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Electronics Forum / Basics / January 2007LED driving; MOSFETs or Transistors?
Hi all, I am new to this newsgroup but have been doing some back reading and this looks like a great resource. I hope to learn a lot, and maybe contribute some! My question is to do with driving a matrix of common anode RGB LEDs. I am building a RGB LED matrix (just 3x3 right now) which I am driving with a ATmega8 AVR. I'm currently using plain old Radio Shack transistors to drive the rows and columns but it seems like I am wasting a lot of juice driving the transistors. Right now I am driving the transistors directly from the AVR, which can source/sink 25mA per pin. This causes the AVR to get really hot and I'd like to make my drivers use way less current. So, can you give me some ideas on what the "right" driver for my LEDs would be? I'm looking at Darlington pairs as an option, but the idea of a MOSFET that basically has no gate draw sounds very appealing. Problem is I don't know what to look for in a MOSFET. I don't need the big 3 pin with metal tab beasts as I'm only drawing a hundred or so mA through the transistor. Are there MOSFETs available in a small signal transistor style package? For reference, 1/3 of one "pixel" of my display currently looks like this: +5v---(E)Row PNP(C)---(A)LED(C)---(C)Column NPN(E)---GND I realize I can put resistors at my gates to decrease the current used by the transistors, but I need almost the full 25mA to switch the amount of current I am using for the LEDs. Any ideas? I'd love to know how the pros would do this and if there are any small package MOSFETs I could use. Thanks, Jason > I'm currently using plain old Radio Shack > transistors to drive the rows and columns but it seems like I am > wasting a lot of juice driving the transistors. Right now I am driving > the transistors directly from the AVR, which can source/sink 25mA per > pin. This causes the AVR to get really hot and I'd like to make my > drivers use way less current. Put a resistor in series with the base ! Mosfets are overkill for such an application. What's the transistor and what's the load current ? Graham >My question is to do with driving a matrix of common anode RGB LEDs. I >am building a RGB LED matrix (just 3x3 right now) which I am driving >with a ATmega8 AVR. ><snip> How big of a matrix are you heading towards? How many columns and rows? It's important to know. >Right now I am driving >the transistors directly from the AVR, which can source/sink 25mA per >pin. This causes the AVR to get really hot and I'd like to make my >drivers use way less current. ><snip> If I gather you correctly, you have an emitter straight to ground and you are directly driving the base! This means that the only current limit of any consequence is the mosfet impedance inside the AVR itself, so you are driving VERY hard into saturation -- a terrible waste. >I realize I can put resistors at my gates to decrease the current used >by the transistors, but I need almost the full 25mA to switch the >amount of current I am using for the LEDs. By this, I take it that you are arguing that you need 25mA of base drive. If so, this means you are probably wanting 500mA of collector current. Really?? At least with a 3x3, I have a hard time accepting this kind of argument. (Although rule of thumb may be 10x for saturation, I find 20x-30x covers quite well with many BJTs.) Add the base resistors for a start. But in any case, where are you headed? I think that's what is important to know. Whatever that end point is, if it is satisfied, then probably so will smaller test configurations also be satisfied. Jon > How big of a matrix are you heading towards? How many columns and > rows? It's important to know. Hi Jon, thanks very much for your response. Right now the largest I am considering is 8x8. The LEDs are RGB though, so it's more like 24x24. > By this, I take it that you are arguing that you need 25mA of base > drive. If so, this means you are probably wanting 500mA of collector > current. Really?? At least with a 3x3, I have a hard time accepting > this kind of argument. (Although rule of thumb may be 10x for > saturation, I find 20x-30x covers quite well with many BJTs.) I was mistaken in my original tests. I thought that I was needing 25mA to get the transistors to switch my full load but after a little experimenting this weekend I found it worked just fine with a 1k base resistor. I was looking for around 500mA (or more!) of collector current. The reason being that these are common anode RGB LEDs, so at any given time a single anode transistor might be powering 9 distinct LED elements. I am using PWM to control the LEDs so I'm actually sending pulses of more than 20mA to each LED. The datasheet says they are fine with 100mA each at a 10% duty cycle so I wanted to use somewhere between 20mA and 100mA pulses to get the brightness I was looking for. > Add the base resistors for a start. > > But in any case, where are you headed? I think that's what is > important to know. Whatever that end point is, if it is satisfied, > then probably so will smaller test configurations also be satisfied. This weekend I made the following changes and now it's all working great: Added 1k base resistors to my PNP anode drivers to reduce the base current from 25mA to ~5mA. Switched my cathode drivers to a Darlington array, with built in 2.7k base resistors, instead of discrete BJTs. It all seems to be working nicely now, and I'm wasting way less current driving all the transistors! Now if I could just get my surface mount USB controller soldered on to the darn board :) Thanks again for your response and help. I think I've got it figured out now. Jason >> How big of a matrix are you heading towards? How many columns and >> rows? It's important to know. > >Hi Jon, thanks very much for your response. Right now the largest I am >considering is 8x8. The LEDs are RGB though, so it's more like 24x24. Um, I think you mean more like 3x8x8, which is 24x8 not 24x24. Luckily. I posted some pictures I made earlier of some assemblies of RGB LEDs I have worked on here. A single RBG LED from it is exposed here, under a small magnifier: http://users.easystreet.com/jkirwan/tricolor%20led%20zoom%2001.jpg The entire panel, a 16x16 matrix, is like this: http://users.easystreet.com/jkirwan/tricolor%20led%20array%2002.jpg Here's a more exposed view of several panels, backs and front, with a proto board nearby for a rough size idea: http://users.easystreet.com/jkirwan/tricolor%20led%20array%2001.jpg Those are about 80 watts per, running full out as bright white, I seem to recall. And that is with separate supplies for R, G, and B and not counting the waste in the power regulation before it reached the panel. Lots of heat dissipation required. I don't know what you are doing, but I'd expect something on the order of 5-20 watts dissipation just in the panel itself without knowing more details. Worse, perhaps, if you use a single supply for the red, green, and blue and need to thereby drop more voltage uselessly in transistors or resistors driving the red ones. You may need to keep that in mind. >> By this, I take it that you are arguing that you need 25mA of base >> drive. If so, this means you are probably wanting 500mA of collector [quoted text clipped - 6 lines] >experimenting this weekend I found it worked just fine with a 1k base >resistor. That makes so much sense to me. Glad to hear it. >I was looking for around 500mA (or more!) of collector >current. The reason being that these are common anode RGB LEDs, so at >any given time a single anode transistor might be powering 9 distinct >LED elements. So all elements may start at the same moment. Are you able to individually PWM the three colors? And do so across all eight of the RGB LEDs you will one day include? (24 PWMs at the same time?) >I am using PWM to control the LEDs so I'm actually >sending pulses of more than 20mA to each LED. The datasheet says they >are fine with 100mA each at a 10% duty cycle so I wanted to use >somewhere between 20mA and 100mA pulses to get the brightness I was >looking for. Okay. Call it 50mA? >> Add the base resistors for a start. >> [quoted text clipped - 17 lines] >Thanks again for your response and help. I think I've got it figured >out now. Okay. Well, if you care for some more thinking about it or some ideas about experiments just say so. If you are considering the idea of doing several 8x8 RGB panels that may be used together, my recommendation is to consider the idea of using three potentiometers to allow you to set the 100% current level for each of the 3 sections, red, green, and blue, and to then use the PWM as a method to adjust the relative brightnesses of each LED. You can get gross coordination between the panels (in terms of white balance) that way. But all that will still be modestly problematic for a 3x8x8 panel. It's a lot to control well. Jon >> Hi Jon, thanks very much for your response. Right now the largest I am >> considering is 8x8. The LEDs are RGB though, so it's more like 24x24. > > Um, I think you mean more like 3x8x8, which is 24x8 not 24x24. > Luckily. Yea, I couldn't decide how I wanted to word that :) We're on the same page though. > I posted some pictures I made earlier of some assemblies of RGB LEDs I > have worked on here. A single RBG LED from it is exposed here, under > a small magnifier: Nice arrays! That looks like an exercise in patience! :) Luckily for me, my goal with this project is low resolution. It's more of an abstract art thing than a honest-to-goodness display. > just in the panel itself without knowing more details. Worse, > perhaps, if you use a single supply for the red, green, and blue and > need to thereby drop more voltage uselessly in transistors or > resistors driving the red ones. You may need to keep that in mind. My DMM tells me I am driving the individual colors at ~20mA after PWM. I don't have the knowledge, or the math, to determine how that relates to the actual current I am putting through the LEDs. I am using 47 Ohm resistors on the cathode of each color and feeding 5v, which tells me I should be doing pulses of ~100mA. If I pull back and measure current drawn from my power supply it's right around 250mA for the entire array so I am thinking that worst case I am burning around 1 watt in the array. >> I was looking for around 500mA (or more!) of collector >> current. The reason being that these are common anode RGB LEDs, so at [quoted text clipped - 4 lines] > individually PWM the three colors? And do so across all eight of the > RGB LEDs you will one day include? (24 PWMs at the same time?) That's right. My PWM scans each row for 255 time slices before going on to the next row. I turn on the power to the row and then loop through my frame buffer. If the color value for that "pixel" is supposed to be on I turn on the cathode transistor for that column. Right now I am able to scan the entire array pretty easily using direct IO from my AVR. Eventually I was planning to switch to shift registers to cut down on the IO pins needed. I haven't sat down and calculated if there will be enough clock cycles to actually do that with an 8x8 :) > Okay. Well, if you care for some more thinking about it or some ideas > about experiments just say so. If you are considering the idea of [quoted text clipped - 6 lines] > will still be modestly problematic for a 3x8x8 panel. It's a lot to > control well. I have been thinking about using pots to get the color mix set up correctly. It just happens to turn out that I get the purest white when I drive the red a little harder than the blue and green, so I am just using the same resistances for all three colors. If you are curious to look, I put up some videos of the array operating the other day. You'll notice a little bit of flashing and shearing in the colors. I need to adjust my software to only modify the framebuffer when the PWM isn't actively scanning. Feels strange to add a "vertical retrace sync" to a digital display! The videos are at: http://www.vonnieda.org/blogdetail.jsp?id=1301 Thanks again for all the help! Once I am happy with things I'll post my schematics here to ya'll to tear apart for me :) Jason |
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