High power IR LED control w/arduino and IR camera - NYCResistor:Microcontrollers

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Message from discussion High power IR LED control w/arduino and IR camera
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Date: Sun, 26 Aug 2012 17:21:37 -0400
From: Guan Yang <g...@yang.dk>
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Subject: Re: [NYCR:Microcontrollers] Re: High power IR LED control
 w/arduino and IR camera
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Or you could use a current mode switching regulator, or a simple bipolar transistor as a current regulator, which would have a dropout around 0.7V.

I have good experiences with this circuit:

http://www.pcbheaven.com/userpages/LED_driving_and_controlling_methods/?topic=worklog&p=2

But I usually use a forward biased red LED instead of the reverse biased Zener, to reduce emitter voltage. Yes, if you don't need a lot of precision, LEDs are great voltage references :-) 


On Saturday, August 25, 2012 at 20:34 , Nick Vermeer wrote:

> Ok.. You should check the voltage drop across one of the IR LEDs. If the voltage drop multiplied by the number of LEDs is close to your battery voltage then the LM317 could be hitting it's dropout. The maximum voltage the 317 will be able to supply is around 2-3v less than the input voltage. If this is the problem, you can use a low dropout regulator instead.
> 
> 
> On Sat, Aug 25, 2012 at 10:26 AM, Sameer <par...@gmail.com (mailto:par...@gmail.com)> wrote:
> > On the MOSFET I actually have source connected to ground and drain connected to the - of the LED chain.
> > 
> > -s
> > 
> > 
> > On Saturday, August 25, 2012 10:23:52 AM UTC-4, Sameer wrote:
> > > Ross-- yes, I'm pretty sure I have enough voltage. I noted earlier that when I string 8 LEDs in series they don't light at all. So I figured that my battery supported the drop for 7 LEDs but not 8.
> > > 
> > > Nick, thanks, I am wiring them according to this web page:
> > > 
> > > http://www.reuk.co.uk/LM317-Current-Calculator.htm 
> > > 
> > > I used the calculator to come up with a resistor value of 1.8 ohms to get me 700 mA. I have the + of the LED wired to the output pin and the resistor between output and adjust. (And power from the battery is wired to +) Then the - of the resistor is wired to a MOSFET (IRLB8721PbF) source pin. The mosfet drain goes to ground and the gate is hard-wired to power/11V. I have the mosfet in there so that I can add a control signal, but for now I want to just leave it on all the time, so this seemed like the best way to do that. 
> > > 
> > > Is that clear? I can upload a drawing if that's not clear.
> > > 
> > > Thanks
> > > -s
> > > 
> > > On Friday, August 24, 2012 10:09:04 AM UTC-4, Ross Williamson wrote:
> > > > Hi all 
> > > > 
> > > > (sorry if this was in an earlier message) 
> > > > 
> > > > You have enough voltage across the regulator for 7 LEDs with a voltage 
> > > > drop of 1.5ish V for each don't you? (about 12-13V) 
> > > > 
> > > > R 
> > > > 
> > > > On Fri, Aug 24, 2012 at 9:46 AM, Sameer <par...@gmail.com> wrote: 
> > > > > Hi everyone-- question about the LM317 -- 
> > > > > 
> > > > > I've been using the LM317 to power the LEDs just fine, when last night at 
> > > > > Resistor it was suggested that I actually measure how much current I was 
> > > > > putting through the LEDs. I did that and it turns out that when I have all 7 
> > > > > LEDs in series I'm only running about 180 mA through each LED. Now the 
> > > > > consensus was that this is probably fine because with all the LEDs running 
> > > > > at full current it could actually cause vision damage (and because it's IR 
> > > > > you won't have a reflex to close your eyes). 
> > > > > 
> > > > > However, I am curious why this is, does anyone have ideas? 
> > > > > 
> > > > > When I run the circuit with no load, I get 704 mA. With just one LED I get 
> > > > > just about 698 mA. I don't think it is because the battery has any internal 
> > > > > resistance/current limiting. I'm using a 2200 mAh 30C LiPo, which means it 
> > > > > can spit out 66A right? So that should be more than enough I would think. 
> > > > > 
> > > > > Thanks, 
> > > > > -s 
> > > > > 
> > > > > -- 
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> > > > 
> > > > 
> > > > 
> > > > -- 
> > > > Ross Williamson 
> > > > Associate Research Scientist 
> > > > Columbia Astrophysics Laboratory 
> > > > 212-851-9379 (tel:212-851-9379) (office) 
> > > > 212-854-4653 (tel:212-854-4653) (Lab) 
> > > > 312-504-3051 (tel:312-504-3051) (Cell) 
> > > 
> > 
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> 
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