# Power Calculations for 20x 10W led panel?



## dakku (Dec 30, 2014)

Hi All,

I am trying to build led panel consisting of 20 10W Led Chips. 

*LED SPECS*
Each chip is rated at:

Wattage: 10W
Voltage: 30-34V
Lumen: 900LM
Current: 300MA


*PSU SPECS
*

Output voltage: DC 12V
Output voltage adjustment range: ±10%
Output current: 33A
Output power: 400W





I am wanting to build something like illustrated in the picture below. Can someone please help me with the power calculations for the required LED Driver/ Boost Converter etc? What do I need to make it work?


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## Steve K (Dec 30, 2014)

Is there a reason that you've chosen this power supply? It would have been nicer to have a power supply that could generate enough voltage to drive the LEDs itself. With this power supply, the 12v output can't produce the 30v (plus) to get current to flow in one of the LEDs. If you did want to use this power supply, your options would be to use different LEDs... perhaps a number of 3 watt LEDs with a 3v forward voltage (Vf), or to use a boost converter between this power supply and the LEDs wired in parallel. Using the LEDs wired in parallel might require a small resistance in series with each LED in order to get them to share current equally.

The idea solution would be to use a current regulated power supply. Oh.. what is the input power? Is it mains power, such as 110V AC power from a wall outlet (in the USA)? There are a number of drivers designed for this task.
http://www.meanwell.com/product/led/LED.html


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## dakku (Dec 30, 2014)

Hi Steve,

Thanks for the reply..

I have not bought a power supply yet, but the reason I was looking to get 12v is so that I can use it for other LED projects (DIY LED Panel using 12v 5630 Strips). 

I bought 30x 10W LEDs that arrived this morning, so this variable is kinda fixed really.
AC Voltage is 220V (UK)

So, if I wanted to drive LEDs in parallel using this power supply, I am assuming I would need:

- 12V to 32V Boost Converter?
- Current Regulator / Driver?

Alternatively, if I chose a different power supply, what would I need to drive these LEDs?


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## dakku (Dec 30, 2014)

If i chose a different PSU. I am assuming I would have to have:

Voltage: 30-34V
Current: 300ma * 20 = 6000ma = 6A

Factoring in some headroom etc, is it possible to get such a power supply? Also, how would I limit the Current? Would I need some kind of LED Driver? Sorry, I am a little lost


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## DIWdiver (Dec 30, 2014)

The link that Steve posted is to a company that makes LED drivers that are powered from the line voltage. You should be able to find one there that will handle your LEDs.


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## dakku (Dec 30, 2014)

ok, I tried hooking up my 10W LED to 12v 10A and it worked (!??) So, I am not sure if these are 30-34v as listed on ebay. Should 30-34v led light up with 12v PSU?
I did some measurements and with 12v PSU one 10W led is running at 12v and 320ma, thats only 3.8W? Is that correct?


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## dakku (Dec 30, 2014)

for anyone else who is looking for help, this thread was most useful for me: http://www.badcaps.net/forum/showthread.php?t=35711

basically, I ended up buying 12v version of 10W LED. No More messing around with boost converters etc. 

Now, how do I current limit each 12v 10W LED to 900ma?


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## DIWdiver (Dec 30, 2014)

It's not terribly surprising that your 30V LEDs light up at 12V, or that they draw less power than rated. It is surprising that they draw about rated current. That seems very wrong to me. They should draw far less than rated current.

Maybe a quick sanity check is in order. A small 10W like a Cree XM-L is impossible to look at from a few feet away. If you dim it to 10% (1W) it's still painful to look at. I'm guessing that your LED is much bigger, but at 3.8W it should still be darn bright. Is it?

Before you go buying much more stuff, maybe you want to learn a little more. For example, there's a big difference between a 12V LED that draws 900 mA and a 900 mA LED that takes 12V. You want to have a good handle on that before you go buying power supplies and LEDs.


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## Anders Hoveland (Dec 31, 2014)

dakku said:


> So, if I wanted to drive LEDs in parallel using this power supply


You generally cannot power LEDs in a circuit in parallel. All the power goes through just one of the LEDs and burns it out. (depending on the LED it could even lead to a cascading failure and burn them all out) 
To power LEDs in parallel, you either need separate power supplies, or a carefully matched resistor in series with each LED to limit the current flowing through each one.




dakku said:


> I am trying to build led panel consisting of 20 10W Led Chips.
> 
> Each chip is rated at:
> Wattage: 10W
> ...


Theoretically, you would need a 600 volt, 300mA power supply! (I doubt you'll find one)
They do make power supplies that have multiple outputs, if you wanted to do four separate strings of LEDs.

If you wanted to wire 20 LEDs in a series circuit, you should have chosen LEDs with a much lower voltage rating, and higher current rating.
-


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## inetdog (Dec 31, 2014)

Anders Hoveland said:


> You generally cannot power LEDs in a circuit in parallel. All the power goes through just one of the LEDs and burns it out. (depending on the LED it could even lead to a cascading failure and burn them all out)
> To power LEDs in parallel, you either need separate power supplies, or a carefully matched resistor in series with each LED to limit the current flowing through each one.
> -



What you say is true if you run the LEDs anywhere close to full power. But if you keep the power (and therefore the voltage) low the inherent series resistance in each LED will be enough to keep then balanced, at least within a factor of two. 
If you are trying to parallel LEDs with different forward voltages (different wavelength emitters) you will have to drive them separately.


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## Anders Hoveland (Dec 31, 2014)

inetdog said:


> What you say is true if you run the LEDs anywhere close to full power. But if you keep the power (and therefore the voltage) low the inherent series resistance in each LED will be enough to keep then balanced, at least within a factor of two.


Yes, this is true. Although, I am not sure how practical this is. There is a strange relationship between power and resistance at these low power levels, when there are two diodes in a parallel circuit. As a low power level goes through one of the diodes, it becomes easier for further power to go through. But then, gradually increasing the current, at some point it becomes easier for some of the current to start going through one of the other diodes. 

If I can be forgiven for using the terminology, running LEDs at very low power in parallel is like a "quantum" effect.

And then obviously, once you start getting much higher above the maximum voltage/current rating of each individual diode, the majority of the current is again going to flow mostly through just one of them.

It would be interesting to see it plotted out on a graph, available current , and distribution of current through each diode in parallel.


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## Illum (Dec 31, 2014)

Did you do this?
http://www.candlepowerforums.com/vb...ht-even-possible-The-maths-dont-add-up-for-me


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## DIWdiver (Dec 31, 2014)

Depends on the LED. I built a driver for a commercial light that has 6 strings of 3 LEDs in parallel, and another 4 strings in parallel. It works so well and so predictably that the customer actually came back and asked that the drive currents be increased above full rated current, plus a 25% boost for short term use.

They were told by the LED manufacturer that as long as all the LEDs on a panel came from the same batch, they'd be fine. I believe that's probably true of most modern high-power LEDs. There are certainly many folks doing 3S3P and larger arrays of XM-Ls for off-road vehicles, and having no problems.


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## dakku (Dec 31, 2014)

Illum said:


> Did you do this?
> http://www.candlepowerforums.com/vb...ht-even-possible-The-maths-dont-add-up-for-me



I created the thread but the project isnt something I have built yet. Still trying to figure out the maths.


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## DIWdiver (Dec 31, 2014)

No, that device is driven WAY below rated current. This is what I'm talking about: https://www.youtube.com/watch?v=vP7fSytFlf0

Though recently posted, it's an old video. It doesn't show the stabilizing legs that spread out, or the RGB color panel that was added later.


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## Anders Hoveland (Dec 31, 2014)

It seems that as long as the supply voltage is kept below the maximum rated voltage, multiple LEDs can be run in a parallel circuit.

I guess this is not true of regular diodes because the voltages going through them are always much greater, to supply the rest of the circuit. But when the LED *is* the circuit...


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## DIWdiver (Dec 31, 2014)

I think that's oversimplifying it, and not exactly true. It's more about making sure you have LEDs that are pretty similar to each other. You could take two brand name LEDs of the same part number, same specs, but if they were from different ends of the voltage range, you could have a problem even if you kept below the max voltage.

The problem arises when the forward voltage of two LEDs in parallel is slightly different. The lower voltage one draws more current, which makes it hotter, which makes it's voltage drop even more, etc. This is called thermal runaway, and can rapidly destroy first one, then the next LED, and so on.

The key is to have enough resistance in the circuit to keep them fairly balanced. But the resistive component of the LED's impedance can be pretty small, so it is possible to have a problem if the LEDs are different. In practice, it never seems to be a problem if you use LEDs from the same batch. Oh, keeping them at close to the same temperature is helpful too.

The voltage range specified by the manufacturer doesn't mean "if you provide a voltage in this range it will be okay", it means "if you operate it at rated current it will be in this voltage range". Those two statements are very different, and it's not only possible, but very likely for the first to be false while the second is true, of the same diode.


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## Steve K (Jan 1, 2015)

regarding operating LEDs in parallel: I've done this on some of my bike taillights. I'll run ten 5mm LEDs in parallel, and it's worked out fine in the 3 times I've done it. No visible difference in lumen output from one LED to the next. 

The caveats are: there's a big difference between doing it once or twice in the lab and doing it thousands of times in production, and ... I don't know if I got parts from the same batch, but it might have happened. If you were going into production, you could pay a little extra to your distributor to ensure that you got parts from the same batch.

On the other hand... I've used "ballast" resistors to ensure that current gets shared properly. If you can spare one or two tenths of a volt to burn up in the ballast resistors, this is a cheap scheme that will be more reliable than just hoping that the LEDs are all matched.


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## Anders Hoveland (Jan 1, 2015)

So the consensus about running LEDs in parallel is it _can_ be done, especially at lower power levels, but it can be problematic, and potentially may run the risk of burning out one or all of them, if the specifications are not right.


http://www.edn.com/design/led/4424539/Overcome-the-challenges-of-driving-parallel-LED-strings


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## SemiMan (Jan 1, 2015)

That article is a joke written by someone with much less experience than many no this board.


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## SemiMan (Jan 1, 2015)

Anders Hoveland said:


> It seems that as long as the supply voltage is kept below the maximum rated voltage, multiple LEDs can be run in a parallel circuit.
> 
> I guess this is not true of regular diodes because the voltages going through them are always much greater, to supply the rest of the circuit. But when the LED *is* the circuit...



Huh???

Voltage through a regular diode is normally about 0.7.

There is also nothing strange about the current/voltage relationship of a LED. 100% well defined and well understood. You not understanding it does not make it strange.


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## Illum (Jan 1, 2015)

SemiMan said:


> That article is a joke written by someone with much less experience than many non this board.



Have to bear in mind many of EDN's publications have probably never been peer-reviewed. I have come across teardown articles where the author called a bridge rectifier a IC controller.


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## Steve K (Jan 2, 2015)

SemiMan said:


> That article is a joke written by someone with much less experience than many no this board.



I hate to agree, but EDN is not as good of a trade magazine as it used to be. The golden years of Bob Pease and Jim Williams (RIP) have passed, and the search for people who can write excellent technical articles on design is in the works. I'm glad that some people are going to the trouble to write articles, but a lot seem to be a bit misguided.


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## SemiMan (Jan 2, 2015)

You had me at Jim Williams but lost me at Bob Pease. Pease was a product of marketing, Williams was the real deal.


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## Steve K (Jan 2, 2015)

Bob Pease wasn't the technical genius that Williams was, but he was good at writing a magazine article in a way that engaged the reader and was educational. I've still got a copy of the troubleshooting series of articles that he wrote back in the mid 90's (or so). Williams was remarkable in that he was incredibly clever and insightful, and was able to put together an excellent article or app note. 

to be fair to EDN, there are a lot of trade and news publications that have been struggling to maintain their former quality. Video blogs and YouTube channels are starting to fill in some of the gaps. I can't decide if the semiconductor companies are doing better or worse in regard to their tech literature.


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