Burning TLC5940 when using external 5V

Other Hardware LEDs and Multiplexing
1

I am running the basic "BasicUse" example sketch from the TLC5940 library.

When I run this on my USB powered UNO, it works fine. At this point I am taking the +5V (and GND) from a pin on the UNO for the breadboard power rails. However, I wanted to use my breadboard power supply. So, with USB unplugged (no power to circuit), I disconnect the +5V coming from the UNO and connect the +5V and GND from the PSU to power rails (GND still connected to UNO).

I then switch on the external PSU (TLC chip feels cold and no LEDs on). I then plug the UNO into USB socket - at this point the TLC5940 gets very hot in the middle of chip and LEDs are not lighting. When I revert to the previous scenario (remove the external PSU) the chip no longer works.

I have tried this with 2 chips and don't want to waste any more!

Can anyone tell me what is wrong? The external supply is a regulated 5V DC 1A wall wart - I've checked this on my multimeter and it's a constant 5.25V

I have the TLC5940 connected as per the diagram in the sketch (below). LEDs are anode to +5V and cathode to TLC5940 OUTx - it works fine until I bring in the external PSU. I don't have any capacitors in the circuit.

---u----
OUT1 |1 28| OUT channel 0
OUT2 |2 27|-> GND (VPRG)
OUT3 |3 26|-> SIN (pin 11)
OUT4 |4 25|-> SCLK (pin 13)
. |5 24|-> XLAT (pin 9)
. |6 23|-> BLANK (pin 10) and 10K Resistor to +5V
. |7 22|-> GND
. |8 21|-> VCC (+5V)
. |9 20|-> 2K Resistor -> GND
. |10 19|-> +5V (DCPRG)
. |11 18|-> GSCLK (pin 3)
. |12 17|-> SOUT
. |13 16|-> XERR
OUT14|14 15| OUT channel 15

2

I have a similar setup, but the external is 12v. I dont connect 12v+ to the chips at all, only through the leds and then into the chips. I power them with arduino 5v. They work fine

3

I don't have any capacitors in the circuit.

Then put some in.

I can only think that you do not have it wired like you think you have or you describe. What you describe sounds like it should work fine.

4

Thanks for the replies.

I rebuilt the circuit from scratch - still problems, but slightly different result. I also put filtering caps across pins 21 & 22. Using the external power, the circuit appeared to work - the 5940 felt cold after about 20 secs (when I last checked it). After about 1 min of running the Knight Rider "BasicUse" sketch (I've only got 8 LEDs connected) the LEDs went off, chip was very hot and had blown. Swapped chip, ran sketch for 5 mins plus on +5V from UNO and looks fine.

I dont understand it. I'm now questioning the output of the wall wart and wondering whether there's an issue with this (I did measure the output when I first had the problem, but maybe didn't monitor long enough). If I try and use a 7805 and caps on this power supply the output is 3.9v (wasn't using this on the problem circuit) - I guess because it's because it's only a 5V supply and there's a voltage drop using a 7805. I'm going to find a 9V supply, add the regulator and try again - I'm not that confident, but not sure what else to try.