# Limit drawn current but not voltage (LED driver for 100W LED)

So I have serial connected 8 LiPo's with corresponding controllers to monitor the battery pack. Now, they deliver 32 volts which is precisely what I need but they deliver way to much amperage compared to what the LED can handle.

The LED specs for input is 32-34V and 3500mA.

So I got reccomended to build/buy a "Buck driver" which looks promising, but I don't know how to calculate the different components and I'm way to much of a beginner to figure it out on my own, so i found a calculator that points me to a list of items that might be suitable.

Problem here is, non of the datasheets mention the range of the amps the circuit can limit within. I would feel much more confident if they mentioned it somewhere before I go about my "learning by doing" quest.

I have no clue where to begin or if the products are correct, I've read up a bunch on buck converters, Constant-current diode's and what not.

I got an example to me saying something like "Lets say you have a car. To maintain the speed you need to have a car that can go fast enough, and a cruise control to set the correct speed", I assume the car is the LED and the circuit I'm looking for is the cruise control. However, this confuses me since the power supply and the LED is already agreeing on the voltage, I don't need a cruise control I need a fan for the bloody engine so it doesn't overheat?

I need help, in any shape or form so I'm turning to the guru's of stack-exchange!

Edit: I've Googled for 4 days and asked engineering friends, and just as I post this I find: http://homemadecircuitsandschematics.blogspot.in/2011/12/make-hundred-watt-led-floodlight.html

Not sure that works for my setup either tho.. I'm so confused by now..

• Are you sure it is 3500ma? (3.5A ??) Can you show the LED's specs? Commented Dec 22, 2014 at 20:04
• @EugeneSh. sure, here it is: banggood.com/… Commented Dec 22, 2014 at 20:24
• Buck converter has not much to do with current limiting. This instructable and linked ones can help you instructables.com/id/high-power-LED-grow-lights-M.k2/step3/… Commented Dec 22, 2014 at 20:35
• The buck topology can be used in conjunction with current sense feedback to implement a buck-type current source. A buck controller with a low reference voltage could possibly work. Instead of connecting the feedback node to a voltage divider, you would connect it to a current sense resistor. Commented Dec 22, 2014 at 21:25
• Amperage is not a word. You mean current
– Will
Commented Dec 23, 2014 at 10:23

The reason you need a regulator (cruise control) is that small disagreements between the LED and the battery produce big changes in current. And there will be disagreements because both the LED and battery voltages can vary -- because of manufacturing tolerances, temperature, state of battery charge ...

The LED module is around 100W. 90%+ of the power will be dissipated as heat. So it is going to require a big heatsink, or a not as big heatsink with fan. Make sure that is part of the design process.

The last link you provided is a linear regulator design. A linear regulator is not a good design for this application because of it being battery-operated coupled with high current. For example, if the design is set to dissipate 2V @ 3A = 6W at low charge. At high charge, the battery voltage can be 25% higher (32V x 25% = 8V higher). The dissipation becomes 10V @ 3A = 30W. That is a lot of power to waste and to heat-sink away.

A switching regulator is more appropriate because it will not just waste the extra voltage away.

Counter-intuitively, with the battery voltage more less equal to the LED voltage, the switching regulator design cannot be in buck topology and would probably be in buck-boost topology. That is slightly more complicated and requires a transformer.

If you increase the battery voltage to always higher than the LED voltage by couple of volts, then the buck topology can be used. It is slightly less complicated, uses smaller components, requires a coil (easier to find) instead of a transformer.

So if you can increase the battery bank voltage, then concentrate on buck circuits for LEDs. Your "calculator" link can lead to usable designs. And in the "calculator", there is the "string current" where you specify the current. There may not be specific current limits specified on the ICs themselves because the limits depend on the components chosen around the ICs. Try other manufacturers also. For example, the TI LM3409 Webench is one that I have used and can potentially produce a design with component selections. Or find a complete driver module as suggested in the other answer.

• Can also lower battery voltage (by changing to series/parallel arrangement, for example) and go boost. Just as simple as buck. Commented Dec 23, 2014 at 18:59
• Actually, 8 * 4.2 = 33.6. Might be able to get away with boost, especially if it is not synchronous (in other words, if there is a Schottky diode drop at 0% duty cycle). Commented Dec 23, 2014 at 19:03
• Thank you for a detail look into your thought processes, i leaned a lot and will take everything in to account. This set me on the right path and I probably won't be needing more help until I have specific problems to solve compared to this overall issue that I had. Cheers! Commented Dec 24, 2014 at 20:18

We'll be able to help. Firstly, here are some guidelines in the right direction. Look through these and 'report back'.

Here is why to use constant current

Here are many possible ideas - look at these to get a general idea and then we can discuss in detail.

You can also buy drivers ready made. Here are some examples - doing it yourself probably costs slightly more but means you can tailor it to just what you want.