IR custom build - use resistor with LED driver?

Hi All, I am building a Spot beam IR LED Flash light with an aspheric lens.

Parts:
Osram SFH 4783 IR LED DATASHEET
1 x 18650 Orbtronic battery 3.7v
1500ma constant current LED driver based on

The driver will handle the current management aspect, so normally no additional resistors are needed. Adding a resistor in between the driver and LED will not reduce the current at all. Current drivers are designed to deliver the designed current "no matter what" (within their ability)

LEDs are not that expensive, but assembling it all cleanly in a light can be a pain. If you are wanting to push the current past the spec, but are unsure about the results, the easiest way is a simple bench test. Just rig up something similar on a table or workbench and run it for 1 week solid and see if it is still working similarly to the initial results.

LED damage from over current or over heating can happen pretty fast. I have ruined more than 1 LED within 3 seconds, some much faster. It is just part of the education process.

I took a look at that ebay driver spec. It has low / medium / high, but the mediums setting is 30 %, and it is a linear driver. It's lower voltage spec is 3 volts, so if that is really correct, (might not be) then it will have really short run time until your battery drops below 3 volts. It has no thermal path to speak of, will create more heat than the LED, and will be less than 30% efficient in your application.

Personally, I think it is the wrong driver for your application.

A closer fit would be a single AA energizer battery with small resistor, or if you really want to use an 18650, find a driver with an inductor.

Hi Harry, Thanks for your advice on this. I will try a few things and reply here with results. I've struggled to find a driver that can run with 3v supply and deliver 1000-1200ma. This was the closest one I could find.

Thanks again.

HarryN said:

I took a look at that ebay driver spec. It has low / medium / high, but the mediums setting is 30 %, and it is a linear driver. It's lower voltage spec is 3 volts, so if that is really correct, (might not be) then it will have really short run time until your battery drops below 3 volts. It has no thermal path to speak of, will create more heat than the LED, and will be less than 30% efficient in your application.

Personally, I think it is the wrong driver for your application.

A closer fit would be a single AA energizer battery with small resistor, or if you really want to use an 18650, find a driver with an inductor.

Click to expand...

I can't agree with much of that. A LiIon cell will maintain above 3V until nearly discharged. In fact some people call it fully discharged at 3.2V. Some say 3.0, 2.7, or even 2.5. But whatever you call it, there's not much life left between 3.2 and 2.5V.

The single AA energizer battery, even the lithium ones, won't ever reach 1.65V at 1A discharge, so that is a non-starter. The data sheet for the Ultimate Lithium shows it starting at 1.6V and dropping almost instantly to 1.4V at 1A load.

The efficiency of a linear regulator (they're all the same, Vled/Vbatt) with a LiIon battery and a 1.65V LED is pretty poor. However, it will never fall below 40%. In fact, it's 40% at full charge (4.1V), and climbs throughout discharge, ending at 55% at 3V. The driver would generate substantially more heat than the LED through much of the discharge, but it never exceeds 2.5W, so it's manageable. But you would need to mount the driver with some thermal compound to a heatsink (or vice-versa).

As far as it being the right driver, that's a judgment call. If you're looking for low heat or long battery life, then definitely not. If you want simple and functional and cheap, then maybe. By the way, you can get the same board at dealextreme for barely half the cost. It's SKU 127688. It is probably literally the same board.

A driver with an inductor will be a switchmode driver, and even cheap ones should be substantially more efficient. Good ones can be 90% or greater.

I found several interesting looking ones at dealextreme, but nothing exactly what you want. SKU 26107 should give you >1200 mA, but it's not really well regulated. SKU 237006 might be better regulated, at 1000 mA. If you're really concerned about low input voltage, SKU 106484 works over a wide range, but only outputs 800 mA. I think you could probably use two of them to get to 1600 mA. They also make 400 mA and 650 mA ones that look the same, probably just a resistor change. That's actually the most promising one in my mind.

The drivers at Dealextreme (or anywhere else that you can get stuff for less than $10 with free shipping) won't be high quality, and efficiency is not super either. I'd expect 70-80%, but could be higher or lower. Regulation is also spotty. But they are cheap.

If you feel ambitious, here's a pretty good list of LED drivers. It can be pretty intimidating at first, but as you enter the parameters you want, the list gets much shorter. http://www.videofoundry.co.nz/ianma...odes=any&size_min=&size_max=&order=drivername

If money is no object, I can modify one of my IS1006 or IS1011 boards for you, to do exactly what you want, as long as it's high/low/off or controlled by a potentiometer. Sticker shock might give you a heart attack though. And they are both linear, so efficiency will still suck.

D

DIWdiver said:

I can't agree with much of that. A LiIon cell will maintain above 3V until nearly discharged. In fact some people call it fully discharged at 3.2V. Some say 3.0, 2.7, or even 2.5. But whatever you call it, there's not much life left between 3.2 and 2.5V.

The single AA energizer battery, even the lithium ones, won't ever reach 1.65V at 1A discharge, so that is a non-starter. The data sheet for the Ultimate Lithium shows it starting at 1.6V and dropping almost instantly to 1.4V at 1A load.

The efficiency of a linear regulator (they're all the same, Vled/Vbatt) with a LiIon battery and a 1.65V LED is pretty poor. However, it will never fall below 40%. In fact, it's 40% at full charge (4.1V), and climbs throughout discharge, ending at 55% at 3V. The driver would generate substantially more heat than the LED through much of the discharge, but it never exceeds 2.5W, so it's manageable. But you would need to mount the driver with some thermal compound to a heatsink (or vice-versa).

As far as it being the right driver, that's a judgment call. If you're looking for low heat or long battery life, then definitely not. If you want simple and functional and cheap, then maybe. By the way, you can get the same board at dealextreme for barely half the cost. It's SKU 127688. It is probably literally the same board.

A driver with an inductor will be a switchmode driver, and even cheap ones should be substantially more efficient. Good ones can be 90% or greater.

I found several interesting looking ones at dealextreme, but nothing exactly what you want. SKU 26107 should give you >1200 mA, but it's not really well regulated. SKU 237006 might be better regulated, at 1000 mA. If you're really concerned about low input voltage, SKU 106484 works over a wide range, but only outputs 800 mA. I think you could probably use two of them to get to 1600 mA. They also make 400 mA and 650 mA ones that look the same, probably just a resistor change. That's actually the most promising one in my mind.

The drivers at Dealextreme (or anywhere else that you can get stuff for less than $10 with free shipping) won't be high quality, and efficiency is not super either. I'd expect 70-80%, but could be higher or lower. Regulation is also spotty. But they are cheap.

If you feel ambitious, here's a pretty good list of LED drivers. It can be pretty intimidating at first, but as you enter the parameters you want, the list gets much shorter. http://www.videofoundry.co.nz/ianma...odes=any&size_min=&size_max=&order=drivername

If money is no object, I can modify one of my IS1006 or IS1011 boards for you, to do exactly what you want, as long as it's high/low/off or controlled by a potentiometer. Sticker shock might give you a heart attack though. And they are both linear, so efficiency will still suck.

D

Click to expand...

Thanks so much. This is very helpful. I'm going to try to use arctic silver to attach the driver board to a copper plate.
I will also try driving this at 1500ma as well.
I'll definitely check this list of drivers also.
I'm not as concerned about heat or battery life for this project because I'll likely only use the light for a few minutes at a time.
Thanks again.

You're welcome.

By the way, 4 AMC7135's give you 1400 mA, not 1500. Just another example of truth in advertising...NOT!