Question about LEDs

[350xfire]

I am going to play with an SSC P7 LED module to see how it works for a dive light. This is a 900 lumen single LED emmiter that is supposed to be one of the brightest out there. My question is:

1. If an LED is rated for a certain voltage and amperage, can you half either the voltage or the amperage and double the other? Example, LED rated for 3 volts at 3A, if you feed 1.5 wolts, can you feed 6 amps without burning up the LED? This will be important when choosing the battery pack.

2. A 2500mAh battery rated at 1.2 volts... Does that mean that at 1.2 volts that battery will theoretically deliver 2500mA in one hour?

3. What are things that would burn up an LED, in other words, things not to do?

Thanks

 

[Roko]

LEDs are diodes, and have a forward voltage that must be met before they will start conducting electricity (eg, turn on)

1) I'd need to see a datasheet on that particular LED to know what the forward voltage is, but usually white LEDs have a higher forward voltage. Since it's rated at 3 volts, that may mean the forward voltage required to turn the LED on is 3 volts. The current rating should not be exceeded, as it will burn out your LED.

2) It will theoretically provide 2500mA steady current for one hour before the battery is disscharged, however the battery voltage will slowly drop while discharging. Once the cell hit's a certain voltage, it's considered "disscharged".. I forget offhand what this is for NiMH batteries....

3) Make sure you have a really good heat sink for that LED.. it'll produce a lot of heat, and if it overheats, that'll burn it up..

Hopefully that helps, and makes some sense.. It's latish and I'm tired.. If you've got any more questions, feel free to ask

 

[Packhorse]

1. A P7 C bin requires 2.8 amps to get 740-900 lumens. This will require about 3.5-4 volts.
If you put more voltage into it it will consume more current get hot and die.

2. Theoretically yes. In practice no. Drawing that much current in 1 hour will result in lost efficency manifesting as heat. If you drew 100ma you should be able to that for 25 hours.25x100 =2500
1v is considered discharged for NiMh I think.
3. Dont put more than 2.8 amps thru it. Make sure you heat sink it properly.

You need to decide on what type of current regulation you want to use. If you use something like the dive LED genie I think you can use any battery voltage above 3.5-4volt up to 30volt. The Genie will convert most of the energy from the battery to run the LED.

If you use a simple AMC 7135 type driver (or 8 of them for 2.8 amp) then you need to match the voltage as close as you can to the forward voltage of the LED. The best would be 3 Alkalines at 4.5 volt or 4 NiMh at 4.8 volt. Even at this voltage the AMC 7135s will get very hot burning off the excess voltage (wasted energy).

Im currently building one see here
And Im using 4 NiMh cells for now. But may use a dive genie and a 12v pack in the future.

 

[350xfire]

Thanks.

So, are you guys saying that you can feed a higher voltage, but not current?

ALSO, What about using Lithium batteries like the Energizer e^2 rechargeables? The AA are rated at 1.5V at 2500mah.

 

[reefduffer]

So, are you guys saying that you can feed a higher voltage, but not current?

An LED, like any diode, is a two-terminal device. You can't independently control both the voltage across it and the current through it. As Roko said above, you'd need to see the datasheet to know the exact relationship, but for any value of voltage across it, there can only be one value of current through it, which is a property of the LED, not of any external circuit.

[There's room for a theoretical quibble about device temperature, which in a sense makes a diode a three-terminal device, but in practice if you're using it to produce significant amounts of light the ambient temperature is a small contribution to device temperature, and so this isn't meaningful in an operational sense.]

If you put some fancy current-limiter in the circuit, it will operate, effectively, by dropping the voltage across the LED so as to conform to the LED's voltage/current properties.

Hope that helps.

 

[Packhorse]

Thanks.

So, are you guys saying that you can feed a higher voltage, but not current?

ALSO, What about using Lithium batteries like the Energizer e^2 rechargeables? The AA are rated at 1.5V at 2500mah.

No that is not what we are saying.
You must put the correct voltage into it so it will not consume too much current. You can not change the voltage with out the current being changed. The 2500mah on the battery is not how much current it supplies at once but how much it can supply total. EG you can use any mah rated cell but the larger ones will power the LED for longer.

You can only direct drive a LED off a battery if the battery voltage is below the Vf (forward Voltage ) of the LED. In the case of a P7 its about 3.5-4 volt. If you want to use a higher voltage then you need a driver board like this (2 infact).
These regulate the current to the LED by reducing the voltage. Have you read my thread about my build up? It may help explain it more.

Are the Energizer rechargables lithium? Im sure they are NiMh. Anyway as long as the voltage is correct and the battery can supply enough current then any battery will work. Although some batteries can not supply their full capacity in less than an hour.
EG 3 x 1.2volt 2500mah cells in parallel will give you 3.6 v 2500mah. Direct driving the LED is OK at this voltage but it will consume 2800ma 2500mah/2800ma= 55 minute run time theoretically but it will probably be 45 minitues and very hard on the batteries. Better going for 5000mah cells.

 

[350xfire]

http://www.kaidomain.com/WEBUI/ProductDetail.aspx?TranID=4338

I bought this one since it will regulate to 2800ma. Made for the P7. What you said makes sense now. So, with this regulator, I can build a 5000 mAh battery pack (just for conversation) and will burn longer.

BTW, the Energizer e^2 are NimH (2500mAh/1.2V). I guess I could put two sets of three in series and then parallel them to make a 3.6v 5000 mAh pack, right? So total 6 batteries. If I put all 6 of them in series, would I see the same effect?

 

[350xfire]

Packhorse, you say that the LED is the same as Salvo's 12W. I wonder, which LED are they using for the 21W? I have the 21W Salvo, have not dove it yet, though.

What I would like for the P7 is to build as a handheld with Goodman handle with 10-12W going to it. Do you know what the best method to accomplish this would be? The Mag Light housing is great, but seems a bit large. I did buy some piezo switches for my light.

 

[Packhorse]

Dont series parallel NiMH or NiCd. You get problems charging. Find a cell with the amp hour you like then series then to the voltage you want.
That driver you linked to is the 2 of the ones I used joined together. But with the way I did it I can have half or full power.
If you go for 3 NiMh or 1 LI Ion you will only get full brightness at the beginning and it will slowly dim down. If you want full power for the whole time you must use 4 NiMh /NiCD. But you will need to heat sink the driver.

I think I was wrong about the salvo. They seem to use the Osram Ostar LEDs. They are similar but require a higher voltage because the 4 emmiters are wired in series as opposed to parallel in the P7.
The 21watt salvo uses the 6 emmiter ostar. Very expensive for only a few lumens more and also needs to be run at 20 volt+.

If you want it hand held get a mag and a D sized Li Ion. You can cut the mag quite short. But you will not get 900 lumen at least not for very long. Run time will be up to 2 hours or so. Problem is using a piezo switch and that driver as a piezo is a momentary switch and the driver needs a toggle so you will need more circuitry or a diffrent driver.

 

[350xfire]

So, question about batteries:
The NiMH batteries I have are rated at AA 2500mAh and the AAA at 900. What are C and D batteries usually rated at? Also, any other batteries worth using?
Thanks again.