I just want to power the stupid thing!

Ok so I've looked all over the net and it's getting all confusing. I have a SCC p7 D-bin LED. And I just want to power the damn thing and get the highest possible lumens from it. I may ad a few more later if I find that it's not bright enough. Is there a clean and simple way to power it? Maybe some parts I can grab from Radio shack and solder together? I'm not worried about heat I can design my own cooling scheme. It's going to be powered by plugging it in not batteries if that makes a diffrence.

use an old phone charger if you are desperate, phone charges are usually rated at 3.7 volts (check the back to be sure) mine is 3.7 volts and 800ma so that should be perfect if you wire each die in parallel. just hack off the charger port thingy and wire up the LED leads, be sure to heat sink it otherwise your LED will have a lifespan of 5-15 seconds (firmly pressing is against a piece of metal is usually sufficient for testing)

:welcome:


What're you gonna do with it: put it in a light or just playing around? As stated you need a good heatsink to not overheat it.

P7's can take up to 2.8 amps easily so make sure your powersupply can output that without frying. An easy way to power it would be to connect it to a computer power supply, the 3.5 volts line should be able to direct drive it but to reach full power you'll need probably around 3.8-4.0 volts so the 5 volt line with a resistor inline would be the best bet for maximum power, now the actual resistance needed is hard to guess but I'd say it'll be about .5 ohms with minimum 1 watt power rating resistor.

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

Thanks for the help guys... I was getting pretty frustrated trying to figure it out myself. What I'm going to use it for is making a replacement bulb for my projector. I doubt one LED will do the trick thats why I mentioned using a few.

I had another question... As far as LED's go I went with the P7 becuase it seamed relatively bright for the price. I've seen 20 watt and 50 watt LED's. Does anybody know how those stack up to the P7 and if they're worth the price?

you may want to take a peak at the luminus devices LEDs, i believe they were specifically designed for battery powered laptop projectors, the SST-90 and CST-90 and the CSM-360 would all make excellent projector lights judging by their output, cooling and power might prove more difficult though (the CSM-360 will do 6000 lumens at spec....if you can get it to spec without going chernobyl on us)

Brain Grenade said:

Thanks for the help guys... I was getting pretty frustrated trying to figure it out myself. What I'm going to use it for is making a replacement bulb for my projector. I doubt one LED will do the trick thats why I mentioned using a few.

I had another question... As far as LED's go I went with the P7 becuase it seamed relatively bright for the price. I've seen 20 watt and 50 watt LED's. Does anybody know how those stack up to the P7 and if they're worth the price?

Click to expand...

The P7 can do up to about 900 lumens if you push a couple amps through it and cool it adequately. A 100 to 150 watt halogen lamp can do a little more than twice that, but the P7 could still do a fairly resonable job, if you can couple it's optical output efficiently.

If you need more lumens, you might consider something like the luminus devices SST-90 or CST-90 as already suggested (if you can get your hands on them). These devices offer high lumens in compact single chip devices, which makes them more suitable for projectors than some other larger multi-chip devices from other manufactures. Other larger multi-chip devices could be difficult to couple efficiently optically, and can suffer from color balance matching issues between the chips.

Cooling your P7 at full power could be done with a section cut from a CPU heatsink from an old socket 7 P4 or Athalon motherboard. You wouldn't need the small cpu fan that would normally be mounted on the CPU heatsink, as long as the existing projector fan provides good forced air cooling for the lamp assembly (just try to mount the modified heatsink so the existing blower can push a good flow of air through the CPU heatsink fins).

For one of the higher power luminus devices chips running at full power, you would need a fairly large heatsink cooling assembly equivelent to one of those high performance air or liquid cooled CPU coolers that overclockers like to use on their PCs (this could be tough to fit into your existing projector housing).

What type of projector are you looking to modify?

I have my old Optoma EP719 Projector that I'll be doing this with. http://www.projectorcentral.com/Optoma-EP719.htm There's the link to it's specs if you're interested. I actually plan to use the old burnt out bulb to house the new LED if possible. I have to wait for it to burn out first haha. But I want to do some tests before that time comes so I don't have down time. This is the projector I use to go to other peoples houses with for back yard theater nights.

I don't think it would be a good idea to mount the led in a glass bulb as it needs a lot of heat sinking to not burn out which won't be there if you just mount it in a glass bulb to get the positioning right.

Well I wasn't actually going to mount it in the bulb it self... I was going to grind away the bulb and use the bulbs reflector to be exact. That way I can still mount a heat sink on it. It will be an experiment. It may not work out but it's worth a try. I have to wait for the bulb to burn out first to try it though. Or somehow find a donor bulb that somebody has already burnt out. We'll see.