# LED circuit for 4 modules

I bought several LED's bulbs last years and several failed because crappy drivers.

I plan to combine 4 of these modules together and make a single custom DIY LED lamp.

Here is a photo of the modules and a aluminium base to fix the modules and serve as heatsink:

It's two 4 W modules and two 3 W modules. The typical specs for this LED is 300 mA current and 3.3 V forward voltage.

Each of these modules, the LEDs are arranged in serial.

I tested using a 12 V, 2 A HDD power adapter and all these modules are working, but I know is to much A and have risk to burn the modules.

Need help how to make a circuit to use this in single DC source, like 12 V DC driver/PSU.

• COTS constant current supply. – Ignacio Vazquez-Abrams Sep 19 '16 at 23:20
• If all the LEDs are in series, and identical, you need a constant-current power source, circa 45VDC maximum. – Whit3rd Sep 20 '16 at 4:54

You have two different voltages to deal with. Some approaches follow. I'm writing more about one of them because I bothered to provide a circuit. My apologies in that regard. It should not be taken to mean I prefer it. It just needs more of an explanation. The other two options are fine.

1. Two supplies: a $3.3V\cdot 3=9.9V$ supply, plus some extra voltage headroom plus two separate load resistors to help balance out the currents; and a $3.3V\cdot 4=13.2V$ supply, again plus some extra voltage headroom plus two separate load resistors to help balance the currents. You might use one primary supply, plus two of those cheap buck converters and adjust them to provide a reasonable rail and then drop some voltage from there, with resistors. You can tweak them for the two different rails easily.
2. Single supply: Enough for at least $13.2V$ plus some extra voltage headroom, plus four separate load resistors to help balance the currents out. This would dissipate more wasted power in the load resistors than solution (1) above. Here you could just use the primary supply.
3. Place all four modules in series with each other, since they all require the same current. This will require $(8+6)\cdot 3.3V=46.2V$, plus some voltage headroom for current regulation. Since $48VDC$ @ $1A$ supplies are often available for around US\$20 on ebay, this might be an approach. You'd need a regulator, which could be composed of something like this:

simulate this circuit – Schematic created using CircuitLab

You can remove and therefore bypass $R_3$, but you'll need to dissipate up to $7W$ in $Q_1$, then. Your call where to put the power. Note that we are talking about tossing away about $8W$ in the control circuit. But your load is about $14W$, so perhaps it is tolerable.

There's not a lot of headroom here. So if the required voltage varies too much, it may not perform as expected with these values as shown. There is about $1 \textrm{V}$ of headroom in $R_1$, so that's all there is to play with for the LEDs. But the circuit can be better adjusted, if you know exactly what the series requirement is.

EDIT: Tony has brought up the idea that $R_3$ isn't necessarily a great choice. He's right. But $R_3$ is completely unnecessary. It's there for one purpose and one purpose only -- to share some of the dissipation. I sized it as large as it should be, so drop it down from there as needed. Down to $0\Omega$ is fine. It can be completely removed, in fact. But the BJT will need to dissipate whatever $R_3$ doesn't. Just a way to trade off where power is dissipated, is all. It serves no other important purpose here.

• No offense but I reverted your "mathjaxifications" in the main question. Wrapping units in mathjax is basically the complete opposite of how SI-units are to be written. Here are the gory details, but essentially: Narrow space between the number and the unit, and "Unit symbols are printed in roman (upright) type regardless of the type used in the surrounding text.". Sadly, MathJax does not have the excellent siunitx package that solves this problem in normal TeX. – pipe Sep 20 '16 at 0:32
• @pipe Thanks. Learn as I go. And I think you are right. I just like stuff accented, I suppose. – jonk Sep 20 '16 at 0:33
• @pipe I'm using \textrm{d} for derivative symbol, too, now. The 'd' is supposed to be upright with the variable slanted. So I use $\frac{\textrm{d} V}{\textrm{d} t}$ instead of $\frac{d V}{d t}$. It's getting pesky to worry out all the details. I'm almost remembering and worrying about the "a foolish consistency is the hobgoblin of small minds" quote. – jonk Sep 20 '16 at 0:36
• Nice! I've even written a short essay on the space between the number and the unit on my profile page. Pretty sure no one reads that though. It's mostly so I have something to refer to when complaining.. – pipe Sep 20 '16 at 0:41
• @pipe You'd better find an in-MathJax method for SI units and appropriate spacing!! While you are at it, get me a way of numbering equations to the far left or far right. I can't seem to figure out how, as they don't include all the nice modules. – jonk Sep 20 '16 at 0:46
• You "could" run the 4 LED modules all off PC 12V +/-2%
• The 3 LED modules need 9V, so mismatched.
• If you ran all 4 modules in series you need a supply of 350mA CC @ 12+12+9+9=42V ( ideal solution )
• gauge current and Pd by how hot they get, if you dont have a meter.
• mount on ALuminum substrate or CPU heatsink for cooler operation.
• They should be hot but not too hot to touch for optimal current.
• If not bright enough increase voltage and add Series R.
• Each 1W LED will be about ~1 Ohm with at least a 50% tolerance. This is the only variable that affects Vf.
• Best quality 1W LEDs will be 2.9V worst 3.5V , check datasheets (singles, not arrays)
• So if using a universal laptop charger set to 14V say then adding 2V for each string needs 2V/350mA = 5.7 Ohms @ 1W This would work for a car but would dim on idle or when ignition is off.

• universal laptop chargers (>=65W) are cheap with selectable voltage
• if 22V is avail then 3LED+4LED =21V nom to 22Vmax then your array is 7S2P which will work hot on 22V if it is 21V then no R is needed.
• Ideally a CC supply for LEDs can be purchased online from some OEM like Meanwell that runs up to 50V to 70V with all in series.

If you want to enter your own values for Vf@1A you can edit this simulation. I chose nominal population values for 1W LEDs.