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Topic: TLC5940 and LED strips (Read 3126 times) previous topic - next topic

jschisler

Hi,

I can't find an answer anywhere so I thought I'd ask here.

I have a 12V LED strip that has 3 LEDs/section.  They are 20mA LEDs.  Can I still drive this with a TLC5940 even if each section has its own current-limiting resistor?  I can desolder each of the resistors and put a solder bridge there if needed, but was wondering if it's possible to leave it alone and have it still work?

I'm not sure how this will conflict with the TLC5940's own current limiting logic.

Thanks.

mike_r

Yes, you can still use them, assuming the strips are common annode. You should still keep a small value resistor on the iref line just to be safe. 100 or 330 ohm's would work fine.

Grumpy_Mike

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but was wondering if it's possible to leave it alone and have it still work?

You will have to put the anodes at +12V and leave the TLC5940 to pull down to ground. You should set the current limit resistor on the chip to the same as the LEDs draw. Note there is a 120mA limit so you won't be able to power many LEDs in the strip.

jschisler

I thought it might work.  I have started to wire it up with the TLC sinking each strip section and all are on a common 12V+.  Then I realized that I didn't know if it would work or not so I thought I should ask.

My intention is to have 2 sections of light strip for each TLC channel.  Each section has 3 white LEDs in series so it seems to me that I should set TLC's IREF to allow 40mA through since the sections are wired in parallel on the strip itself and you can break up the strip every 3 LEDs.

Thanks for double checking my logic.

funkyguy4000

Grumpy mike, Is that 120 mA limit for the entire TLC5940 or just at 1 pin. 1 pin right?
Accelerate to 88 miles per hour.

Grumpy_Mike

That is per pin. But don't think you can do this on all the pins because your power dissipation gets breached before you can draw this from all pins.

jschisler

But I should be good at 40mA/pin I hope.  I can add heat sinks if needed, but I understand the DIP version of this chip is pretty tough.

Grumpy_Mike

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But I should be good at 40mA/pin I hope.

Well it depends on the input voltage and the voltage the LED is dropping. Do the sums on the power dissipation for the actual situation you are using.

jschisler

I'll give it a shot:

I have 16 LED strip sections.  Each section has 2 x 3 LEDs (in series) connected in parallel.  The power source is 12V.  The LEDs are white, so I am assuming 3.3V since I don't have the specific data on them.  Each set of LEDs in series runs at 20mA.

12-(3.3*3)=2.1V

2.1/.020=105 ohms which is 0.042W (I took this last value from an online calculator)

Since there are 2 sets of 3 LEDs on each channel, I assume that each channel needs to dissipate 0.042 * 2 = 0.084W * 16 channels = 1.344W total?

The TLC5940 datasheet says the PDIP version of the chip can dissipate 2456mW so it looks like I'm safe.  Did I do this calculation right?

Grumpy_Mike

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Did I do this calculation right?

No sorry, see page 13 of the data sheet.
You don't need to include the power dissipated in the resistor. You have not said what value you are setting the current limiting resistor at.

It is the two lots of LEDs in parallel that is the worry, I am not convinced they will share evenly.

jschisler

Sorry, I'm setting IREF to 1k which gives me (according to the datasheet):

Imax = V(IREF)/R(IREF)*31.5
Imax = 12/1000*31.5 = 0.378A

I'm going to try out a test on the weekend to see if this does indeed work.  It'll be ideal if it does since I want to be able to adjust lighting levels in my project vs. just on/off.


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