Paralleling LED Strings "“ Make Sure You Understand the Risks
Maybe I should write this really small so it gets ignored. I started thinking about this kind of late so did not try and remember what I read on parallel, but here is some basic information:
Since I started the summary I have been thinking more about the parallel configuration.
As a general rule we have been saying avoid parallel configurations. After thinking I wonder if the Meanwell's are meant to driven in parallel. The advice was to ignore the internal voltage setting potentiometer. But if we set the voltage to the measured voltage when running at the desired current then when a string is lost the Meanwell us unable to increase the voltage to get the additional current out. I don't think any additional safety precautions are needed.
I think this was mentioned before, but just did not sink in for me. So where is the problem if both internal adjustments are PROPERLY set with running parallel strings?
And kcress pointed out what I forgot:
The problem is shorts in a string.
With a short the required voltage of the string drops and the current increases, a vicious circle.
All the streetlights and signal lights run parallel with no protections and the strings all burn out eventually. I was just noting yesterday that about 1 signal light in 3 has failing LEDs showing. My town embraced LED signal lights the minute they came out. So what, 6 or 7 years? And now failures are starting to occur. If you are fine with retooling your fixture in that period it will probably work fine to parallel with no added protections.
So I think with a properly adjusted Meawell and some fuses you should be ok.
So for those interested here are notes CJO (who was interested in this as he read the threads) took:
Series or Parallel
Fairly simple actually. With the same power supply, in this case for 4 LEDs with a forward voltage of 3.3v, of 15v.
The series circuit, uses less current draw from the power supply, and a single resistor.
The parallel circuit, uses a higher current draw from the power supply, and larger multiple resistors.
Another way to look at it, you need a larger voltage, lower amperage, power supply for series circuits. A smaller voltage, higher amperage power supply for parallel circuits. Assuming the same number of LEDs.
With power supplies, having less than the total forward voltage of all the LEDs, it is necessary to use a series/parallel circuit. In this particular case, the power supply is easy to come by, and the closest standard size resistor 120 ohms, is the exact size needed. The other circuits use the next closest higher resistor. (results in dimmer LED output, because less current will flow.
So from a design point of view, you design for the type circuit that will give the best results, in this case, although the series circuit is close enough, the series/parallel circuit with a 9 volt supply, will perform the best. (LED output wise, power is a secondary concern-- except for dissipation, depending on the size of the array)
See circuits below. If there are math errors, it is the calculators fault (it is an old TI-36X)
The greatest difference is seen in the total power (wattage) for the circuit.
----see attached diagram----
(kcress)
You should only have one driver for each string. Period.
Any other scheme risks all the LEDs as soon as one fails shorted.
Two stings in parallel will toast them all. Why?
If you are running two stings in parallel and each string is, for example, 700mA, your driver would need to put out 1400mA. Now if one LED shorts the driver will continue to drive 1400mA into the two stings. But the string with the shorted LED will have a different voltage requirement than the good remaining string. This causes what is termed as "current hogging". The good sting will either go dim or OFF completely while the bad string may have 1200mA running thru it. The remaining LEDS will fail in seconds.
Once the entire string with a short in it has blown or one of the LEDs fails OPEN the driver will then focus on driving the 1400mA thru the remaining good string. Every LED in that string will also fail within seconds in a domino effect.
One driver per string!
Drivers cost money.. How do you deal with this?
Two ways: The first is to string far more LEDs in a string. Using a 36V or 48V driver or at least 24V. 12V borders on the ridiculous.
48V/2.2V = 21 LEDs
36V/2.2V = 16 LEDs
24V/2.2V = 10 LEDs
Alternatively you could run strings in parallel but you would need to put a fuse in series with each string. As soon as an LED fails shorted that string would hog current, exceeding the fuse rating, and the fuse would blow. Promptly the full current would try to run thru the adjacent parallel strings and those fuses would also promptly blow. It would take some careful fuse selection however.
How to test forward voltage of individual LED's:
Wire them to a driver (you can do lots at once if you want). Run them at your target current. Read voltage across each one with a multimeter, by probing right at that LED's solder pads.
Really, the only trick to it is not blinding yourself. It helps if you have optics or an optic holder that leaves the pads exposed, because then you can easily look at the LED from the side without getting blinded.
(der_wille_zur_macht)
Parallel strings are not ideal in these applications, for a few reasons. First of all, if there are any tiny differences in characteristics in your various LEDs, you'll have inconsistent performance. If you have one string that ends up requiring 10.3v to drive at 700mA, and another string that requires 10.5v to be driven to 700mA, then the driver will end up over driving one string and under driving the other. Since even a small variation in voltage can lead to huge variations in light ouput, this might mean poor performance from some LEDs. In practice, I've seen 3 - 4% variation in drive voltage to achieve a target drive current from LED to LED (even in the same bin) which is enough to make me worry about performance in parallel applications.
A: (der_will_zur_macht) Daniel, fusing the parallel strings would prevent failure, but it leads to some other (potential) issues:
1) If the LEDs in one string have a different total forward voltage at a given current than the LEDs in the other string, they won't balance out well. This would be especially true if you mixed different colors/types of LEDs on the same driver. I'd want to carefully "bin" the LEDs I was using (set up a test station where you could drive a single LED for a few seconds to record it's voltage at a given current) to avoid this.
2) As you get more LEDs on a driver, you start to lose control resolution. Maybe this isn't an issue on a very large tank, but on a smaller tank, if you had drivers doing 12, or 24 LEDs each (for example) you quickly lose resolution to the point that it would be hard to implement the sort of control people are starting to show interest in. For an extreme example, I have a nano rig with 16 LEDs run at very low current. This is two of my DIY drivers, 8 LEDs each. A driver capable of doing all 16 wouldn't even let me dim blue and white separately.
From the sounds of your posts, neither of these would be huge stumbling blocks for you, but I wanted to point them out in case others were following along.
Binning LED's, adding Fuses (Kress)
You could theoretically run 4 strings of 48/3.5 = 13 LEDs.
Or 52 total.
You would be limited to 1.3A / 4 = 325mA per string.
To do it right though you'd need to do some additional work.
It would consist of some detailed meter work.
You would set up a string on a Mean Well and set the string current to 325mA using an ammeter.
Turn it on and wait until the string is warmed up. As you wait, use a Sharpie to number every one of them. Once warm measure the voltage across each one and write it down in a numerical table.
Do this for all 52.
Now take this table and mix and match the values to end up with the same total voltage in each string. You could do this many different ways. Use, say, the highest 5 with the lowest 6 if that works. Or just match across one low one in each string then the next higher one in the next string, etc, etc.
Once you have them grouped build your 4 strings.
You need to build the strings normally BUT you need to add fuses in each string.
Something like a 375mA fuse. Digikey F1504-ND in a holder F1467-ND.
Now when a LED opens or one shorts the fuse will open protecting the rest of the string.
Note that if any fuse opens they will all open, so keep spares.
If you can't pull this off as described, don't run parallel strings.
(der_will_zur_macht)
Skeptic, it's an easy problem to solve. Set up a "test station" with a constant current driver that can power a few LEDs at a time at some reasonable current, while allowing you to probe each individual LED with a multimeter. Turn the test array on, test the voltage drop across each LED, and write them all down. Then, arrange your LEDs into groups such that the total voltage drop for all groups is as close as possible. That's what I meant about "binning" your own LEDs. It should take an hour or two max, and it's cheap insurance if you're running parallel strings.
.2v CAN be quite significant (like 100mA!!!), but I'm not sure you'd see variation that high unless you randomly stacked things up in the worst possible way.
GEORGEDOPE; If you want to use that driver with parallel strings, go ahead. Add a 5 Ohm 10Watt resistor in each string. This resistor will help balance the string currents due to the inevitable mismatched string voltages.
Next turn down the voltage limit on the driver until you just detect some dimming. Then turn it back up a wee bit.
Now when one of the three 700mA strings opens the remaining two will theoretically have 1050mA sent thru them. But to have this happen the voltage must rise significantly, except, you just limited that voltage rise with the adjustment. The limitation will keep the current below 1050mA by a significant amount.
The only thing you aren't fully protected from is a shorted LED in one of the strings. The math shows that with those resistors you would probably see one string increase about 50mA and the other two drop by 25mA. Someday I will check that on a build. For piece of mind you can add a 3/4A fast blow fuse to each string.
Partial Summary
First, I don't think there is any difference between the cool white and royal blue (XR-E). Both are 3W, have a forward voltage of ~3.6v, and take a max current of 1000mA. However, I've read that the royal blue (and white to a lesser extent) are best run at a slightly lower current. Say 700mA. This extends the life of both types of LED with only a slight decrease in light output. This means that both colors could be on the same driver. But, most people don't so they can dimm them by color and turn them on at different times to simulate sunrise/sunset. I'll try and get confirmation on this.
The Meanwell, specifically that in the group buy ELN-60-48P, is a line voltage constant current source. It provides a constant 1.3A (-25%,+3%) to the LED load on it. It can handle up to 48V max on the load. And the P means it is externally dimmable with an analog signal. The line voltage part means it just plugs into the wall (90 - 240v), no additional power supply is needed. The max constant current is changeable by an internal potentiometer -25% or +3%. This means it can output from 1A to 1.4A or so.
LEDs have a current requirement and a forward voltage. For the Cree XR-E that is ~1A and 3.6v (on average). In series, voltage adds and current is the same. Thus, the Meanwell can power up to 13 XR-E in series at 1A (with the max current at -25%).
It can also run two parallel strings of 13 LEDs at 700mA per string with the max current set to +3%. Current divides in parallel so a total of 1.4A is being provided but each string gets 700mA.
A word of caution with parallel strings of LEDs. This is a recipe for disaster. If you aren't comfortable building this next item, don't run LED strings in parallel. What happens is if one LED in one string dies, then that string shorts and all of the current is sent through the other string. This either kills part or all of the LEDs in the other string or it severely limits the life of the LEDs in the second string. The higher current will work but it will also heat up the LEDs a lot. That is how they die.
But, you can do parallel strings as long as you build in a current mirror. Evil66 found and posted this in the Meanwell thread on nano-reef. Basically it forces the same current in both strands. If one strand shorts then it shorts the other strand too. The Meanwell thread:circumventing the filters is generally frowned upon
). Generally it is agreed that each driver should drive one string as kcress says (I think this was all him):
