[SOLVED] ULN2803 with LED with 12V VCC

[DianWardiana]

Hi, i have circuit shown below:



It going right as what i want. but the problem is ULN2803 is often broken after circuit running for 1-2 days. Anything wrong in my circuit?

Thanks.

 

[KlausST]

Hi,

"broken" means exactly what?
* A cr.ack in the plastics?
* erroneous OPEN of one channel or all channels?
* erroneous LOW of one channel or all channels?
* anything else?

***
The ULN2803 usually is relatively robust.
* check overvoltag/undervoltage peaks in 12V supply.
* check for very good GND connection between: microcontroller_GND - ULN2803_GND - 12V_GND
* are there somewhere inductive loads?
* ESD problems?

BTW: it´s a very power consuming way to drive LEDs. I assume about 5 times the LED power is dissipated as heat in the resistors.

Klaus

 

[DianWardiana]

Broken is mean the ULN2803 does not work again.
And there is no inductive load, just LED at there.

Thanks.

 

[xenos]

What frequency do you use to turn it on/off?
What is the driving voltage? Ttl levels or less?

 

[DianWardiana]

i don't know frequency in my circuit. the driving voltage is 3.3V.

Thanks.

 

[betwixt]

3.3V is only just enough to turn the ULN2803 on, it's internal bias circuit is optimized for 5V. However, for low current loads like the 8mA per channel your circuit passes, even 3.3V should be sufficient so it should be running WELL within it's rating and no damage should occur.

Are all the channels 'breaking' at once or is it just one that stops working?

Brian.

 

[DianWardiana]

all the channels stop working. And on some ULN2803 that stopped working i saw a hole in the top (i think it has burned).

Thanks.

 

[xenos]

Well, you don't drive transistors in hard core, however, for so small currents, driving with 3,3V should be adequate.
To blow a ULN the way you describe, you have to short circuit one of its outputs to the power supply or increase the voltage source above the limits.
Upload photographs of your contraption and cabling you use and of any connectors you use.

 

[DianWardiana]

And I'm forgot to convey an information. I use UTP cable. The length of cable is about 20m.

Thanks.

Tomorrow i will upload photos.

 

[xenos]

You better tell us all the details.
Is the cable in parallel with a high energy cable? (significant capacitive charge and overvoltage)
You already have an inductive load (cable).

 

[KlausST]

Hi,

A clean overvoltage protection:
* connect 100nF ceramic capacitor as close as possible to PIN9 and PIN10.
* add a 10uF electrolytic capacitor in parallel (only important if there are other loads connected to this 12V line.)
* add a 20V zener in parallel. (cathode to +12V)

Klaus

 

[baileychic]

What wattage Leds are they ?

 

[DianWardiana]

I used 3 pairs cable for load LED, and 1 pair is for 12V and GND.
Whether cable can be the problem? As datasheet said UTP which i used can carry 48VDC.

Thanks.

 

[DianWardiana]

Hi,

A clean overvoltage protection:
* connect 100nF ceramic capacitor as close as possible to PIN9 and PIN10.
* add a 10uF electrolytic capacitor in parallel (only important if there are other loads connected to this 12V line.)
* add a 20V zener in parallel. (cathode to +12V)

Klaus

i will try that. And Anode of zener connect to PIN10 or to Ground?

Thanks

 

[DianWardiana]

I'm used 0805 SMD red, blue and green. as datasheet said, wattage is about 44-64mW.

Thanks.

 

[KlausST]

Hi,

pin 10 = 12V = cathode = 100nF = 10uF(+)
pin 9 = GND = anode = 100nF = 10uF(-)

All three additional devices are in parallel.

Klaus

 

[betwixt]

What wattage Leds are they ?

Baileychic - with 1.2K resistors in series with each LED and a 12V supply, the current cannot possibly exceed 10mA per channel so it isn't the LEDS causing the problem, it's either a wiring/construction fault or excessive voltage causing latch-up.

Brian.

 

[DianWardiana]

Hi,

pin 10 = 12V = cathode = 100nF = 10uF(+)
pin 9 = GND = anode = 100nF = 10uF(-)

All three additional devices are in parallel.

Klaus

Excuse me, so the circuit would be like this?


Thanks.

 

[betwixt]

Excuse me, so the circuit would be like this?

Exactly - but be sure to fit the capacitors and Zener diode at the ULN2803 end of the cable, preferably close to the IC pins.
Although not essential, it would be considered good practice to connect the unused inputs (pins 7 & 8) to GND as well. There are already pull-down resistors inside the IC but if something nasty enough to kill it is present, it would be a good idea to make sure it doesn't enter through those pins.

Brian.

 

[DianWardiana]

ok thank you very much for the help.
I will give the information in next 2 or 3 days, to make sure the new circuit is work :wink:

Cheers,
Ian.