9V battery, but the result is the same. But funny is that only the MAX7219 shuts itself down - the arduino keeps running.

here the link: http://tinyurl.com/yhxfwss

thanks for your help.

btw: did you move the topic or did i post it wrongly?

I'm going to guess that your 5V power supply to the MAX7219 is dipping below 4 V, which is the minimum required for it to operate.

Assuming that the 5V supply has sufficient current sourcing ability, you may be able to fix this by adding a capacitor near the power supply pins of the MAX7219.

thanks for the hint - but it does not help. it's even worse. with the capacitor, only around 40 LEDs can be lit simultaneously.

Maybe the power source can't provide the needed current. i'll try an external one.

If adding a cap there makes it worse, that would be a big hint that there isn't enough current available. Try hooking up 3 AA cells to your LED array; that should be better.

i tried using an external regulated power source which should be able to provide 2A. but same effect. i'm pretty clueless - i could increase Rset to dim the display a little bit and see if this makes any difference.

>i tried using an external regulated power source which should be able to provide 2A

And was it hooked right to the array, or still as an input to the regulator on the Arduino?

Also, from your photo, it looked like you might be sending 3.3 V to the MAX7219.

I send 5V, that's also what the volt-meter shows. I connected the external source to the V+ of the MAX7219 and the V- to GND. And I disconnected the V+ from the arduino.

I did some more tests. I had to set Rset to 100kOhm in order to get all LEDs concurrently running. Which seems very high. according to the spec, the LED current is 100 times higher as Iset, so that would only be 5mA.

Then I tried to measure the current the circuit is using:
PSU
5V+ ---- [ 75ohm ] ---- V+
5V- ------------------- GND

Arduino
GND ------------------GND
CS ------------------- CS
DIN ------------------- DIN
CLK ------------------- CLK
5+ --

And measured the voltage drop between 5+ and V+ over the 75Ohm resistor. When all LEDs are on, I got 1.6 V / 75 ~ 20mA, when no Leds were on, I got: 2.7mV / 75 ~ 36uA. Interestingly, when i turned arduino off, I got 2.0 V / 75 ~ 26mA

I also measured a frequency of 3.78kHz.

When I disconnect the external power source I still measure some negative voltage over 5+ and GND, coming from CS.

then I got the idea taking CS LOW again after latching the data:

//latch the data onto the display
digitalWrite(SPI_CS,HIGH);
delayMicroseconds(1);
digitalWrite(SPI_CS,LOW);

and then it works with my 27kOhm resistor!! but not very stable :-( Maybe I have to add a diode somewhere?