# Is it OK to drive a LED matrix with constant current?

I bought one of these LED matrix modules as described in this Adafruit PDF: https://cdn-learn.adafruit.com/downloads/pdf/32x16-32x32-rgb-led-matrix.pdf

The PDF says "they do not have built in PWM control of any kind. Instead, you're supposed to redraw the screen over and over to 'manually' PWM the whole thing."

The information on Aliexpress, where I bought the module, says that the refresh rate is 400Hz and the power consumption is 1400W per square meter, which is 36W for my 160*160 cm module. At a scan rate of 1:32 this yields 0.28W per pixel and that's 56mA at 5V. Now I don't know if these RGB LEDs are driven at a constant 56mA (19mA per color) or if I'm supposed to manually create a duty cycle of 50% because the LEDs might be driven at 112mA, or if the duty cycle is created by the refresh rate.

The question is, if I turn one line on full white and do not manually PWM it, will it burn the LEDs? Or does the module itself maintain a 50% duty cycle at 112mA or 100% at 56mA which should be OK for RGB LEDs?

• In the PDF, they write the 64x32 panel will take up to 4A. That one has 2048*3 LED chips, so they are driven at (average) less than 1mA per piece. Your 23mA is a peak consumption which is okay for standard LEDs. For calculating heat, you have to go with the average. – Janka Jul 13 '17 at 23:18
• They say "running full tilt (all pixels set white), can require nearly 4 Amps of current! Double that figure for a 64x32 matrix." I read this to say that a 64x64 needs $16\:\textrm{A}$, worst case. So: $\frac{5\:\textrm{V}\cdot 16\:\textrm{A}}{64\cdot 64}\approx 20\:\textrm{mW}$ per white pixel. Running from $5\:\textrm{V}$, this means $4\:\textrm{mA}$ for all three RGB LEDs. Pretty shockingly low. I worked on custom outdoor 16x16 RGB panels and they were $80\:\textrm{W}$ each. But probably a lot brighter. Also, they supplied three separate RGB current-set (at 100%) resistors. – jonk Jul 13 '17 at 23:40
• So my question is this: Why is the info on Aliexpress saying $1400\:\textrm{W}$ unless what you bought on Aliexpress is NOT the same as what Adafruit is selling? It's interesting that Aliexpress gives a figure that high, because with the RGB panels I worked on it would require 16 of them to make a 64x64 and this would require $80\:\textrm{W}\cdot 16=1280\:\textrm{W}$ for a similar number of RGB pixels. Which is darned close. In fact, I'd call it the same for all intents. But this is NOTHING like the figure Adafruit is giving. What gives? – jonk Jul 13 '17 at 23:45
• @jonk 64*64 still only draws 8A, because of the lower scan rate. But I think I made a mistake. The 64*64 module has a scan rate of 1:32 which means that only two lines are turned on at a time and average power consumption is 600W and maximum is 1400W. But that many Watts are consumed by only two lines powered on at a time. That means 128 white pixels draw 1400W = 2.18 Amps per pixel which doesn't make sense. – uzumaki Jul 13 '17 at 23:55
• @jonk Update: It's 1400W per square meter. One module is 160*160 cm. In the end that makes 56mA per pixel. – uzumaki Jul 14 '17 at 0:02