Does running parallel circuits for LEDs decrease their output?

Question

Sorry if the questions here are elementary but my electrical knowledge is limited.

I am planning to build an aquarium light fixture and I am having trouble grasping how to best use a driver.

The driver I am looking at is Mean Well ELN 30-48D or the Mean Well ELN 60-48D. The diodes I am looking at are CREE XT-E 5w with 2.85-3.4v and I should be able to run 4-14 on the MW 30-48 and 9-14 on the 60-48.

Now what I don't know is if I run 2 parallel circuits of 14 LEDs in a series, would this effect the intensity of the light being produced? And also of the 2 drivers listed above, the only difference I understand is the minimum number of LEDs which can be in a series. What would be other factors to consider when looking at these 2 drivers?

A little more information. I am trying to build 2 individual identical lights over my reef aquarium and I am trying to figure out if i can have 14 of the same LEDs on light A and 14 on light B working from 1 driver without compromising the color temperature and intensity which I am trying to achieve. If the parallel circuits (28 LEDs) will be as intense as 1 series of 14 then that will save me some money on buying drivers.

If the parallel circuits create the same light how many parallel circuits would the driver be capable of running?

I know it is a lot of gibberish in there and I have probably left out crucial information that is needed to answer my questions, but like I said, limited electrical knowledge.

Thank you in advance.

Michael

Answer 1

From what I understand of your question, you are asking if you can do this:

The short answer is: No, you can't.

Now for the explanation, using random example values.

Given LEDs that require 100mA and 3V, and 14 in a chain purely as an example.

Given a perfect world, with perfect LEDs, each chain would require 100mA at 42V. The two chains therefore would require 200mA at 42V. And connecting a constant current 200mA that can supply at least 42V would work perfectly.

However, we don't live in a perfect world. LEDs don't all come with a precise forward voltage. They all vary around the rated forward voltage. For 3V LEDs the forward voltage may be anywhere between say 2.9V and 3.1V.

So what is the total forward voltage of each chain now? Well, who knows? In a worst case scenario you may have one chain with a forward voltage of 40.6V, and the other with a forward voltage of 43.4.

Now a constant current supply will output the needed current, but the voltage will automatically set itself to the required level for the circuit. In this case the forward voltage of the LED chains. But which forward voltage? 40.6V or 43.4V? Well, the lower one always wins. So the voltage the supply would give would be 40.6V. That means the chain that wants 43.4V would only be getting 40.6V.

So what would the effect of that be? Well, again, worst case scenario - one chain just won't light up at all. The diodes won't get enough voltage to start conducting. But that's not all. You're providing the current for two chains, but only one is activating. So where is all that current going? Yep, it's all going through the one chain. That chain is getting double the current it should be getting - and LEDs don't much like that. Expect your fish to get showered with bits of LED dust as they explode.

But the worst case scenario isn't likely to happen. Instead it is more likely the LED chain with the higher voltage will illuminate, just not as bright as the other, as it partially conducts. So some of the current will go down that chain, but still most of the current will go down the chain with the lower voltage. That chain will still be getting way too much current and the LEDs are still going to blow.

Of course, the more LEDs you have in a chain the more the forward voltages average out. It would be terribly bad luck to get all the LEDs in one chain as a low forward voltage and all the LEDs in the other as a high forward voltage. The chains would probably end up fairly close together in voltage. So it might appear to work fine at first glance. However, one chain will be slightly higher current than the other, and thus will be being stressed slightly more. And when that chain blows the other chain gets all the current, so that one doesn't last much longer either (known as a cascade failure).

So what to do?

Well, there are two simple answers and one complex answer:

1. Have a chain of 28 LEDs in series and use a higher powered supply.
2. Have the two chains completely separate and use separate supplies for them.
3. Employ some other form of extra current limiting or balancing arrangement.

You'd think that option 3 would be as simple as a small resistor in each branch to help balance the currents, but no. That may allow them to light better, but certainly wouldn't protect one branch in the case of a failure of the other. You'd basically need to create a constant current sink or source circuit for each branch. There are chips that you can buy to do that for you if you want to make your own driver circuit up.

Answer 2

I'll disagree with Majenko on the final conclusion, and I think he's missed an important point (well, several, actually).

First, let's take a single string of 14 LEDs. As you've calculated, this will need a 48 volt driver. This, as it happens, is not necessarily the case. The maximum 3.4 volt forward drop you're working with applies at a current of 350 mA. However, if you look at the LED data sheet, you'll see that the maximum current is 1.5 amps. This makes sense, since the LEDs are nominally 5 watts, and running at about 3 volts. So, if you hook up a 30-48D with dim set at max, you'll get a maximum of 0.67 amps (and the drivers you specify all primarily control current, as long as the voltage needed to do it doesn't get too high or too low). You can use the dim feature to reduce this, but that's the default. At .67 amps your LED voltage drop will get higher, and the Cree data sheet doesn't say how much. But let's assume that it's within driver limits: you can, after all, increase the upper voltage limit of the driver by twiddling a pot in the driver.

So you can run one chain on a 30-48D. How about a 60-48D? Yup. Maximum current is now 1.3 amps - still less than the 1.5 amp LED limit. Notice that you'd really need to start paying attention (if you haven't already) to properly heat-sinking the LEDs.

But now for the big question - can you put two strings in parallel? Aaand the answer is yes. But. It won't do you any good. As majenko has written, one string is going to hog some or most of the current. Actually, the longer the strings the less the expected voltage difference. This is just because you expect a random selection to trend toward the average, and the more units the more "average" you expect. Worst case still applies, though. However, since one string alone can handle the output of a 60-48D, as long as you're able to handle the temperature of the LEDs you'll be within spec.

So why won't it do any good? Because the TOTAL current through the two strings is the same as through one. Remember that the driver controls current, and so the current stays the same, and an LED puts out light which depends on current. So, if you have two identical strings, each will get half the current a single string would get, and each string will be just about half as bright.

There are relatively minor effects. LED efficacy (light/amp) actually decreases a bit at higher currents, so two strings in parallel will put out a bit more light than one, but this is not noticeable. It's also easier to heatsink two strings, each at half power, than one at full power, and you do want to keep the LED temperature down if you can help it, so that might be a reason for using two strings. Although I'd suspect that, for the money, a better heatsink is cheaper than two less-effective ones - but that's up to you.

Answer 3

The driver I am looking at is Mean Well ELN 30-48D or the Mean Well ELN 60-48D."

You may wish to search with Constant Current LED Driver Power Supply. Your current choice Mean Well ELN 30-48D is a fixed voltage PSU.

The Constant Current LED Driver PSU for CREE XT-E 5w should be around 1A or somewhat less, I think. The voltage range of the PSU to choose depends on the number of LEDs.