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Topic: MAX7219 and 8x8 LED matrix modules - Measured current used (Read 380 times) previous topic - next topic

marco_c

The question of how much current these modules use comes up on a regular basis so I thought I would try an experiment and actually measure the current usage. I used my Parola module (forum discussion) as the test device, but this is similar to the more generic modules and I believe the results are transferable.

The MAX7219 has a resistor that sets the maximum current supplied to the matrix (Rset in the datasheet). My matrix had Rset = 62k, in line with the datasheet suggested values. I know that some of the cheap generic modules have very low values for Rset, which would significantly increase the power/current required by the module.

A digital multimeter was used to measure the current into the module (ie, nothing special).

Results were not surprising, but confirmed by the measured data:
1. The current was linearly proportional to the number of LEDs lit, irrespective of whether the LEDs were rows or columns. I ended up discarding the values for columns due to this symmetry.
2. The current was linearly proportional to the intensity setting of the LEDs.

With no LEDs lit the current drawn was 2.6mA and with all LEDs at maximum brightness setting it was around 67mA. This last value was surprising as it is considerably less than the often-quoted 200mA per module. I expect that Rset comes into play in a significant way at this extreme end.

Below is the data and attached are some charts.

Code: [Select]
Supply Voltage: 4.9V
Rset: 62k

Int  0R   1R   2R   3R   4R   5R   6R   7R   8R
0   2.6  2.9  3.0  3.4  3.7  4.0  4.3  4.5  4.8
1   2.6  3.4  4.2  5.0  5.9  6.7  7.5  8.3  9.1
2   2.6  4.0  5.3  6.7  8.0  9.4 10.8 12.1 13.5
3   2.6  4.5  6.4  8.3 10.2 12.1 14.0 15.9 17.8
4   2.6  5.0  7.5  9.9 12.3 14.8 17.2 19.6 22.0
5   2.6  5.6  8.5 11.5 14.5 17.4 20.4 23.3 26.3
6   2.6  6.1  9.6 13.1 16.6 20.1 23.6 27.0 30.4
7   2.6  6.6 10.7 14.7 18.7 22.7 26.7 30.7 34.6
8   2.6  7.2 11.8 16.3 20.8 25.3 29.9 34.3 38.8
9   2.6  7.7 12.8 17.9 22.9 28.0 33.0 38.0 42.9
10  2.6  8.3 13.9 19.5 25.0 30.6 36.1 41.6 47.1
11  2.6  8.8 14.9 21.0 27.1 33.2 39.2 45.2 51.1
12  2.6  9.3 16.0 22.6 29.2 35.8 42.3 48.8 55.2
13  2.6  9.8 17.0 24.2 31.2 38.3 45.3 52.3 59.2
14  2.6 10.4 18.1 25.7 33.3 40.9 48.4 55.8 63.2
15  2.6 10.9 19.1 27.3 35.4 43.4 51.4 59.4 67.2


Arduino Libraries http://arduinocode.codeplex.com
Parola for Arduino http://parola.codeplex.com

Riva

Nice work, are the MAX7219 chips you use genuine or fake ones as I have read somewhere there is a difference (fake draws more current).
Once I almost saw Elvis but then my shovel broke. :(

marco_c

Arduino Libraries http://arduinocode.codeplex.com
Parola for Arduino http://parola.codeplex.com

Pedro147

Good job Marco. It's always nice when theory aligns with measurable results in the real world
http://www.pedroduino.com

Paul__B

My matrix had Rset = 62k, in line with the datasheet suggested values.
How is that specifically "in line" with the datasheet?  The datasheet offers a wide variety.

I know that some of the cheap generic modules have very low values for Rset, which would significantly increase the power/current required by the module.
Certainly would. :smiley-lol:   Normally 10k for the eBay specials, for a segment current somewhat in excess of 40 mA (well within specs for both the MAX7219 and most 8 by 8 LED matrices), the specified minimum value for RSET being 9.53kΩ. ( :smiley-eek: )

With no LEDs lit the current drawn was 2.6mA and with all LEDs at maximum brightness setting it was around 67mA. This last value was surprising as it is considerably less than the often-quoted 200mA per module. I expect that Rset comes into play in a significant way at this extreme end.
How could this possibly be surprising?  According to Table 11, given a segment voltage (red) of say, 2.5V (albeit I would actually expect it to be lower, under 2V), RSET of 62k gives a segment current of just under 10 mA, eight times which would be 80 mA.  67 mA sounds close enough to me, no surprises at all.

A fully lit matrix with a 10k RSET though would arguably draw 320 to 350 mA, still within specs.  (Not tried here.  For simple experimentation I have never found full brightness to be necessary, and you cannot then use multiple modules - I generally play with three - with USB.)

Using the PWM function to operate the matrix at lower brightness levels means that this is also the peak current, which is why that 10µF (or better) bypass capacitor on each MAX7219 is so critical.

Pedro147

Someone Seems to have a chip "on their shoulders" and always seems to offering very negative replies to other peoples posts  :smiley-cool:
http://www.pedroduino.com

louwin

I run 8 (2 by 4) modules (on USB) ALL the time and my current peaks at .61A (with ALL 512 LEDs lit, on medium intensity). I started a thread a while ago asking for confirmation(or repudiation) of my current usages but nobody seemed to be interested.  :(

I came away with the suggestion that current should only be measured with an osciloscope not the cheap ammeter that I had.  :(

Till then I was faced with the usual claims of 170ma (sometimes 200ma) per module. I actually bought 2A USB wall chargers to drive my 8 module displays.  I have 5 by 4 modules arriving SOON and I intend to play with 12 module displays (maybe 16 module displays?). It is good fun with Marco's libraries....   :D

It is refreshening to see your very professional approach to the situation, Marco.  :)

As always thanks for your libraries Marco. :D
Newbie, bought a Uno because I had a $10 Paypal voucher  :)

Chamier

Hi there
I tried to make a module for a led matrix 8x8 2 inches square, common cathode, red by copying the diagram from the mini 7219 module, but i'm not able to make it work. Does anybody can share gerber files for this project?
Of course i'm a newby.
Thanks in advance.
 

CrossRoads

Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

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