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Topic: tlc5940 and led strips (Read 3432 times) previous topic - next topic

fungus


I thought the TLC5940 could drive (sink) up to 120mA per channel with VCC of 5V.


Only if you're very careful and leave less than 0.8V per channel for the chip to regulate/dissipate.

(ie. use an external resistor to drop the incoming voltage down to under 0.8V, let the chip do the rest...)

0.8V * 120mA * 16 channels = 1.5W dissipation - about as much as the chip can safely handle without adding fancy cooling.

No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

Grumpy_Mike


Ok i discussed this with a friend and the result was that i didn't understand what i was measuring more or less.

but now that i understand what i'm measuring i am still not sure what to insert in the power dissipation equation for voltage. i think i will need more testing nonetheless.

Ok now you understand what you are measuring what do you get?
That is with the anode to 12V, blue cathode to TLC5940 output set to full on, what voltage is on that output? Make sure the TLCs current limit resistors set for twice the current you are drawing. For a better measure then use a scope because with the output full omit pulses off for one click cycle and that will artificially raise the on voltage. You should see a small voltage.

tulpix

hello

sorry that this reply took so long, was swamped with work and needed some time to familiarize with the chip.

i did some testing. first with a 22k Ohm resistor (had no closer value to 2k) theoretically letting 17,7 mA through and second with a 470 Ohm resistor one letting potentially 83 mA trough.
then measured Volts and Ampere

for a 10 cm strip (3 leds) i get between led strip anode an red cathode:
1) 11,68V (directly attached to the anode and the cathode) 18,9mA
2) 12,05V (same measuring as above) 18,24 mA

directly connecting anode to +12V and cathode to ground gives 18,25 mA and 12,28 V
power source gives me 12,30V

so am i right that only ((12,30 - 12,05) = 0,25V goes into the tlc chip which by my calculations would mean i am on the safe side for power dissipation?
my calculation is based on: 20 mA (into the chip) 0,25 V(into the chip) and 12 outputs used (4 led strips per chip)

to make clearer what i measured i attached a picture

thanks

fungus


hello

sorry that this reply took so long, was swamped with work and needed some time to familiarize with the chip.

i did some testing. first with a 22k Ohm resistor (had no closer value to 2k) theoretically letting 17,7 mA through and second with a 470 Ohm resistor one letting potentially 83 mA trough.
then measured Volts and Ampere

for a 10 cm strip (3 leds) i get between led strip anode an red cathode:
1) 11,68V (directly attached to the anode and the cathode) 18,9mA
2) 12,05V (same measuring as above) 18,24 mA


Those numbers don't look right. Normally more volts = more amps.


so am i right that only ((12,30 - 12,05) = 0,25V goes into the tlc chip which by my calculations would mean i am on the safe side for power dissipation?
my calculation is based on: 20 mA (into the chip) 0,25 V(into the chip) and 12 outputs used (4 led strips per chip)


That number, OTOH, does sound about right. The strips are designed for 12V, your power supply is putting out 12.3V... 0.25 is pretty close to 0.3.

0.25V for the chip to dissipate is plenty of margin. You should be fine connecting a string of 15 LEDs to each pin (ie. 5 groups of 3 LEDs, 20mA per group = 100mA).

No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

mjkzz

#19
Mar 06, 2013, 01:43 pm Last Edit: Mar 06, 2013, 01:47 pm by mjkzz Reason: 1
Most of strips are voltage based, not current based, I mean, they have resistor built-in to limit the current. TLC5940 is a constant current IC so I am not sure you really need it, if you want to control color of the strip, just use PWM to drive three MOSFETs [edit] for each strip. Your power supply must be able to supply enough power, of course, otherwise, voltage at power supply will drop.

Just a thought.
Search for "mjkzz" on eBay :-)

Grumpy_Mike


Most of strips are voltage based, not current based, I mean, they have resistor built-in to limit the current. TLC5940 is a constant current IC so I am not sure you really need it,

We have been through that at the start of the thread. He is not using the constant current bit of the chip just the PWM bit. This chip gives 16 PWM channels which is more than you can get on the Arduino. This is why the power dissipation is so low, because the constant current part is not kicking in.

mjkzz

Search for "mjkzz" on eBay :-)

tulpix

Hello

thanks for all the input.
so it seems i can proceed with the project.
i will solder cables to the strips and hook them up in the next days.


thanks again for the help

Skorn

Hi,

Are you opposed to using a SoftPWM library for your project?  The following Library is great:

http://arduino.cc/forum/index.php/topic,121772.msg916796.html#msg916796

I used it to complete a 6 x 10w RGB Lamp Cluster project controlled by 1 Atmega 328p that SoftPWMs 18 pins/channels at 110Hz. The chip which is running at 16mhz has no performance issues and accurately handles IR command input on another pin, and does the required calculations to support dynamic lighting sequences/modes without a stutter.  Going this route would reduce your solutions hardware footprint alot, and you won't have to deal with the issue that comes with using the TLC5940 /inverted PWM: the lights won't fully dim to off...  They'll always be lit just a very little...

http://arduino.cc/forum/index.php?topic=88243.0

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