I have a question about using the PL9823 led in an matrix.

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(Anders Erlandsson) #1

I have a question about using the PL9823 led in an matrix. I have seen that everybody at http://instructables.com using this are connecting a 100nf cap at every led. But i can’t see the reason for this, because all caps are parallel connected and just acting as one big cap. 10 leds +cap = 1uF. 100leds =10uF, so why not just use one big cap around 10-220uF at the powersupply? Or am i missing something here?

I have this problem with my matrix that all but the 3 first h-lines are garbled (i have 493 leds 29x17). I would guess more that it is i need more power wires connected, not a bunch of caps??
I have a 5v 35amp supply. With wires both at the left side and the right side of the display.

(Jarrod Wagner) #2

If you’re using a strip each IC should already have a cap on it.

If you’re using bare LEDs, the data sheet calls for a cap on each IC (in the same manner).

Adafruit dictates that a large cap should be used as a PSU buffer to protect from current in rush.

(JP Roy) #3

Sorry but you are missing something here.
Actually, your maths are right but you are missing the effect of resistance that is there even in the heaviest wire gages.
A small decoupling capacitor, close to a device acts as a small but fast acting charge reservoir for that device. It can act faster because of the minimal resistance between it and the device. The results are: A reduction in voltage fluctuations on the power distribution that could potentially affect the device operation.
Using 1 cap per device is a technically sound guideline. You could potentially get away with 1 cap per 2, 4 or even more devices and even go for the single huge capacitor but you risk getting excessive noise and ringing on the power distribution.
From your description, I am only guessing you have some DC voltage drop in your power wiring. That can be fixed with shorter and/or heavier gage wires. With a voltmeter, probe along your wiring to see if you drop too much voltage.
Normally, you must not get lower than 4.5 Vdc anywhere even with the maximum current load.

(Daniel Garcia) #4

(FYI - as of about 2 minutes ago, the FastLED master branch has a definition for PL9823, since this is the first i’m hearing about them)

(Anders Erlandsson) #5

Thanks for explanation. I’ll add caps. I have used 1.5sq-mm solid copper wire between leds for power.

(Anders Erlandsson) #6

I have 1 bad led somewhere i think. if i drive the lower 6-8 lines everything look ok. If i drive only the first 4 lines then i can se that 3 lines are ok, the 4th is more and more garbled toward the end. so one of the leds in line 4 must be bad.

I have checked the voltages all over the grid, and all are 4.95v with 5v from the supply (PC supply)

(JP Roy) #7

@Anders_Erlandsson
If you think of adding caps to your matrix, have a look at how I did this in my 8X8X8 LED cube. I used surface mount caps.

(Anders Erlandsson) #8

I have actually already bought caps (pack of1000), i just did not think i would need them.
Have already started to solder them.

I have a thought about the flickering of later lines. The return from one line to the other ( i have them zig-zag left to right, then next left to right. I think they pickup noise, so i am going to try shielded wires instead.

(JP Roy) #9

Have you looked at the 5V lines for any DC voltage drops !?
If you are sure about the DC levels then complete the soldering of the decoupling caps before you try going to shielded wires.

(Anders Erlandsson) #10

I have good 5v all over. I have added caps on every 3rd line now and it is getting much better. Will keep adding caps.

But i can’t get the mega2560 to output anything from glediator or Jinx. Using Uno or Nano works. But i need the extra memory of 2560.
I want to make a prog to read files from sd-card AND also read serial from glediator.
I have no experience of programming so i don’t know why 2560 won’t work.