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

Hippynerd

http://bildr.org/2011/08/74hc595-breakout-arduino/
heres an example of someone running 595s on 3.3 volts from an arduino. They are even running LEDs w/out resistors.

Then I read this
http://arduino.cc/forum/index.php/topic,26476.0.html

dhenry

1) Your hc595 is not a perfect voltage source, as it has significant internal resistance. As the current draw goes up, its output voltage goes down (when outputing 1), reducing the current draw. The same mechanism works with a mcu's pin.

2) Your led does not have a constant voltage drop: the voltage drop goes up, albeit slowly, when the current through it goes up.

Put the two together -> you don't get smoke when powering a led with a  hc595 directly.

Hippynerd

I've been looking about the internet for more info about 3.3v, and found this about the nano...

Power can be supplied to the Nano via the USB cable; feeding 5V directly into the 5V pin, or 7~12 (20 max, not recommended) into the Vin pin. You can only draw 3.3V at up to 50 mA when the Nano is running on USB power, as the 3.3V is sourced from the FTDI USB>serial IC. And the digital I/O pins still allow a current draw up to 40 mA each.

Im pretty sure that 50 ma wont be near enough, but If I resister the 5v input from the USB to 3.3v that may work. That way, the shift registers are powered directly from the USB, instead of through the arduino.
Its still going to need 17 resistors, which is messier than i would like, but less messy than 50 resistors.

Hippynerd

#18
Oct 29, 2012, 07:21 pm Last Edit: Oct 29, 2012, 08:12 pm by Hippynerd Reason: 1
Revisiting resistor options...
Ideally (from the pretty lights perspective), each LED would have a current limiting resistor (thats 192 resistors!), but because of many reasons, its very impractical to put 3 resistors on each LED, but not as impractical to put one on each LED column (or SR output pin, same thing) (48 LEDs), but even that is a lot parts, space, and complexity. Only one plane at a time is lit, so theoretically, it should be the same, but then I think only one LED is actually lit at a time, so it seems that resitoring the planes (common) (like I did on my other cube), would be viable.

I have noticed that on the other cube, when it lights up all the LEDs on a plane, they are not as bright as when it lights up only a few leds per plane. this cube will have 48 LEDs on a single plane, and my guess is that it the dimming would be more dramatic.

HRm... resitoring the USB power input wont work, because the current will vary depending on how many LEDs are lit at one time, I think i would need a 3.3 Voltage regulator to run the CRs at 3.3v.
A driver chip would be nice, sadly I havnt found one that will work with common cathode.

Hippynerd

#19
Oct 29, 2012, 10:10 pm Last Edit: Oct 29, 2012, 10:12 pm by Hippynerd Reason: 1
I found a 3.3v voltage regulator (from an old wrt router that died), snatched a few parts from it (Diode, coil, voltage regulator, cap), and soldered them up into a tiny regulated power supply.

This site tipped me off to the VR
http://kioan.users.uth.gr/wireless/wrt54g/supply.html

And looking over the datasheet, they had a typical circuit, which happened to be exactly how Linksys used it, so I just took the parts connected to the VR, and applied them based on the schematic in the datasheet http://html.alldatasheet.com/html-pdf/87877/ANACHIP/AP1501-33K5/1780/7/AP1501-33K5.html

I wasnt sure about the input cap, so I omitted that part, and it seems to put out 3.4v with no load.
The 12v brick I used said its .3a@12v, the VR is rated at 3a. Im not sure what my current needs are, but I think this will be more than plenty.


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