Driving parallel led series with 12v deep cycle Help.

I have been searching for free software and circuits trying to figure this out on my own but have failed. I'm trying to power 18 3w green LEDs in 6 series of 3. They have a fv of 3-3.4v and 750ma. They will be mounted on aluminum square tube inside acrylic tube and submerged in water. Heat hopefully shouldn't be an issue due to the aluminum caps exposed to the water. Trying to find the cheapest most compact circuit that can provide all the series with a constant current. I have combined a few circuits I found online but I'm not sure if it will work.

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Regarding the current regulator, composed of Q1 and Q2: You may want to keep an eye on the stability of this circuit. I've seen similar arrangements oscillate rather badly. The use of a mosfet as the pass transistor adds a lot of gain to the control loop, which can reduce the stability. Adding a resistor between R3 and Q1-b is one way to reduce the loop gain.

I assume that Q3 through Q8 are supposed to form a current mirror. I haven't tried that... it does rely on the transistors being closely matched and being thermally coupled. I think I'd just try using a resistor in each string. For a circuit like this where you've got the same voltage across each string, the series resistor should work as well as a current mirror.

Thanks for the reply!

Is there a simpler way to get 4.5 amps evenly to all 6 series that the above setup. I'm not very knowledgeable with circuits and there is so much online im not sure which direction to go. I tried running the series parallel with each series having its own resistor but some of the series were dim while other bright.

As a general rule, using a resistor in each string helps compensate for variations in the LED voltage/current characteristics. If the LEDs are well matched, a home project can sometimes avoid the need for the series resistors. I've used ten 5mm wired in parallel on bike tail lights and they shared current very equally.

My guess is that either the LEDs are not well matched (perhaps they are low quality?), or that there is something about how they are wired or connected to a heatsink that is changing their behavior. For instance, if the LEDs aren't all the same temperature, that will change the voltage across them at a given current. Or.. if there were some high resistance connections to a LED, that would reduce current in that string.

Do you have any additional data? What was the value of the resistors? What was the range of currents in the strings (i.e. what was the max and min)? It may have been that the supply voltage was fairly low and some strings barely had enough voltage to forward bias the LEDs. Lots of possible causes.

In this sort of situation, it's very helpful to measure the voltage at all of the points in the circuit, and then write these voltages down on the schematic where they were measured. It's easier to notice where the voltages just don't make sense, indicating where a part might be failing, a bad solder joint might exist, or there could be something wired incorrectly.

I agree with Steve K that stability can be a problem, and that a resistor per string is probably better than the current mirror. You need to plan on half a volt or more across the resistor for it to work properly. But you may find that with a proper driver and construction, you don't even need the resistors.

I'm quite surprised to see that the Vf of the LEDs is 3 volts or more. Greens are usually closer to 2 volts. Do you have a part number for us to look at? The rest of this post will assume that the 3.0-3.4V spec is correct. If it isn't, then ignore most of this and I'll have some other comments.

I notice that the Vbe of the 2n5088 is more like 0.7-0.8V at the design current (around 70 mA). This is going to drive a lot more heat into R3, which should probably be a 5W resistor, not 3W. It should also be 0.75V/4.5A = 0.166 ohms. The nearest readily available values are 0.15 and 0.18 ohms.

Vbe of the 2N6715 is 1.2V at 1A. It will be pretty near this at the design current of 0.75A. Add this to the drops across R3, Q2, and R4 (unspecified) and you get over 2V. You see that you are rapidly running out of voltage to run the LEDs with a 12V supply.

Even worse, since this is to be run off a deep-cycle battery, you want it to run down to 10.5V or so. This is barely enough to run the LEDs, even without considering the losses in the regulation circuitry. I think you really need a low-dropout regulator. The schematic for one of my proven low-dropout regulators can be found here: http://www.candlepowerforums.com/vb...10A-linear-LED-driver-New-and-Improved!/page2

(Check post #33)

This can be simplified somewhat in your case, as you only need one 'mode', and probably don't care about precise regulation as battery voltage changes (plus or minus 10% is normally not detectable to the human eye). I can help you with simplifying it and selecting appropriate parts if you want to build one. Total voltage lost across this circuit could easily be 0.3V or less, which is perfect for your application.