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Topic: Arduino Uno, TLC5940 and CAT4201 how do I make them work? (Read 1 time) previous topic - next topic

XenoSaga


why not look at the cat4101 - less additional hardware needed to accomplish same thing


Yes I had a look at that driver too but exactly as Chagrin stated, I would waste a lot of power and would need a heat sink especially for strings that only have two LEDs and need 6 V.


Yes, that's it, Xenosaga. Word of caution though - while the datasheet states that 90K is the maximum recommended Rset value I believe that that translates to the datasheet's stated minimum 50ma of LED current (never fully off). The datasheet never fully answers that question and only shows current up to 35K (Fig. 7 pg. 3).


Thanks a lot! I think I will just get the components and see how and if it works and if not I will bug you guys with some questions about that inverted input on CTRL ; )

Greetings

XenoSaga

Hello there,

I finally got around to building the circuit as depicted in post #3. As stated by Chagrin earlier the LEDs do not fully switch off.
To make matters worse, even at full PWM Input from the TLC5940 the LEDs (350 mA) are orders of magnitude dimmer than
a standard 5 mm red LED.

In the light of the not-switching-off issue, is there a possibility to invert the input from the TLC5940?

Greetings
Tristan

Skorn

#7
Mar 07, 2013, 11:55 pm Last Edit: Mar 08, 2013, 10:29 am by Skorn Reason: 1
Hi Tristan,

I made some related comments recently in a thread regarding the "always on" issue with the TLC5940 here:  

http://arduino.cc/forum/index.php/topic,144330.15.html

The PWM output on that chip can never be truly at 100% duty cycle, only slightly under.  As youve figured out, the PWM is inverted due to the fact the chip is a current sink so the LEDs can never be fully turned off.  If that is a manditory requirement to your project You'll need to modify your design.  Four high level ideas I have off the top of my head are:

1.  You can have the microcontroller cut the power to the LEDs using a relay/switch of some kind.
2.  It should be possible to invert the 14 TLC5950 PWM signals using a transistor for each.
3.  You could implement LED drivers that use P-FETs to switch current.
4.  Do away with the TLC and Impliment a simpler design using a SoftPWM solution as I explain in the post I made in the thread linked above.




XenoSaga


Hi Tristan,

I made some related comments recently in a thread regarding the "always on" issue with the TLC5940 here:  

http://arduino.cc/forum/index.php/topic,144330.15.html

The PWM output on that chip can never be truly at 100% duty cycle, only slightly under.  As youve figured out, the PWM is inverted due to the fact the chip is a current sink so the LEDs can never be fully turned off.  If that is a manditory requirement to your project You'll need to modify your design.  Four high level ideas I have off the top of my head are:

1.  You can have the microcontroller cut the power to the LEDs using a relay/switch of some kind.
2.  It should be possible to invert the 14 TLC5950 PWM signals using a transistor for each.
3.  You could implement LED drivers that use P-FETs to switch current.
4.  Do away with the TLC and Impliment a simpler design using a SoftPWM solution as I explain in the post I made in the thread linked above.


Hi Skorn,

thanks a lot for your input. First thing I'm trying now is via Darlington arrays according to your suggestion #2. But while waiting for the parts I'm going
to try the SoftPWM, sounds intriguing. And as you mentioned in the thread, reduces the amount of parts needed.

Greetings

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