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[Reply #15 on:]

Ok...slowly i understand the circuit...and your calculation.
Due to evp's advice i will have to switch the led!!!
Just because of the price - the shop sells it for 2,14€ vs. 10€ this saves me so much money! - and this comes in handy with our problem (when i got everything right).
If i switch to this one http://www.satisled.com/3w-rgb-led-emitter-on-star-1w-for-each-color_p234.html i'll deal with 3.6 max v loss. So just to be sure i understood this correctly i have 5 - 3.6 - 0.65= 0.75 V Vsat. To get the red one up to the same V i burn 1V with a 2.8 Ohm resistor. The friction in the transistor is reduced to 262mW.
So lets give me another try with a part http://www.farnell.com/datasheets/1504602.pdf
Vsat is -500mV (i can provide 750 in the worst case), Powerdissipation can be over 1W (i am below with 262mW) Did i forget something?

Just to ask it: is it really that bad to drive an led via resistor without ccs??? I can't imagine how http://www.satisled.com/10w-9w-rgb-led-emitter-3w-for-each-color_p236.html those are driven... (they get special drivers i guess as dc42 mentioned in his first post).

[Reply #16 on:]

If i switch to this one http://www.satisled.com/3w-rgb-led-emitter-on-star-1w-for-each-color_p234.html i'll deal with 3.6 max v loss. So just to be sure i understood this correctly i have 5 - 3.6 - 0.65= 0.75 V Vsat. To get the red one up to the same V i burn 1V with a 2.8 Ohm resistor. The friction in the transistor is reduced to 262mW.

To calculate the maximum power dissipation in the transistor, you need to assume the minimum voltage drop in the LED (3.2v for the green and blue) and across the current sense resistor (say 0.6v). This gives 1.2v (assuming a 5v supply), which at 350mA gives a dissipation of 420mW. To this, you need to add the dissipation due to base current, but that should be only 20mW or so.

So lets give me another try with a part http://www.farnell.com/datasheets/1504602.pdf
Vsat is -500mV (i can provide 750 in the worst case), Powerdissipation can be over 1W (i am below with 262mW) Did i forget something?

Vce(sat) for that part is quoted at 500mA collector current and 50mA base drive. You can't provide 50mA base drive because your TLC5940s would get too hot. You'll need to run with something like 10mA base drive instead. Looking at that data sheet, the transistor looks like it may not have sufficient hfe at 350mA to guarantee a low enough saturation voltage. Also note that the power dissipation limit assumes 6 sq cm of copper as a heatsink, per transistor.

Just to ask it: is it really that bad to drive an led via resistor without ccs??? I can't imagine how http://www.satisled.com/10w-9w-rgb-led-emitter-3w-for-each-color_p236.html those are driven... (they get special drivers i guess as dc42 mentioned in his first post).

No, it's not that bad to use just a series resistor, especially as your new LEDs now give you 1.4v minimum voltage drop across the resistor. [The higher the voltage drop across the resistor, the better the current regulation, at the expense of more power dissipation in the resistor.]

If you use the resistor, you might wish to consider using mosfets (e.g. http://export.farnell.com/toshiba/ssm3j02t-te85l-f/mosfet-p-ch-1-5a-30v-sot23/dp/1714372 or http://www.farnell.com/datasheets/25641.pdf) for switching the leds, because this keeps the dissipation in the TLC5940 low, at the cost of needing a resistor (say 1K) to pull the mosfet gate high when the TLC5940 output turns off. Make sure the series resistors are adequately rated, e.g. the one for the red LED will dissipate up to 1W in this configuration.

Yet another possibility is to use a constant current driver in which the transistor passing the current is replaced by a mosfet - thereby avoiding problems of saturation voltage and allowing the TLC5940s to be run at a low current so as to keep them cool. This may be the best solution.

The other thing you might want to consider is whether it is still worth using TLC5940s, given that they are not cheap and you are unable to get the benefit of the constant current output.

[Reply #17 on:]

Ok...weekend is over(i need weekend) - now i have to do PROGRESS
I just made some decisions with my collegue who works with me on this project.
First of all i'd like to thank you two very much for your support!
We will do the "small version" with normal leds - we will face some other problems when it comes to etching for sure, so time is an expensive factor.
We will build it modular - so we can extend the actual controllerboard with a "driverboard" which contains the small solution of powering the 3W led by MOSFET and resistor.
The circuit design must be Pin (2.54) compatible. We can use our parts even if etching fails (which is quite hard when we work with smd).
And there is another question (yeah im not annoying ):
The posted MOSFET is smd - due to our decision it won't make it to the basket. I found another one which seems to be similar for me http://www.farnell.com/datasheets/14454.pdf - one thing i noticed: the GS threshold voltage is -2 to -4 on the recommended RTR030P02 (second link, previous post) it is -0.7 to -2. Will this work out with my -5V? I read that the on resistance is 0.32 ohm - do i need to subtract the volts i lose in the mosfet?
In the attachment i painted how i'd wire it. Please ignore the calculation - the correct one is: ((5V - 0.32ohm*350mA)-Urgb)/350mA = Rrgb
Thanks a lot!!!

[Reply #18 on:]

That mosfet isn't suitable, it's designed for 10v gate drive. Look for one that has Rgs(on) specified at 4v or 4.5v.

You need a resistor from mosfet gate to +5v. The value is not critical, I would try 10k.

Your calculation looks OK but bear in mind that the Rds(on) value quoted on the data sheet is usually the maximum value at the specified gate voltage, and it might be somewhat lower in practice.

[Reply #19 on:]

Finally we found one (yeah i really hope 4 eyes got the right one now)
http://www.farnell.com/datasheets/14454.pdf
Did you mean Ugs(on)/Vgs(on) here?

Look for one that has Rgs(on) specified at 4v or 4.5v.

[Reply #20 on:]

I meant Rds(on) @ Vgs. From that datasheet:

RDS(on) Static Drain-Source On-Resistance VGS = -10 V, ID = -3.5 A    Typ. 0.32 Max. 0.41 Ω

So it's not suitable, Vgs(on) isn't quoted at 5v or lower. Try http://uk.farnell.com/international-rectifier/irlib9343pbf/mosfet-p-to-220-isol/dp/9933867. If you go to the datasheet you will find that it has Rds(on) quoted at Vgs = 4.5v as well as 10v.

[Reply #21 on:]

Just for my basic understanding of reading a Mosfet's datasheet:
Rds(on) is the Resistance between Drain and Source when i connect the given "@" voltage.
Now according to Figure 12 of http://www.irf.com/product-info/datasheets/data/irlib9343pbf.pdf i know where i made the mistake:
I always thought the Mosfet becomes conductive when i have the right voltage difference between source and gate. But obviously things are not that easy
The problem is that the resistance gets very high when the Voltage difference between source and drain is too low! *lightbulb* So Vds(on) significantly influences the resistance of the Mosfet.

Are there more tripping hazards like this?

[Reply #22 on:]

No, that's not quite right, it's not the drain voltage Vds that affects the resistance, it's the gate voltage (Vgs) and the drain current (Ids).

When the gate-source voltage (Vgs) exceeds the gate threshold voltage, the mosfet starts to conduct. However, the resistance between source and drain will only be low up to a certain current. That current depends on the Vgs, the gate-source voltage. A higher Vgs keeps the resistance low up to a higher current. You can see this effect in fig. 1 and fig. 2 of the data sheet for the IRLIB9343PBF, the curves flatten off on the right hand side.

The actual limiting current for a given Vgs can vary quite a lot for a given type of mosfet. However, the manufacturer tests the Rds(on) at one or more combinations of Vgs and drain current (Ids), as shown on the data sheet. So you need to pick a mosfet where the quoted Rds(on) is low enough, at a test current at least as high as the current you want to switch, and at a Vgs that is no higher than the Arduino can provide (i.e. 5v).

[Reply #23 on:]

So just to learn something...
this one fits too: http://www.irf.com/product-info/datasheets/data/irlz24pbf.pdf
It is npn so the values of the pwm will react the other way round...

[Reply #24 on:]

Yes, that's specified at Vgs = 4v and 5v, so it's suitable for driving from an Arduino pin (a resistor between the pin and the mosfet gate of 100 to 150 ohms is recommended when driving power mosfets).

btw bipolar transistors are npn or pnp, but mosfets are p-channel or n-channel. An n-channel mosfet is connected it in the ground side of the load you are switching, rather then the +5v side. I wouldn't recommend driving an n-channel mosfet from a tlc5940 because reversing the pwm changes the range from from 0 - 4095 to 1 - 4096 so you can't turn the led off completely (or so I have heard, and it sounds entirely plausible).