Is this the transistor I need to drive LEDs?

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MooreOrLess
 
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Is this the transistor I need to drive LEDs?

Post by MooreOrLess »

Please help a programmer do hardware. I am new to the Arduino, but I love it because now I can interact with the real world outside the box of my desktop PC.

I have a batch of 8x8 common-anode bi-color (red-green) LED matrices. (datasheet: http://oomlout.com/DATASHEETS/LED-8X8M-03.pdf) I want to use TI constant current sink drivers (TLC5940 or TLC5916) to control the columns, so I need to figure out how to switch each of the 8 rows. To provide 20mA for each LED in a row (16), I need to switch up to 320 mA on the high-side of the circuit. In spite of all the tutorials and examples online, I cannot find any clear examples that show how to select the right part.

I have done my research, and it seems that I need a PNP transistor or mosfet to source the rows under the control of Arduino output pins or a shift register. The partfinder lists 2N4403 and PN2907 as general purpose transistors that would meet the current requirements. Is this the sort of transistor I need for this application? Input will be most welcome.

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zener
 
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Re: Is this the transistor I need to drive LEDs?

Post by zener »

Technically since those parts are a matrix they are both common anode and common cathode... Looking at the Fairchild datasheet for those 2 transistors, they look identical, except for the Vcesat, which is better on the 2N4403 (looks like a typo on the PN2907 since the graphs are the same) In any case the SOT-23 has higher power rating than the TO-92. 320mA is pushing it a bit but you probably wont be at 100% duty cycle. You will drive each transistor with a IO line (8 total) unless you want to get fancy and employ some pin reduction techniques. Your TI part looks like it is serial interface so maybe 3 or 4 pins.

nothinrandom
 
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Re: Is this the transistor I need to drive LEDs?

Post by nothinrandom »

You can always go with a power mosfet, which will definitely be a better heat dissipator and be more robust when compared to a BJT. Because mosfets are voltage driven and not current driven, it's perfect for your arduino application using pwm or any digital pin. The really cool thing about a power mosfet is that it might just dissipate less heat. Power dissipation is calculated by this equation P = RDSon*I^2. You can find mosfets on digikey with RDSon less than 1mOhm. (320mA)^2*0.001Ohm is like no power loss whatsoever (stay cool). Here's the caveat. Think of a mosfet as a capacitor, and you need to charge it up in order for it to work properly. Some of these mosfet still state the charge at the gate, so you will need to place a resistor in series to the gate in order to properly charge it (think of a RC circuit). Anyway, tell me how fast your switching (PWM default is 490Hz), and I'll help you pick a mosfet from digikey.

MooreOrLess
 
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Re: Is this the transistor I need to drive LEDs?

Post by MooreOrLess »

BANNED:
I am looking to switch each of the 8 rows in turn and handle PWM and current limiting on the other side with the TLC5940. I really don't know exactly how fast the rows will need to be switched, since this is my first time trying this. I need to figure out how to supply the load on the high side before I can experiment with the refresh rate to try to create a flicker-free dispay.

I feel a bit foolish, asking questions without knowing more about what I need, but this is the learning process. I tried to research as best I could, but it seems like I am trying to do this differently than most of the examples I found. Thanks for your help.

MooreOrLess
 
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Re: Is this the transistor I need to drive LEDs?

Post by MooreOrLess »

Zener:
Thank you for helping me try to figure out some of my questions with the hardware. I feel comfortable with the programming, and I am trying to reduce the hardware to a set of pieces that I can understand the same way, like a Lego set. Thankfully, many of the ICs feel like that to me already.

You are right, I will only be running 12.5% duty cycle on the row drivers, and I'll be trying PWM on top of that. This is the first baby step on the road to full RGB matrices on a large scale. That will require some beefier switching to handle up to 24 LEDs per row, and even more power as I envision arrays that are driven at 25% or even 50% for greater brightness.

And it will never be enough, I will always need more power, MORE POWER!!!

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