IR LED Driving Solution

[dsholz]

Hey everyone.

I'm looking for a good way to get a stable light output from a 3 watt IR LED, using two 1.2v nimh AA batteries. Although I can just use a resistor, I don't always get a stable light output (espsecially as the batteries drain). It seems like this would be an ideal situation for a current regulator, however, the LED requires 2 volts @ 1.5 Amps, and I haven't been able to find a regulator that would work for this.

Can anyone help me out? Maybe there is something on the market that I've missed? I would be willing to build my own regulator, if that is what is required, but I don't have a EE background, so I have no idea what parts or design would be required for this.

 

[MikeAusC]

The AMC 7135 will give 350mA as long as there is more than 100mVolt across it.

You can get 4 in parallel on a driver board from DX for about $5 with free postage - this will give you 1.4 amp as long as the batteries stay above 2.1 volt output.

 

[Steve K]

I've built a linear current regulator using a LM10. This hand little part has an op-amp and a voltage reference built into it, and can operate from supply voltages as low as 1.1v. About all you have to do is add a NPN transistor and a current sense resistor. The NPN will use up about 0.3v, and the sense resistor will need 0.2v, so it'll need 2.5v to work with a 2v LED. Maybe you could substitute a mosfet for the NPN, but it seems unlikely that you could drive it very hard with only 2.4v to work with.

To do much better than this, I think you'd have to find a good CMOS IC designed specifically for this sort of function.

Or maybe find a switching power supply that would do it? I've used a nice little boost converter from Zetex that drives 4 yellow leds in series from a single AAA nicad. It only drives them with 20mA or so, though. There's no reason that someone couldn't design one that could deliver more current, though.

Looks like TI offers some converters that can deliver about 1 amp. Not bad.

regards,
Steve K.

 

[dsholz]

Mike:

DealExtreme says each of these drivers have 4x AMC7135 ICs in them, so would one of these work?

http://www.dealextreme.com/p/amc7135-1400ma-regulated-circuit-board-for-diy-flashlights-20-pack-3178

I'm a bit worried that it says that the recommended voltage range is 3.7v. Is that not a problem?

If these ICs aren't appropriate, can you direct me to a proper one on the site? I don't see any single IC versions.

 

[MikeAusC]

Yes, these are the ones I was thinking of.

The AMC7135 will provide the specified current, provided the battery voltage is higher than the LED voltage by 0.1 volt and the battery voltage is less than 6 volts. The chips just wastes the excess voltage by converting it to heat.

Because they're mainly used with White LEDs, they suggest using a single LiIon cell - i.e. 3.7 volts.

 

[dsholz]

Thanks!

Do I just hook the LEDs and power source directly into these? Or does there need to be a added resistor as well?

Also, do you happen to know if there is any place aside from DealExtreme that sells something like this? (something outside of Asia, so that I could get some a bit more quickly)

 

[MikeAusC]

No resistor needed.

Kaidomain may be faster than DX.

 

[dsholz]

Thanks! Ordered from both on 5/8, Kaidomain just arrived, DealExtreme just shipped (ouch).

Could someone tell me where to solder the positive and negative LED leads to on these things? Here's a picture of what I got: http://dsholz.com/amc7135.jpg

Also, on the bottom side, the inner ring is positive (to be connected to the batteries) right?

Someone said "you can set it to 3 modes (L-M-H) by soldering the two right legs together on the bottom row (looking at the board with the ATMEL chip at bottom)", is this referring to the two top dots on the copper plated (bottom) side? I assume the mods just switch sequentially as the power is turned on and off?