Real power usage information about 8X8 LED matrices

Other Hardware General Electronics
1

I'm sorry if this question appears to repeat others but I cannot understand the results I have read from other answers.

Starting at the beginning. I want to design and build a clock using a Nano and 8 8X8 LED matrices and an RTC of course. To be different I want my clock to be programmed with all family birthdays etc and to display information (scrolling if necessary) using the 8 matrices.

I have bought these

I have a Nano on a 830 bread board driving 3 matrices (as I expected 3 was the most a USB connection could handle). I am measuring current drawn with a cheap USB power thingy.

The power used was almost negligible! First surprise! :astonished:

Article from this site says

"Internally the MAX72XX multiplexes the rows of the matrix..." etc etc

and also says (I quote)

PeakCurrent=(8 x LedCurrent) + MAX72XX-Supply
PeakCurrent=(8 x 20mA) + 10mA = 170mA

I expected my 3 matrices to use about 500ma. It read a peak about 80ma!

So I added a 4th matrix. Granted I am only displaying patterns on ONE matrix at a time but I do draw a rectangle across ALL 4 matrices.

My "cheap" meter says I am drawing a peak of 90ma! For four matrices! When all 64 LEDs in a matrix are lit it draws 60ma!

Is my cheap meter completely wrong? I am a programmer, not an electrician. I know Ohm's law and a bit more.

ALL the research I have done disputes my results.

Has anyone driven 4 matrices and measured the current draw? And has results to prove or disprove mine?

Please excuse any perceived challenge to anybody's previous results or calculations. I am respectfully seeking confirmation or rejection of my results.

From my results, eight matrices will still draw way less than 500ma. I don't test this for fear of being wrong and blowing something. :wink:

Any comments, disagreements or confirmation will be happily received. :smiley:

2

What pattern were you displaying?

3

About 20 different patterns almost every pattern ending with all 64 LEDs on one matrix being on. And ONE rectangle over all four matrices, 76 LEDs in all. Across the top (and bottom) of all 4 matrices, down the left side of the first module and the right side of the fourth module.
The 90ma peak is over all the patterns but not very often. The power goes as low as 60ma quite often....
I don't just take one reading. I watch the whole sequence of patterns over about 2 minutes. The sequence starts and ends with a big X on all 4 modules, two crossed diagonals.

4

Okay! With my heart in my mouth I wrote a pattern to turn on (slowly) ALL 64 LEDs in ALL FOUR matrices. Slowly so I could reset the Nano if the reading went too high.

Ran the sketch and the current peaked at 15?ma. I don't know what the last digit is as the meter reads 0.15A. So in the worse case the reading could be 159ma.

So 159ma for 256 LEDs lit which is less than the predicted 170ma for ONE row of lit LEDs(eight)!

I am using the excellent LedControl library to drive the matrices.

Could some curious person please repeat my experiment and confirm or refute my results?

Thanks in anticipation :smiley:

P.S. The brightness is set at 3 (out of 15) :slight_smile:

5

P.S. The brightness is set at 3 (out of 15) :slight_smile:

Well you've answered your own question. Set the brightness to max and try again.

6

You appear to be on the right track MarkT :slight_smile:

As I quick test I set the first module to brightness 3, the second to 5, the third to 7, the fourth to 9 and the fifth to 11. Mainly to see is it made a difference in current drawn and whether the varying brightness was noticeable?

There was no perceivable difference to my old eyes but the peak reading went UP to 0.19A with the big X on all five modules. An increase but still nowhere like the predicted draw.

With ALL modules set at 8 (halfway) the peak was 0.14A no difference to brightness 3 all round.

Finally at full FULL brightness with ONE big rectangle over FIVE modules it peaked at 0.20A

Right track but still doesn't explain it :frowning:

7

louwin:
Finally at full FULL brightness with ONE big rectangle over FIVE modules it peaked at 0.20A

Did you keep an eye on the USB voltage at that time?

louwin:
I am measuring current drawn with a cheap USB power thingy.

Do you mean this?

8

Yes, that's it....

And yes, I kept my eyes on it for 4 or 5 minutes. The display cycle is about 2 minutes then it repeats all the patterns. The meter goes into a voltage display every now and then but the current peaked at 0.20A. The voltage read 4.94 to 5.12 up and down....

I have no idea how accurate the USB meter is?

9

Depending on the spec of your LED matrix and the RSET resistor value you used you may not be running each individual LED at 20mA. Also as the matrix is multiplexed the current reading will only be an average.

10

Paul__B:
Did you keep an eye on the USB voltage at that time?

Do you mean this?

I'm studying about it in Electronics. Thank you!

11

A cheap current meter like that will not like the pulse current that your multiplexing gives you therefore you will not get a very accurate reading. You need a scope and to measure across a small resistor, that will show you the peaks. Alternatively try a large smoothing capacitor across the supply to the matrix.

12

The OP already knows an 8x8 array is 8 LEDs - 8 x 20mA mentioned in the posting if you
read it...

13

Thanks for your replies :slight_smile:

I have read all I can find but there isn't much about current drawn by LED matrices :frowning:

In the Parola library thread there is mention of allowing 200ma per module but elsewhere in the thread someone talks about powering 8 modules from the MC's VCC quite successfully.

8 X 170ma OR 8 X 200ma is a LOT more than I am seeing. I see 0.20A for FIVE modules at various brightnesses?

Please don't get me wrong, I am NOT challenging anybody else's figures, just trying to make sense of my own? :wink:

IS anyone else in the position to put a few matrices together and measure current drawn?

14

MarkT:
The OP already knows an 8x8 array is 8 LEDs - 8 x 20mA mentioned in the posting if you
read it...

Who is this aimed at?
Just because the LED's are 8x20mA does not mean setting them to full brightness on the MAX7219 they will draw 20mA unless the correct RSET resistor value is used.

15

Sorry, this YouTube shows six modules being run off a Uno's (?) VCC. No indication of the current drawn but, by popular reckoning, this SHOULD be drawing 6 X 170ma OR 6 X 200ma judging from some of the patterns drawn. 1.02A to 1.20A?

Can you draw that much off a Uno's VCC? OR is it drawing MUCH less? 8)

16

Can you draw that much off a Uno's VCC?

No.

OR is it drawing MUCH less?

Given the above then probable yes.

Have you actually tried any of my suggestions?

17

Sorry Mike....

I am a retired Computer Analyst/Programmer who discovered Arduino 2 or 3 months ago.

I have resistors because they are needed for LEDs. I have An Uno, a Mega and 8 Nanos. I have a ton of sensors, IR Receivers and transmitters, lots of remote controllers, thousands of LEDs. I have ordered stuff I need for this new hobby and waited the 5 weeks for delivery when necessary.

Believe it or not, I don't have an oscilloscope and I don't have ANY capacitors. I haven't needed them so far. If I had them I wouldn't know how to use them. :frowning:

I just want to know if I can safely plug 8 LED matrices into a Nano to design and build a special digital clocks for my grand children. I have 12 LED modules to play with and JUST have ordered 8 more, side by side ones as they appear neater to connect together.....

The programming I can handle, the electronics not so much.

Why do my 5 matrices use so little power when all around me say I should be using 4 or 5 times more power than I am using? That is why I started this thread :slight_smile:

I have a 5V 2A power supply I am playing with for my test. I have pulled an old power supply from an old computer to see if I can use that on my testbed.

This weekend I will try 8 matrices and risk a Nano. I babysit a lot so leave long complicated tasks for the weekend.

I settled on 8 because I read that that is all that can be strung together.

Sorry, I haven't tried your suggestions :frowning: