LED's aren't hard to get working reliably as long as you remember three things.
1. Get the Plus/Minus polarity right
2. Limit the current to the rated value.
3. Use an adequate heatsink for the power the LED is running at.
The recommendation to just hook the P7 up to a 3.5 volt high current power supply is a little dangerous because the P7 can draw a huge amount of power and destroy itself if the power supply doesn't limit the current somehow.
If you have a good heat sink, you can probably get away with direct driving the P7 from 3 alkaline AA batteries in series (should be able to find a 3 battery series holder at radio shack), because the batteries internal resistance will act like a dropping resistor, and limit the current to an amp or less.
If you need something a little more sophisticated than the 3 AA batteries in series, you will have to take a few minutes to try to actually UNDERSTAND the device you are working with.
The first thing you need to understand is that an LED device is NOT like an incandescent lamp, where there is a simple fixed drive voltage; instead LED devices have a sharp voltage threshold that shifts with temperature, which makes driving them from a fixed voltage source very difficult.
A drive voltage even a little below threshold, and the LED will draw no current and do nothing, and at a voltage only a little over the threshold, the LED will draw massive current and destroy itself.
So to safely drive the LED you need to LIMIT THE CURRENT somehow.
The simplest way to limit the current is to use a simple series resistor.
This will protect both the LED and your power source from drawing too much current.
The value of the Resistor is VERY VERY VERY easy to calculate.
Just take your power supply voltage and subtract the LED's specified threshold voltage (about 3.3 volts)
This gives the voltage drop that the resistor needs to create.
For a 3 alkaline series battery source at 4.5 volts, the resistor will need to drop about 4.5 – 3.3 = 1.2 volts, and the batteries will supply the needed ohm or two of resistance (due to their internal resistance).
For a 5 volt regulated 'wall wart' style power source, an external resistor will be needed to drop about 5 – 3.3 = 1.7 volts.
Knowing these values, we can then decide how much current we want to push through the LED and pick the right resistor to drop the appropriate voltage at that current.
The desired Voltage drop divided by the Desired Current gives the needed resistor.
Let's say you want to drive the P7 at 0.5 amps from a 5 volt source to start with for testing (0.5 amps is not driving it at maximum, but it will keep the heat generation lower so you can use a reasonable sized heat sink).
Assuming that the resistor should drop about 1.7 volts, if we divide that value by 0.5 Amps, then we get a value of 3.4 ohms for the series dropping resistor.
1.7 volts / 0.5 amps = 3.4 ohms
(this is the famous E/I=R ohms law)
A 3 to 3.5 ohm resistor would be perfect, and any value from 3 to 5 ohms would be close enough.
For the power rating on the resistor, we calculate that the resistor will dissipate 0.5 amps x 1.7 volts or about 0.85 watts, so a 1 watt rating would be the minimum you would want to go with. (Three 1 ohm half-watt rated resistors in series would be fine).
So, if you want to power your P7 from AC, just find a 5 volt 1 Amp or so rated regulated wall wart (you may have an old one laying around from a broken CD player or some other device), then get three 1 ohm half watt rated resistors from Radio Shack, and you are in business.
I went through the math to explain the dropping resistor calculation, because it will be needed for anything more complex than the simple 3 x AA direct drive hookup.
For example to drive 3 series connected P7's at 1 AMP in an automotive application, you could use a single 4 to 5 ohm 10 Watt rated dropping resistor.
The 3 series connected P7's would drop 3.3 x 3 or about 10 volts.
The car's electrical system can supply up to about 14 volts.
The difference is about 4 volts, which divided by the desired current of 1 Amp gives value for the dropping resistor of 4 ohms.
The only issue with this arrangement is that the 3 P7 LED's would dissipate a total of about 10 watts, so you would need a pretty substantial heatsink to keep from frying them (the 4 or 5 ohm 10 Watt resistor will also get quite warm, and would need to have good ventilation)
Hope this helps.
- Luminescent
