Prop controlling variable current for LED driver?

Joms

I have been working on the project for awhile, but running into too many walls. Taking a step back, I am looking to drive 9 or so LED's from a prop. They are high power LED's that require about 700mA of constant current. (see link below for specs)

From my trial and error, I am guessing I will need to run about 3 in series, and limit the current to 700mA per array of 3.

My final project will have 12-800mA (red), 9-800mA (green), 9-700mA (blue), 6-700mA (white), in which I will try to use a prop to control the brightness of each color...

MY QUESTION- Does anyone know if there is a digital to variable current driver IC available? Something that might be I2C in, and variable variable current out? Am I more concerned with current then voltage? Does anyone know any good information I can read on this?

THANKS IN ADVANCE!


http://cgi.ebay.com/3W-High-Power-Blue-Led-Without-AL-Base-Board-48pcs-/380317571478?pt=LH_DefaultDomain_0&hash=item588cb19996

StefanL38

Hi Joms,

allegro has various LED-driver-chips.

most of them are for current of 150 to 350 mA.

http://www.allegromicro.com/en/Products/Part_Numbers/6210/index.asp

You need 700mA.

Brightness of LEDs depends mostly on the current.
Your powersupply should provide the voltage like written in the specs of the LED

Current can be easily controlled through PWM.
For creating PWM there is an object in the obex which can create different independent PWM-signals
for ALL 32 IO-pins. So 9 IOpins is a piece of cake.
If 9 IO-pins is too much you could use a shiftregister like the 74HC595 which requires 3 IO-pins (there is a PWM object for this chip too in the obex)

You could use a simple I2C IO-expander too. Then you need a PASM-driver to be fast enough to do the on/off-switching of the IO-expander-outputs

All of the above suggestions would need a FET (Field-effect-transistor) as the powerswitching element.
You have to choose the FET carefully that the FET can be driven directly from 3.3V (or maybe 5V through a 3.3V/5V-buffer
(Gate to Threshold Voltage less than 5V)

With a quick look I found this type BUZ10 and IRF510
Some months ago somebody mentioned a "well suited" FET for the prop but I can't remember the type-number

Another suggestion is to use a LED-driver-chip for small currents which creates PWM at the outputs himself and feed the PWM into MOS-FETs (Metal Oxide Semiconductor Field Effect Transistors)

best regards

Stefan

Rayman

an IGBT might be more cost effective that a mosfet for this current level..

kwinn

I would suggest using a bipolar transistor circuit similar to the one attached. You will need to replace the 2n3904 with a transistor that can handle the required current and power and select a resistor (Ri) for the desired current. The input can come directly from the uP, from a shift register, or other circuit. It has the advantage of providing a constant current regardless of the input voltage to the led so the unregulated supply can be used to power it.

tonyp12

here is a list of 700mA to 1A LED drivers.
http://www.mouser.com/Semiconductors/Drivers/LED-Drivers/_/N-7zhqfZscv7?P=1yzvmmbZ1yzu5uaZ1yzu5u9Z1yzufvzZ1z0wt49Z1z0ws57&Ns=Pricing%7c0

They probably all support PWM dimming.

Joms

Thanks for all the replys...but bring up more questions...

I guess I am liking the idea of an actual driver IC because I am going to be making a large quanity of these, that it will be less components. I was looking at the following link I found thru the mouser link...

http://www.diodes.com/datasheets/AL8805.pdf

MY QUESTION - If I use the above unit, looking at the specs, could I run 3 LED's in series, per driver IC? So I would basically have 700mA @ 12v going to the LED? Or does each LED draw 700mA, and putting them in series would make it 2.1A?

If I look at the data sheet correctly, I could hook a prop pin thru a 1k resistor to the control pin on the driver IC and use PWM for dimming?

THANKS!

kwinn

The 3 leds in series will draw 700mA (same current goes through all 3). The input voltage must be high enough to overcome the sum of the 3 leds forward voltage drops plus whatever the driver chip requires.

tonyp12

this is rated 3A
http://www.mouser.com/ProductDetail/Texas-Instruments/TPS61500PWP/?qs=QZKT0uTfj2LCtVhZh7Wjpw%3d%3d

btw the leds on ebay are not mounted on aluminum back plates, they will overheat.
if you design the pcb to act as giant heatsink, it may work

Clock Loop

I know high power leds like these very well, I have made a design myself that used luxeon 3 leds and rebel leds. The maximum rating on these leds is around 750ma. If you push the leds at even 1/2 that current the leds will produce so much heat that you will burn them out very quickly if you do not use major heatsinking and fans.

Treat these leds like they are a computer processor, because they will heat up just as much, and they do not have internal protections, with exposure to high heat the leds will deteriorate until they don't work or emit very little light.

All manufacturers of these leds boast high lumen output, large current capability, but they all hide the fact that you will need a massive heatsink, fans, and possibly even an air-conditioned air supply depending on how many leds your working with in an enclosed space(enclosure)

You need to keep the leds ROOM temperature to keep them from burning out within 2 years. (another claim that these manufactures boast is 10 year life span, but they fail to mention that they use active air-conditioned cooling for their testing)

bradharper

I'm still getting some of the management aspects (enabling/disabling) worked out, but the STCS2 is a driver that I use and will certainly do what you're looking for. There's a single current limiting resistor that can vary the constant current up to 2A. The part count is pretty small, and you can use the schematic as is in the data sheet - which also happens to show the calculation for the 700mA you need.

http://www.st.com/stonline/products/literature/ds/13968/stcs2.pdf