basic LED help

[graemeh]

You don't need a constant current power supply, an ordinary power supply with enough voltage and current will be sufficient.

The LDD700 produces a constant current of 700ma for the LEDs. When you select the voltage remember that the LDD700 drops some voltage. Check the data sheet but it is probably around 1.2 volts, so make sure the power supply voltage is higher than the sum of the forward voltages for the LEDs.

You mention having strings of ten LEDs with a forward voltage of 3.4-4v, so a total of 34-40v per string. You're probably better to get a higher voltage power supply, say a 48v supply.

 

[salty joe]

That's all great news. I'll get the power supply ordered. Thanks all.

 

[salty joe]

The red LEDs have a Fv 2.5-3V and the blue Fv is 3.4-4V.

If I got a 36V power supply of sufficient wattage, could I run 4 strings of 10 red and 4 strings of 9 blue?

In this case, a 350W power supply would not use any more energy than a 120W, would it?

It looks like each string is isolated and if one LED goes, that whole string shuts down anyway. Do I even need fuses?

Both red and blue have Fc 700mA.

 

[zachts]

The red LEDs have a Fv 2.5-3V and the blue Fv is 3.4-4V.

If I got a 36V power supply of sufficient wattage, could I run 4 strings of 10 red and 4 strings of 9 blue?

In this case, a 350W power supply would not use any more energy than a 120W, would it?

It looks like each string is isolated and if one LED goes, that whole string shuts down anyway. Do I even need fuses?

Both red and blue have Fc 700mA.

You should be fine with 10 reds and 9 blues (test out a blue sting you might be fine with 10 there as well on a 36v power supply, I've never yet seen an LED running at 700mA exceed 3.6v even the cheap ones, but it is possible, usually the fall in under this, but never assuem, test. It won't hurt the LDD to try and run too many LEDs, they just won't light up full power.)

Using an LDD for each strin is like using a fuse on the "parralell" builds. You do not need a fuse for each string.

 

[salty joe]

Thanks, this plan keeps looking better and better.

 

[perkint]

A 350W PSU run at a low load (say 100W) might use a bit more power than a 120W but on most of the data sheets, if you look at the stats, the difference will not be massive. You'd probably find more difference buying a decent power supply instead of a cheap one, than you will by running a power supply in its "most efficient load" compared with below that. If you see what I mean...

Tim

 

[salty joe]

I do see what you mean, thanks. The power supply on the way says it's 85% efficient-at least not ridiculous.

The LEDs and thermal grease arrived, as soon as the insulating washers get here the LEDs will go on the heat sinks. In the meantime, is it important or even worthwhile to measure and record the Fv of every LED for this build?

 

[Gorgok]

Measuring individually is hard, as you would have to setup a string and power it fully to test it, then let it warm up as well. I would just use the max values of the datasheet and use that for calculating Vf and the length of string you are capable of having.

Lower than the max, there isn't much point in measuring for a LDD build. The LDD drivers can regulate around 2-52V, or in your case 2-Vmax of the psu. Unless you go over the Vmax the LDD will handle any string of LEDs attached to it. Low numbers of LEDs at higher input voltages are inefficient, as in at 48V input with a single LED the datasheets claim around 75% efficiency, while closer to ideal situations the LDD is capable of 95+% efficiency.

Other drivers had a high value and a low value that you had to fit in between, or they would not run or run properly.

One thing to remember, the LDD does consume 3V for itself. So you have to account for that extra from the PSU that won't get to the string.

 

[salty joe]

If I understand, I could send 33V to the LDDs which would give about 30V to the strings. The red Fv is 2.5-3V. 30/3=10 So, 4 strings of ten red in series.
The blue Fv is 3.4-4. 30/4 = 7.5. Would it be probably safe to go with 4 strings of 8 blue?
I wanted to use 40 LEDs on each scrubber, but if that's not doable with the 8 LDDs that are on the way then I'll make one side shy.

The power supply on the way is adjustable between 32-33V and 39-40V.

Thanks for chiming in Gorgok.

 

[Gorgok]

8 blues per string is probably fine even with that high a max Vf. That sounds really high though, at what amperage is that number given?

I haven't read what the LDD does when under-powered for the string attached. It may blink on, realize it fails to regulate the current (for lack of voltage) and shut down repeatedly. If this is the case you could adjust the PSU higher to see if it stops. I doubt it will be an issue though.

 

[salty joe]

That sounds really high though, at what amperage is that number given?

.

700mA

 

[perkint]

No need to measure the fVs, as has been said. I would try 10 of each. If there is not enough voltage on LDD-Hs (probably same on LDD-ls) the LEDs will just slowly flash on and off. If that happens, take on of the LEDs out of the string.

Tim

 

[salty joe]

Will do. Thanks.

Would it be wise to adjust voltage on the power supply to just above the point of making the LEDs flash?

 

[perkint]

Yeah. LDDs are most efficient when supplied exactly the voltage they need (total fV + 3V) but the fV varies with temp so I'd adjust it up a bit more than what stops them flashing. And keep an eye on them for a while

Tim

 

[Gorgok]

I wouldn't worry too much about hitting the exact point where they run stable, the difference in the efficiency at the very end of the curve is small. Based on the datasheet even around 9V too much (at 36V) supplied to the driver will hit 95% efficiency while cranking it down to ~0V will get you 97%.

 

[salty joe]

Good to know, thanks.

I looked up how much power the arduino needs/uses and saw numbers like 350mA, but it seemed to depend on what the arduino is doing. I think the max it could handle was considerably more.

This arduino will be controlling two sets of 40 3W LEDs, with separate dimming and switching back and forth between the two scrubbers. Only 1 scrubber, 40 LEDs, will be lit at a time.

Do you think the 5v 550mA wall wart I found gives us enough wiggle room?

 

[Gorgok]

You are only using pwm pins to control the leds, it doesn't take much. I don't think you'd have issues running lots more with that power, at least until you try to add a wifi adapter or something (which does use a bunch of power).

 

[salty joe]

Sounds good, thanks. I'll try to keep things as simple as possible and use a cable between my iPad and the arduino.

Each scrubber has 4 pieces of 1" aluminum U channel for the heat sink. There is about 3/4" between the U channels. If I installed a polished aluminum sheet between the LEDs and heatsink, do you think that would greatly interfere with heat flow? I could use a dap of thermal grease between the polished aluminum and heatsink as well as between the LED and polished aluminum. The LEDs will be screwed on.

I'm trying to minimize light leakage, plus a reflective surface can't hurt.

Wish the parts would hurry up and get here!

 

[zachts]

the aluminum sheet will reduce/eliminate airflow up between the heatsinks which will greatly reduce thier effectivness. You want to keep that open air space for cooling. also the reflective sheet will provide little to no benifit as all the LED light is emited down and away from the heatsink.

A reflective skirt around the perimiter of the fixture would help contain light, but it need not be reflective as not much light is emitted straight out to the side from the emmitters.

If you want to direct and contain the light down into the scrubber using optics would be your best bet. Either 60 or 80/90 degree depending on how close to the water you are. Optics will let you run things more efficiently and at lower power as more of the light will be focused down where you need it.

You are planning to have a splash gaurd somewhere right?

 

[salty joe]

All right then, I'll give up on the aluminum sheet.

The LEDs are only about 2 1/2" from the algae screen, so I don't think I need lenses. The LED rig is hinged and sits in an acrylic 'boat' that floats. The LEDs will swing out the way, pull the boat out, then pull the screen. The design has more to do with working with available space than anything else. I do feel that a floating splashguard allows for one less reflective surface the light has to penetrate.

I was going to wait until at least the LEDs were mounted before posting a pic-if you guys are interested in seeing it now, let me know.

Thanks all.