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 [Urgent]Overheating LED driver chips
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By: sleepingdust (offline) on Sunday, December 06 2009 @ 04:12 PM PST (Read 3494 times)  
sleepingdust

Hi all,

I'm currently involved in a project of using peggy2 to illuminate a "smartsurface" with RGBW LEDs. We are using Ethernet cable to create "extensions" for LEDs in order to have all 625 of them filled a 6 ft. tall, 3 ft. wide human-shaped surface. The Ethernet concept was confirmed by the vendor and the longest cable we are using is about 3 ft. Each Ethernet cable has 8 wires and we grouped RGBW LEDs as a unit. The resistance from the cable haven't caused any trouble for us to light up the LEDs. So it's confirmed that cable+LED works as well as LED on its own.

However, after we soldered some of them on the peggy2 board and powered it with the power adapter purchased from the same vendor, nearly half of the unit didn't light up and there isn't a single unit that has all its RGBW LEDs lighted up. They either had only one or two on. I didn't upload any new sketch. So according to the manual, factory pre-programmed sketch is supposed to uniformly light up all the LEDs.

Here comes the worst part: the two LED driver chips at U4 & U5 heated up very fast (they got super hot within ~5 sec). I checked the components and everything is soldered correctly according to the instruction except my teammate placed the holder* of 328 microcontroller in the opposite direction but the microcontroller itself is in the correct position with its half-moon matched to the drawing on the board. I don't think holder's orientation would matter but I think elaborating what we did in detail would be convenient for you to help us.

We tried to test each connection on the board with a single DC power supply (30V 1A), with a voltage of 2.7V and a current of 0.01A. Sometime we saw when connecting to a LED, let's say R2 of the RGBW unit#2 shown below, the LED R1 of unit#1 that is two row above dimly lighted up, even though nothing was connected to it. I'm not sure if it's normal or not.
Row 1: W1, B1
Row 2. R1, G1
Row 3. W2, B2
Row 4. R2, G2

We only have 5 days till the final installation day, any help would be much appreciated!

Sleepingdust


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By: Windell (offline) on Sunday, December 06 2009 @ 05:18 PM PST  
Windell

There is an issue with the recent Peggy 2 boards in that they do not really behave uniformly when presented with a partially populated board. We are in the process of changing the manuals and other literature to reflect this, and we may soon have a software fix for it as well.

The basic problem is that voltage output from the chips goes high when they try to power an LED that isn't there, and then they dump that power into the next LED that they come across. This isn't harmful to the LEDs, but it does create a nonuniform appearance, and can possibly warm up the chips a bit. You can work around this by either (1) fully populating the board (2) reprogramming the board to light up just the LED locations that are populated or (3) various more complicated ways... but I suggest that you go ahead and keep building it.

You can also test this by test-inserting regular LEDs into the matrix to see how they behave. If you rest five LEDs in a column, in neighboring rows, you'll see that the top one is much brighter than those in lower rows, and this effect goes away when the whole matrix is populated or reprogrammed.

I have not seen the chips get dangerously hot because of this, so there may be something else going on. If you are running anywhere close to full current, the chips will get warm, but should not activate the automatic thermal shutdown. Have you modified the board to run at higher than normal current?

I'm not sure what you were doing trying to test the circuit with the power supply, but it doesn't necessarily sound safe. The chips and transistors may not tolerate this, and it's hard to tell what is what it would mean if you saw some LEDs light up.

One word of advice that I'd definitely like to give you: don't panic, and don't start doing things to your board (like poking and prodding it with external voltages) that you might regret later. It will probably be fine once you finish putting it all together.


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By: sleepingdust (offline) on Sunday, December 06 2009 @ 07:07 PM PST  
sleepingdust

Thanks for that bullet-speed reply!

The LED driver chips get really HOT just in the matter of seconds. By hot, I mean it actually almost burned a layer of skin from my finger tip. If it has a high temperature safety shutdown, why the LEDs were still lit up when it got burning hot? But I think it's better to take the time and try to either populate the whole board or re-programme them just to avoid those problematic pixels.

About using a external power supply to test each pixel, I did it when the whole board was unplugged. All I did is using the two probes from the power supply (Red and Black) to touch the + (square) and - (circle) of a single pixel (with about 2.7V and 0.01A, pretty small for not frying any component) to check if the section from the Ethernet cable to the LED units works. Since we have already soldered them to the board, it's quite hard to de-solder each of them to troubleshoot the connections. But I will definitely take your advice and keep building the whole matrix.

By "only some LED lighted up during the run", I mean when I powered the whole board with the adapter that came with it (no external power supply is used), some pixels were inactive.

One more thing, I noticed that my board is slightly bended, it's very very tiny due to the weight of many strands of cables connected to it. Do you think it will potentially mess up the circuit board inside?

Thank you!

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By: Windell (offline) on Sunday, December 06 2009 @ 07:23 PM PST  
Windell

The safety shutdown is only activated when it *really* overheats inside-- it's designed to not break the chips from overheating; it will definitely get too hot to touch before that shutdown. Frown

With respect to testing LEDs, the important thing to do is to put a 10k resistor (or so) in series with your low-current test to help prevent unexpected trouble. Many chips are not tolerant-- at all -- of reverse voltage. So, be careful.

>some pixels were inactive.

Can you identify if it's due to the same problem with the rows? It would be helpful if you can identify which pixel it is, and how that relates to others on the board, to understand what's going on. You can also run one of the example sketches to try and light up just specific LEDs to test them.

I wouldn't worry about bending the circuit board much... but reinforcing it to minimize bending is a good idea if it's hanging like that. Surface-mount boards are much more sensitive to bending. Most likely failure mode from minor bending is unseating the chips in the sockets.


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By: sleepingdust (offline) on Monday, December 07 2009 @ 10:32 PM PST  
sleepingdust

Hi Windell,

It turned out that my teammates put the 2 LED driver chips at the wrong place. So the chips that are supposed to be on U4 U5 were placed on U2 U3 and vice versa for the other two. So the overheating chips I mentioned earlier are actually 74HC154 chips that control the columns of the display.

I've run several sample codes as well as the RGB gradient codes from the RGB peggy2 page. Several rows were not lit up and some were dimly lit. Do you think placing chips at wrong places would badly damage the chips themselves?

Since we only have three days till our final installation in the gallery, we are thinking about buying some new chips. Does those 4 driver chips have any firmware programmed inside? Some of my friends might have the same type of chips but I am not positive on that. I will probably order some from evilmadscientist.

Thank you!

Sleepingdust


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By: Windell (offline) on Monday, December 07 2009 @ 11:14 PM PST  
Windell

>It turned out that my teammates put the 2 LED driver chips at the wrong place.

Yikes-- That'll do it! If you plug those in at the LED driver locations, one of their outputs is connected to ground-- a short circuit that may explain the overheating. Those chips are not "power" chips, and I don't believe that they have any thermal shutdown mechanism; they could easily be damaged. The other two chips (LED drivers) could also be damaged the same way. (Also: your LEDs will not light up anywhere near uniformly-- I'll guarantee that.)

The 74HC154 chips control the rows, and the LED driver chips control the columns. If you have rows out then it could be due to a problem with those chips. In any case, I'd strongly recommend replacing all four chips as a first step just to make sure that everything else is okay. If it is, then you could swap in the old chips one at a time (in the right places) to see if they are okay.

Only the AVR (the ATmega328P) has firmware on board, so you can replace the other four chips with off-the-shelf components (or all five chips, if you have an AVR ISP programmer).


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