Go Down

Topic: LED Driver running multiple LED's (Read 951 times) previous topic - next topic

AeroDavid

Jun 10, 2015, 02:37 pm Last Edit: Jun 10, 2015, 02:38 pm by AeroDavid
I apologise if this topic has been covered however I've been unsuccessful in finding the information I need.

I am running a constant current LED driver(TCA62724FMG) and hoping to power 6 LED's component. The LED Driver can run up to a maximum of 150mA to each internal LED (RGB). The LED that I wish to use is the Osram Opto LRTB G6TG. Each colour(RGB LED) has a forward current of 20mA so my guess is it'll be okay.

My question is, would I be able to run additional LED's without running over working the LED's or perhaps not not supplying enough power to the LED's. I've decided to connect the LED's in parallel that can be seen in the attachment. The two component data sheet can be found here:

Driver http://www.mouser.com/ds/2/408/DST_TCA62724FMG-TDE_EN_21212-369086.pdf

LED http://docs-asia.electrocomponents.com/webdocs/1280/0900766b81280943.pdf


Any guidance will be appreciated.  

boolrules

Hay, thanks for turning this up. I've been half-assed looking for a equivalent 5050 I can get as a die. This may be a possibility. It doesn't have the single serial line (BOO) and, after a quick look, I see no easy way to mux these. Oh, well.

You may possibly get away with your 6 LEDs in parallel. You will need series resistors to limit and share current equally.
I don't see how more than 6 20mA LEDs would be reliable.
It looks like you have to use a parallel connection as you are limited to 5.5V max. If you add transistor drivers, you're probably negating the constant current feature and the whole point of using this chip.

Have you purchased? How much are they?

Headroom

Running these LEDs in parallel is not likely to work reliably using this chip. Also the dimming resolution of 4-bit pe channel (16 dimming states per channel) is only half of what you'd get from a regular Arduino PWM pin. And 8-Bit can already be somewhat limiting.

You'll be much better off using individual 2812b LED's.

Higher resolution per color (8-bit)
PWM and constant current controls are integrated into each individual LED. No extra chip needed.
Can be run of 5.5 V
Can be operated using any of the 2812b LED libraries.
http://trippylighting.com

http://ledshield.wordpress.com/

AeroDavid

#3
Jun 11, 2015, 01:25 am Last Edit: Jun 11, 2015, 01:28 am by AeroDavid Reason: added more information
Thank you for the feedback guys.


Have you purchased? How much are they?
I was given these as spare parts hence I'm trying to work with what I got. Another reason is because the LED will be driven by 1 constant current driver over the i2c bus and I don't want to use any more additional pins on my microcontroller.

Headroom, the amount of dimming states per channel isn't my main concern unless attempting to run 6 of the LED's at max PWM will cause the current output to be limited and not getting the maximum brightness. Unless you have another reason as to why this is not a good option.


I really appreciate the posts guys.

Headroom

The main reason that this is not a good application is that it is not common practice to run LEDs in parallel.
As has been suggested in a previous comment when connecting LEDs in parallel you can try to limit the current through the LEDs by placing a litter current limiting resistor in series with each individual LED. But resistors cost as well.

There is nothing wrong with using components you already have but my suggestion to use individual WS2812b LEDs uses no more wires than your I2C approaching you can use existing libraries rather than trying to figure out how to talk to that LED chip that I am assuming you'd also have to buy.
http://trippylighting.com

http://ledshield.wordpress.com/

AeroDavid

As has been suggested in a previous comment when connecting LEDs in parallel you can try to limit the current through the LEDs by placing a litter current limiting resistor in series with each individual LED
Thanks for the reply Headroom.

Each PWM (ie, PWM0, PWM1, PWM2) outputs a maximum 150mA at 5.5V. I'm assuming it's 5.5V is quite low, connecting the LED's in parallel is okay. Now the current, referring to attachment on my initial post, the 150mA that comes would be dissipated evenly with all the 6 LED's so roughly a maximum of 25mA through each LED. My question is why is it necessary to add resistors right before the LED? If not, what would happen.

On the latter, worse case I'll have a look into the WS2812b because they look promising and well suited for my project.     

boolrules

Parallel LEDs will not naturally share current evenly. Process differences guarantee this. The one or two that pull the most current will begin degrading soon. And then...
Also, they most probably do not want 5V across them. I don't know what your LEDs are, but I would bet they would not like 5V. You fix that by a suitable resistor value to pass the required current and drop the required voltage.

Paul__B

I was given these as spare parts
There may have been a reason for that!

So the suggestion to go and get some 2812 LEDs may still be the better approach. :smiley-lol:

Headroom

5V across these OPs LEDs are perfectly fine. The reason for the resistors is not to drop the voltage. A LED is a constant current device with a temparature dependent forward voltage that usually raises with an unavoidable raise in temperature.
You can connect this LED to a 12V constant current power supply and it will be fine. Only the forward voltage will be dropped. Your power supply has to work harder and is less efficient but there will be no harm to the LED.

As explained already, LEDs do not share current evenly. As opposed to a normal light bulb where she resistance of the tungsten filament increases as the heat raises and in effect the current decreases, an LED is a semiconductor device and the current increases with increasing temperature with a rapid snowball effect.
Either the LED burns out very quickly if the power supply can deliver enough current or the power supply is going to get damaged.

When several LEDs are connected in parallel the resistance at ambient temparature is not perfectly equal but subject to normal manufacturing tolerances. The one or two LEDs with the lowest resistance will consume the most current and due to the very quick snowball effect will burn out quickly.
http://trippylighting.com

http://ledshield.wordpress.com/

Go Up