I need help creating an LED circuit. I'm trying to make an infrared flood light using 50 IR LEDs. The 'light' is to be powered by a single 9 volt battery and last approximately 6 hours before draining a battery. The LEDs in question use a forward voltage of 1.3 volts, have a 1.2 volt drop and draw 50 mA each. The data sheet can be seen here LINK.

There is currently a light in production that uses these same specs, so I know it can be done, I just can't figure out the proper configuration and resistors to use. Any help would be GREATLY appreciated.

Thanks!

 

You're misreading the data sheet. Those IREDs want to be operated at 20mA. If you push 50mA through them, they won't last long. Using the 50mA figure, a 9V alkaline battery would run the IREDs poorly for 1 hour or less.

http://www1.duracell.com/oem/Pdf/Mn1604.pdf
http://led.linear1.org/led.wiz

 

Like I said, I know it can be done, look here.

 

So, lets see...

You want 50 Leds driven at 50 mA and a Vf of 1.3V.

Each LED is going to dissipate 1.3V * 50mA = 65 mW of power.

Fifty such LEDs are going to draw 65 mW * 50 = 3.25 W.

Lets say you power them all in series, using a 100% efficient step up converter to supply 1.3V * 50 or 65V at 50 mA (still 3.25 W).

Our fictional 100% efficient converter will draw 3.25W/9V = 361 mA. At low battery condition of 7V, it will draw 3.25/7V = 464 mA.

So, lets pretend on average we draw 412 mA.

At this level of current, an Energizer alkaline battery has < 400 mAH capacity.

Your LEDs will run, at most, 400 mAH / 412 mA = 0.97 hours.

Add in inefficiencies, and I'd be surprised if you do any better than 15 to 30 minutes.

I think you need to review your design specification.

 

Ok, assuming that I've gotten something wrong with the specs of the LEDs used in the flood light, what sort of LED and circuit WOULD I need to use to achieve the supposed 6 hours on a 9v battery? (NOTE that there is also a 25 IR LED model that touts 12 hours on a 9v)

Thanks!

 

You will need to keep the (average) battery discharge current less than 70mA, including inefficiencies.

Therefore, you can power, *at most*, about 560mW, for 4 hours. You can work the math backwards from there.

 

And this only applies at temps > 25C. Colder temperatures will significantly reduce your runtime.

 

I think you need to review your design specification.

What design specification? All I saw was a marketing blurb.

If you run the LEDs at a pitiful 5mA each (Why would you want to?), you may be able to get a 6-hour run time.

 

The link doesn't refer to what type of 9v battery is used. You may be thinking "transistor radio battery" and they may mean a battery pack of 6 "d" cell batteries.

 

What design specification? All I saw was a marketing blurb.

If you run the LEDs at a pitiful 5mA each (Why would you want to?), you may be able to get a 6-hour run time.

I was referring to *his* design specification in the original post.

 

I'm not trying to market anything...I just thought that close to $100 was silly to spend on something that could probably be produced for less than $25 in parts.

I am fairly confident that they use a single 9v battery to operate their equipment, as I've seen them used in various TV shows.

I just couldn't figure out how they were able to get these types of run times.

 

I was referring to *his* design specification in the original post.

I know. It's more like a concept. Maybe it uses one of these giant 5000mAH 9V batteries (46mm X 46mm X 60mm).

If you use the nominal figures of 1.3Vf and 20mA, the 50 IREDs need 7.8WH to run for 6 hours. That would drain 3-4 standard alkaline 9V batteries. The commercial models are either running a weird big battery or battery pack or they're running at a very low power. I like how the blurb reads "Imagine 2 times the power!". Well, yeah, you would have to imagine it because that's the only way you could experience it.

 

I know. It's more like a concept. Maybe it uses one of these giant 5000mAH 9V batteries (46mm X 46mm X 60mm).

That would provide a max 12 hours operation...less inefficiencies.

 

I understand that this all sounds weird and impossible, but I contacted the creator of the item and asked "What are the specs of the IR LEDs you use for your 50 LED IR Flood Light? I am having trouble understanding how you can get 6 hours of run time from 50 LEDs using only a single 9v battery."

I got a reply from the maker stating "The Flood lights actually last longer than that. I am not sure want you are wanting to know here. These are our own design and patent pending. I can not give out specs but they do last over 6 hours and the 25 infrared illuminator last over 12 hours of constant use."

So, I replied asking "Do these run on a single 9v battery or some combination of C or D cell batteries?" To which he replied "The lights run off of 1 9v and that is it."

So, ultimately, the reason why I came here to ask you all for help is because this all sounds nearly impossible to achieve and I figured that your knowledge would be able to shed some light on how they are accomplishing such a feat.

Thanks!

 

There is no magic here.

If what he says is true, they are running the 25 leds at a *much* lower current than you specified, and using a high efficiency step up converter.

The answer to your original question is: "its impossible". No ifs, ands, or buts about it.

But if you change the question, things become possible.

 

Isn't PWM a big part of the solution to this enigma? I would guess the LEDs are pulsed at low duty cycle but "high" (maybe even 50mA) current.

 

Isn't PWM a big part of the solution to this enigma? I would guess the LEDs are pulsed at low duty cycle but "high" (maybe even 50mA) current.

No.

In general, light intensity is a linear function of current (with some variation due to temperature).

Operating an LED 100% at 25mA produces about the same amount of light as at 50% and 50mA (on average).

Now, the intensity *is* higher during the on times, but the "average" intensity is the same.

Also, I assume he is illuminating an area for some kind of IR imaging or video application. If this is the case, the pulsing could cause flickering of the video if it is not synchronized with the frame rate appropriately.

 

It would be possible to create a synchronized, pulsed IR emitter but that would probably be an accessory matched to the camera by the camera maker. It would be a clever and efficient thing.

 

... the pulsing could cause flickering of the video if it is not synchronized with the frame rate appropriately.

I was thinking of un-synchronized, but higher frequency, say 30kHz. That wouldn't flicker, but of course wouldn't save power like a 1-pulse-per-frame scheme would.

 

I think the only way this would work is the "one pulse per frame" method, and less than the standard frame rate for continuous video. It is clear that a standard 9V battery doesn't have a chance of running this load continuously.