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Topic: RGB SMT LED Cube, resistors, drivers, and shift registers. (Read 24 times) previous topic - next topic

Hippynerd

That seems a bit excessive, I would have to heat it, to soften it, to straighten it, then harden it to help keep the structure of the cube. I certainly wont be heating up galvanized wire till its glowing red.

If i were to heat wires, I think I would use music wire, but use a lot of heat to tin them (like with a solder pot, or small torch.)

I really like the look of the copper, but its too soft for cubes, they really think they need a bit more durability than copper or aluminum provide. The music wire is also small enough that you can barely see it.

Hippynerd

I think i have worked out controlling the columns on the cubes, but I need some help with controlling the planes. One cube sinks(Shift register/resistor cube) the planes, the other cube sources(constant current cube). 

It looks like the mosfets that I purchased are totally not appropriate for what theses, which kinda sucks, but moving on, I need to figure out a good way to sink, and source 5v upto 1 amp.

Based on what i've read, it seems like there are basically 2 viable options, Darlington arrays and Mosfets. It sounds like mosfets are a better option, because they waste less electricity.

http://learn.adafruit.com/rgb-led-strips/usage

So, it seems I should get an N channel mosfet or maybe I could use a couple ULN2803 chips for sinking the planes on the shift register cube.

How about sourcing 5v?, should I use a P channel mosfet? What are other options?

I understand that I want a fet with low RDS, typically measured in millionths of an ohm, What are all the factors I need to consider when shopping for mosfets?

Crossroads posted a way to sort through the mind boggling inventory at digikey, That is a good start. I tried to apply the concept at mouser, but that didnt work out well. Ebay, Thaishine, and Tayda all seem too difficult to figure out what part I want. 

Newmark has a good looking search interface, but Im unsure exactly what things to select. I was able to knock the list down to 100 parts for both n and p channel at 1amp.

I think i also need the parts to be logic-level, that is to say, they can be switched with 5v.

Jameco has lots of parts, but the search interface is kinda difficult.

If I knew exactly what i was looking for, and exactly what specs I should be looking for would make it easier,

I am also uncertain about packages. for 1amp at 5v, I dont think i need any heatsinks, but I would rather buy a part that wont need one, if that is an option. Smaller  (as in takes up less space), is better than bigger. SOT23 isnt too hard to solder, so even that isnt too small.


I've modified one of my charliecubes to use 100 ohm resistors, and it is noticably dimmer, Im going to try to measure some LEDs with a few different resistors sizes and calc the current use, and try to get them closer to 20ma. Nearest I can tell, I will have to settle for less current on the blue/green, If I drop to 50 ohm, that should put the blue/green at about 20ma, but the red should be over 20ma. Because of the layout, you can only use one size resistor and keeping red within spec means putting blue and green considerably lower current than red.

If you are considering trying music wire, I have found that for this size cube, 015 is too delicate (you can do it, but its very difficult), and 025 is more than sturdy enough, so Im going to try 020, I think it could be the ideal size for this size cube.

Tom Carpenter

#72
Jan 19, 2013, 12:36 am Last Edit: Jan 19, 2013, 12:45 am by Tom Carpenter Reason: 1

I understand that I want a fet with low RDS, typically measured in millionths of an ohm

I think that is mOhm, or milliohms, i.e. thousandths of an ohm.


These are what I chose for my 12x12x12 cube. I ended up with 72 of them, and each one supplies 2 rows of 12 RGB LEDs, totalling 24*3=72 diodes per transistor = 1.44A peak current.

http://uk.farnell.com/nxp/nx2301p/mosfet-p-ch-20v-2a-sot23/dp/1894738?Ntt=1894738
I chose them for (a) price, (b) size, and (c) they are rated at 2A drain current at a Vgs of -4.5V making them suitable for 5v logic levels.

Ideally one with a slightly lower on resistance (these are 0.1Ohm at -4.5V) would be better, but these were a good price and I only lose 0.144v which would have been lost over the PWM constant current sources I have wired to the Cathodes. In my case theThe power lost is 1.44^2 * 0.1 = 200mW, which is well withing the power dissipation the transistor can support.
~Tom~

Hippynerd

Ah, now that you mention power dissipation, I remember reading something about power dissipation, I think it was nick. He mentioned a bunch of things that were a bit over my head, and hard to comprehend. There is something about gate capacitance or some such thing that I should also consider?

I think nick also posted some nice tiny FETs that were rated at 4amps, which seemed very surprising to me that such a tiny thing could handle such current.

So, it seems i will get the best results with a mosfet in both circumstances?


I decided I should test and measure the current on the common cathode LEDs that im using on the charliecube. I would like to find ideal resistors for them, by my calculations 200 ohms should keep the LEDs safe, but you dont know unless you measure. I soldered some resistors on to some LEDs, and hooked them up to a 5v power supply. I found that the blues were running at 20.9mA. I measured the power supply, and found that it was actually 5.25v.

I tested the test fixture running off usb, and it was 4.17v on the Vin pin, but a little higher on the 5v pin. I tested the cubes running on 500ma, and 850ma power bricks, and they were both different too. and both well under 5v. The highest voltage reading I got was on the 850ma on the 5v pin at 4.8v

If the LEDs are getting 4.2, then they need much smaller resistors than if they are running at 5, but since every way that I power them, they run at different voltages, i cant figure out the right size resistors, other than,200 ohm more than enough. At 5v, 200 ohms (2 100 ohm resistors) should be limiting the current to 14mA on the red. At 4.2v, those same resistors are limiting the LEDs to 10mA. If I change to 50 ohm resistors(100 ohms), and the LEDs are only getting 4.2v, they should be at 20mA, and the blue and green will be well under 20ma. but if they are getting 4.8v, then 100 ohms will only limit them to 25mA, but the blue and green will be down to 12ma.

Somehow, i dont think these LEDs are going to ever see 5v, no matter what.

Hippynerd

Yet another crazy cube idea. one of the nice things about this setup is that you dont use too much current, and you dont have to worry about mosfets, It does require more parts than the charliecube, but its only 16 transistors. Because it only lights up 12 LEDs at a time it only uses 240 mA, but it has a duty cycle of 4%, so it wont be very bright.

Here my modification of a 5x5x5 transistor cube (thanks crossroads), that I changed to 4x4x4 RGB. I didnt quite draw all the lines in, but hopefully enough is there to get the right idea.


At this rate, I may never get the original cube SMT constant current cube finished, or the shift register-resistor cube built.

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