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Topic: tlc5940 and led strips (Read 3 times) previous topic - next topic

tulpix

thanks

yes sink voltage was the wrong term. i meant the voltage that goes into the ic after the forward voltage drop of the leds.
because i need to put that in the power dissipation equation.

i decided to buy the ics and the strips and try.

will post my results but this could take 2 weeks

thanks for the input on this matter

Grumpy_Mike

I think the problem is that data sheet assumes you are using the chip to control the current and so the FETs in them are operating in a linear mode. What you want to do is to operate those FETs in a switching mode. The heat generated then is just the current through the on resistance of the FET. This is tiny compared with the voltage drop restricting the current so it is not in the equation. So set the chips current control to twice what you strip takes. Then turn one on fully and measure the voltage on the output, preferably with a scope. Use this voltage in that power calculation. This will be a very small voltage.


tulpix

#13
Feb 06, 2013, 07:47 pm Last Edit: Feb 06, 2013, 07:53 pm by tulpix Reason: 1
Hello

so i got the strip and i cut an 10 cm strip of it and did some testing. but the readings somehow confuse my understanding of how this should work :(

if i measure my power source directly without anything hooked up, i get 12.30 volt. which is ok since its a 12 volt power source
if i connect the common anode and the blue cathode directly to the power source  (without resistors or anything) i get 12.16 volts on the blue line. which seems high. shouldn't leds have a higher voltage drop than 14 mV ?

i then used a transistor setup. common anode to the power source, 3 transistors (one for each cathode ), no resistors and cycled the colors with the arduino and i only measured the blue line again.
then i get:
if blue is off, 4.20 volts
if blue is on without any other colors 12.12 volt
all colors on 11.9 volts

this confuses me

has anybody any advice on why this is or if i am doing something wrong ?

thanks

EDIT: i always measure between anode and cathode (holding the measuring wires directly on the anode and cathode wires)

tulpix

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.

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