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Topic: Blown tlc5940nt when connecting an ext 5 volt power supply (Read 3097 times) previous topic - next topic

kilowack

Hi folks,

I currently have a tlc5940, just one, wired up in a standard multiple led configuration as per the tutorial on the chip on the foundations section of the website. When powering the chip through my arduino board and computer, via the 5 volt pin, everything works fine. However, when I tried to power the tlc chip with a 9 volt power supply pumped through a 7805 mosfet, which puts out 5 volts, I've noticed that the 5940 gets very very hot, and then it no longer works. I've torched two chips so far. Three left to go, and I'm not too keen on torching those three!

I'm at a loss, because I want to hook this up to a 9 volt battery  for a portable device.
Is there too much of a current draw (it is powering about 14 led's), or is it the sum total current draw of the entire load, eg. chip, led, mosfet, resistors? Everything is connected to a common ground onto the arduino. The setup is exactly like the tutorial on the tlc5940 chip in the foundation section.

madworm

Draw us a schematic of how you have it wired up, we're not good at guessing.

Maybe you'll find what is reversed in the process of doing so.

Grumpy_Mike

Quote
a 9 volt power supply pumped through a 7805 mosfet, which puts out 5 volts,

The 7805 is not a mosfet, it is a voltage regulator.
Have you put the capacitors on the input and output of the regulator? If not the chip is probably oscillating which could burn out the TLC5940 especially if there is no supply decoupling on that chip either.

kanta

I have a same problem too!!
I'm wondering if I had a very basic careless mistake or not...

I believe I've connected exactly same as this image (images of official TLC5940 library page don't work at this moment...)
http://la.thepublicschool.org/sites/la.thepublicschool.org/files/breadboard-arduino-tlc5940_close.png

Basically it works but sometimes one or some dots are missing or super weak brightness.
Or sometimes all of LEDs don't respond. In this occasion, TLC5940 is going to be very hot, and it spends around 500mA!
It happens both with Arduino's 5V supply via USB, and with external 5V power.


Now I'm expand it for 22 full colored LEDs with TLC59401 (no EEPROM version, but same protocol as 5940) and still have same problem =(
I have 30 copy of manufacture-implemented PCB boards but behaviors of them are different. Like perfect(around 20 of 30), or some dots missing, or most of dots are missing, etc.
Each of them has its own behavior but the strange thing is when i check it next day, sometimes it comes back, or becomes wrong with completely same setting.

Any suggestions for attached diagram??? Your 5940 project works fine??






Grumpy_Mike

The problem with connecting the variable resistors like you have is that it is easy to over drive them. You should have a series resistor to ensure that you never get it too low. Anyway 2K is too low a value, you have a minimum current of 20mA.

While you show decoupling capacitors on the circuit, it is important how they are physically arranged, they have to be close to the chip and they have to be the ceramic type. 

kanta

Thank you for your advice.
Yes, a series resistor is a thing I forgot, I had to do that. LEDs which I use, can accept up to 200mA, typically 150mA, and I use them with around 120mA, so it shouldn't be problem.

I'm curious everybody can work fine with TLC5940 without problems? Or is it kind of delicate chip?


Grumpy_Mike

Quote
and I use them with around 120mA, so it shouldn't be problem.

Yes 120mA is the current limit for this chip for each channel, so you will get a problem running it at anything over this.
Even at this you could easily exceed the power dissipation for the chip depending on the other factors of your circuit.

Quote
Or is it kind of delicate chip?

Any chip is delicate if you run it above it's maximum ratings.

kanta

thanks, that's true, i'm realized my setting is on limitation...

Over-current is one big possible reason but I'm still thinking about these things.
-Same situation (overheat the chip) has happened with weaker current setting too, 1 TLC5940, 8 small LEDs and R(IREF)= 4.7k ohm.
-A friend of mine had this problem with Sparkfun's PWM shield http://www.sparkfun.com/products/10615
-Someone on an old thread said pulling up BLANK pin could fly the chip http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1278208311/all

I'll inform here when I find some progress about this.

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