im trying to design my own power supply that will drive 20-38 high power LEDs for a reef tank. the problem im running into is that i would like to have a fixed current of 700mA with the output voltage variable based on the amount of LEDs connected. (i want a 3-stage panel incorporating 20 then 29 then 38 LEDs) How can i go about this? I've read up on bridge rectifiers and using a capacitor to maintain voltage. Any help would be greatly appreciated.

 

... i would like to have a fixed current of 700mA with the output voltage variable based on the amount of LEDs connected.

How did you arrive at that?

Each individual LED needs a specific current. A typical LED might specify a max current of 30mA, and you'd want to operate it at 20mA or less to extend its life.

Multiple LEDs can all be run at the same current if they are placed in series. This requires the power supply to exceed the minimum voltage - each LED will need about 3v - and also requires something to limit the current. A resistor is typically used.

For as many LEDs as you want, it's not likely your supply will have enough voltage to drive them all in series and perhaps they have different current requirements anyway. In this case you establish multiple strings - LEDs in series - and place them in parallel. For instance you might have many strings of 3 LEDs with a resistor, all strings in parallel, driven by a 12v supply.

Any more specific design requires the specifications of the LEDs you have, plus any power supply parts you already have.

 

They are high power LEDs. Max current is 1000mA, optimal at 700mA. The forward voltages are 2.3V(30 LEDs) and 3.0V(8 LEDs).

As for the 3 stages i mentioned, stage 1 would be 20 LEDs @ 50.2V, stage 2 would be 29 LEDs @ 72.3V, and stage 3 would be 38 LEDs @ 93.0V.

I do not have any parts yet as im unsure of what i need to complete this project. Using a bridge rectifier connected to an AC source i can get ~120V DC right?

 

More like 170v, but mains-direct circuits are not allowed here for safety reasons. It would be better to use a transformer for isolation, or consider using a computer PSU or a laptop power brick. These will give you isolation, regulation, and plenty of power with built-in over-current and overheat protection. Much cheaper than you can build.

That said, you're going to need a lot of power. 87 LEDs at about 2-3W each. Are you prepared to heat-sink all these?

 

i understand that i would need to take some measures to ensure safety. laptop PSU and other LED drivers are all quite expensive. A power supply that would support my voltage and number of LEDs is easily $80.

my main issue is current, being that these LEDs are quite specific.

I would really rather build this myself as it will be tailored to my needs, how can i go about this while making it safe?

sorry for the confusion, its a total of 38 LEDs, at 2-3V each, i listed those numbers as totals for each channel being that i wanted to run a linear system and have only one series.

I have a 12 x 6" aluminum heatsink with 2 92mm case fans attached to it.

 

Well, an old PC PSU can be obtained for free if you head to the local recycling center, or just ask your friends if they have something in the basement. Depending on the specs, it might well have all the power you need.

Someone here might be able to recommend what you should look for. Basically, you want as much current at 12V as you can get. Some supplies have a lot of 5v current but not so much at 12v.

 

yes but that would not be specific, i have a laptop PSU that is 19V @ 4.74A, more than enough power, but not in a current that will work for me.

not to mention the fact that the current must remain the same because the voltage has to be able to fluctuate.

so what i really need is to design and build a power supply that will output a current of 700mA, 1400mA, 2100mA, 2800mA or any other multiple of 700mA, with the ability to vary its voltage based on load.

 

Ah, well that's not so hard.

Let's suppose we're using only LEDs with a nominal Vf of 2.3V and we want a 700mA current. The PS voltage is 19V, meaning you could put as many as 8 LEDs in series: 8*2.3 = 18.4V with 0.6V left over.

To limit the current through the string, in theory you need a resistor that drops 0.6V at 700mA. Apply Ohms law V = IR. 0.6 = 0.7x x=0.86Ω The power dissipated by that resistor is I^2*R or 0.7^2*0.86=0.42W. So you'd want a resistor rated for at least 1W to ensure long, safe life of the resistor.

A 1Ω resistor would drop 0.7V and possibly your string of 8 LEDs might not light if there isn't enough Vf. Vf is kind of a squishy spec, though, so you'd really have to test it to know for sure. You might have to redo these calculations for 7 LEDs and see how it works out. Or 5 strings of 6 to use all 30 of your LEDs. Just be sure to check the power dissipation in the resistor, and use one spec'd for at least twice that amount.

Five strings at 0.7A each will consume 3.5A, which will warm up your power brick but should be within it's specs. I wouldn't want to run it continuously much higher than that, so you may need another one if you want to use those other 8 LEDs too.

 

its the power supply i currently use for my laptop, i was using it as a reference.

would it be easier for me to build a constant voltage supply capable of up to 2.8A at 28V? also how can i factor in my cooling fans which run at a different current of 500mA and 12V each?

 

For the fans, you could put them in series and use a 24V supply, if you're going that route. I wouldn't hesitate to to use 28v, but that does put them at some risk of a shortened life.

Is it easier to build a supply than use a commercial one? Not in my book. The biggest hurdle for a simple (not SMPS) DIY power supply is getting a transformer with enough juice. They're the most expensive part.

 

how difficult is it to build an SMPS power supply?

 

Pretty challenging I hear. I've never done one myself.

Maybe someone else here has more encouragement to offer.

 

Just a note on the 700ma constant current drives. I am doing a 12V, 3x Cree high power, desk lamp. Using the AL8805 1A (max) led driver, the driver and associated parts at Mouser are around $4. There can be up to 8 white leds at 30V on the AL8805 driver. If this was an all white led build that would mean 5 drivers. Plus the budget pcb's at around $10-15 for a total of $30-40 lets say. Premade Meanwell or BuckPuck drivers can be had for around $10 each.

 

It is not that hard to build a SMPS power supply. If you want a chain of 6 or so LEDs you could get by with some simple analog regulators that would be much cheaper though.

Every color drops a specific voltage, but as you know current is what matters. The totals on the voltages can not exceed the power supply voltage though.

LEDs, 555s, Flashers, and Light Chasers

If you want to build a SMPS current regulator there have been several good tries here, but analog is much, much cheaper (many less parts). You would need a separate regulator for each chain of LEDs.

Feel free to post me a PM to respond to this thread, and I'll follow up (always publically though, see my signature).

 

i can only find meanwells for $16 each and i would need atleast 2, being that the sites ive found do not sell internationally. i live in canada. plus my shipping costs im at almost $50. we have various electronic parts suppliers here where i feel i can probably buy mostly all the parts i would need to construct my own power supply. i found an inventronics driver that would work perfectly, only problem is its $79.95. Unless you guys happen to know some good retailers online where i could get drivers?

Bill im doing a 38 high power LED panel, your article was great but it seemed like its only for low voltage applications. how can i tailor this to these high power LEDs at a current of 700mA?

 

The basics don't change. Again, we have schematics for SMPS that have not been properly tested, though I have build a few on a home brew PCB. It would be a bit of work, but you could duplicate this if you wanted. The big advantage for SMPS is it does not get hot, and it uses almost all the power from a power supply efficiently. The LEDs themselves will still get hot.

Look up grow lights, I have helped with lots of articles on those, and they are very similar.

300W LED Grow Light

Need help with simple LED grow light circuit: Cash reward

Simple LED Project