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Topic: TCL 5940 + RGB LEDs in parallel? (Read 3480 times) previous topic - next topic

rfrankel

I'm a CS guy with a very basic understanding of electronics.  I read a few pages of search results but couldn't find an answer to my exact question.

I want to drive many common anode RGB LEDs off one TCL 5940.  Obviously I can do 5 LEDs (since there are 16 outputs) but I'd like to be able to do more.  Is there a way to have the same three PWM outs on the 5940 (say OUT0, OUT1, OUT2) sink multiple RGB LEDs?  Unless I have a really fundamental misunderstanding, it's not possible to put common anode LEDs in series (the notion doesn't even seem well-defined), so I guess I'd want to put them in parallel.  But I'm not sure what this does to voltages, currents, etc.  Can I do this, or am I limited to one LED channel sunk per PWM output?

My ultimate goal is to build room lighting, with several wirelessly-connected nodes (controlled by a single server), and if I could keep adding lumens, while having just a single TCL 5940 per node, that'd be ideal.

For reference, the LEDs I have are http://www.ebay.com/itm/160579743206?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649 and the datasheet is http://www.noodlehed.com/ebay/datasheets/10mm_RGB_LED_CA_Diffused.pdf

Thanks!

Noorman

There are ways to multiplex pwm channels but you will need some external electronics to achive this.

A way to do this is use eg a 4066 to control which led circuit is used.
You have to beware that latency in the switching or the program can cause flickering of the leds so you absolutely need to optimize for speed.

I know there are many ways that lead to results, so try to dig in the multiplexing bit.

Good luck!

Grumpy_Mike

#2
Aug 31, 2011, 10:00 am Last Edit: Aug 31, 2011, 10:03 am by Grumpy_Mike Reason: 1
Wiring LEDs in parallel is not a good idea because they will not shair current evenly and one will burn out. If you must then you will have to use a resistor in each LED. This makes the constant current output on the TLC chip a bit redundant.
It is not clear if these LEDs have to be controlled separately or if they can do the same thing.

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A way to do this is use eg a 4066 to control which led circuit is used.

Only a fool would use this to control an output. All of the disadvantages of multiplexing with none of the advantages.

rfrankel

Thanks for the replies.  I'd rather not multiplex and worry about flickering.  I think I understand the issue you describe, Mike, with current sharing...but how would I calculate the resistance needed on each LED in that setup?  Also, would the resistors go on each cathode (3 per LED) or each anode (1 per LED)?

Quote
It is not clear if these LEDs have to be controlled separately or if they can do the same thing.


Sorry, they can do the same thing; I don't need to be able to address them separately.  If I were to do wire them in parallel I'd have five control groups per 5940, but within each control group (of parallel LEDs) there'd only be one color (i.e. one set of three values).

dc42

The resistors would go on each cathode. I'd choose a value that drops 0.5v to 1v  at the maximum current you want to use per led, e.g. 33 ohm or 47 ohm if the max current is 20mA. This will only work if the leds you connect to the same 5940 pin are all of the same type and colour.
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rfrankel


The resistors would go on each cathode.


Thanks.

I'd choose a value that drops 0.5v to 1v  at the maximum current you want to use per led, e.g. 33 ohm or 47 ohm if the max current is 20mA.


How did you compute that?

dc42


I'd choose a value that drops 0.5v to 1v  at the maximum current you want to use per led, e.g. 33 ohm or 47 ohm if the max current is 20mA.


How did you compute that?


The higher the resistors, the better the current balancing between the LEDs will be. However, if they are too high, then the voltage across the LED + the voltage across the resistor + the saturation voltage of the 5940 will be greater than the supply voltage (which I assume is 5v), in which case the resistors will reduce the current to the LED.

The 0.5v to 1v was a guess on my part, but here is a calculation. Most LEDs have a voltage drop in the range 1.7v (red) to 3.4v (white and blue). The 5940 specs are quoted at 1v. So in the worst case you have 5v - 1v - 3.4v = 0.6v left to drop across the resistor. Therefore, plan for no more than 0.6v across the resistor, unless you are driving the LEDs from more than 5v.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.


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