LED driving; MOSFETs or Transistors?

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

How big of a matrix are you heading towards? How many columns and rows? It's important to know.

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.

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

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.

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.

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

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:

The entire panel, a 16x16 matrix, is like this:

Here's a more exposed view of several panels, backs and front, with a proto board nearby for a rough size idea:

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.

That makes so much sense to me. Glad to hear it.

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?)

Okay. Call it 50mA?

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

Yea, I couldn't decide how I wanted to word that :) We're on the same page though.

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.

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.

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 :)

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:

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