Electrical help required in DIY LED circuit

[lifesci]

I'm gonna keep this short, so if any additional detail is required, please let me know.

I want to run multiple LED's on one driver. The driver I'm looking at is rated at 100W, and the LED's at 20W each. The supplier asks me what specs I want on the LED, i.e. what combination of voltage & amps (since afaik many combinations of volts*amps can give the same wattage, i.e. 50v * 1amp, 25v * 2amp, etc). How do I respond to this? I don't know how these different specs will affect driver output. I'm guessing there is some degree of math involved. Any help here would be greatly appreciated.

Secondly, my knowledge on electrical jargon is very basic, so I have a few questions.
1) The driver provides constant current; how is the voltage regulated? Or does it not matter?
2) If I add a 6th 20W LED to the circuit, what happens? Does the driver output more voltage to compensate for the additional LED? Or are all 6 LED's underpowered?

Thank you very much.

_________________________________

In case anyone is wondering what I'm doing.. I've built an aquaponics system that employs photosynthetic bacteria at the base for which I need specific wavelengths of light (and that can only be achieved efficiently using high-powered LED's). Its a research project at university.

[510christie]

Hey Lifesci

The main thing you need to know is the forward voltage of each led - add the voltages up if you are running them in a string. The forward voltage can be found in the led data sheets. Your driver needs to be able to supply at least that voltage plus a bit more - it gets a bit involved but be sure your driver can supply at least 10% more current than you need to drive the leds at their maximum.

The voltage the driver supplies is not critical as long as it exceeds the total forward voltage of your led string. As you noted, the drivers job is to regulate current.

You can only add another led (I am assuming you mean in series) if the additional added voltage does not exceed the drivers limit. If you are running the leds parallel, then adding an led doesn't affect your voltage - however, the current is split between parallel strings. So if you are supplying 1 amp to one led (or series of leds) adding another led (or string) in parallel will half the current each string sees to 500 mA (without affecting the forward voltage).

If you are running parallel leds (or strings) off one driver it is critical that the total forward voltages are the same in each string.

Hope that is not too confusing!

[lifesci]

Hi Christie,
Thank you for the informative reply. I, now, understand the voltage part, but am still confused about the current. I will be using series instead of parallel. My question now is whether the current is also split along the length of the circuit after passing each LED? Like you said to employ enough driver voltage to match the aggregate of all the LED's +10%, does it work the same way for current? If driver output is 1 amp, must the 5 LED's be rated at 1 amp each or 0.2amp each? I guess I'm asking if current remains constant throughout the whole circuit, unlike voltage which is lost at each LED? Must I add the current requirements of all the LED's to arrive at the 1 amp provided by the driver?

[kamloops_reefer]

You'll probably get the answers your looking for posting on dedicated DIY LED forums. There is lots of information out there and I found LED group buy was very useful, they have their own dedicated thread on nano reef.com

the DIY led build was confusing for me at first, but after buying everything and putting it together I would probably be able to do it twice as fast a second time

[mike31154]

Perhaps the DIY route is not the best option if you're having that much difficulty getting a handle on designing a circuit that will work for your application. If you have some or all of the parts already, it would be a shame if you cooked a bunch of LEDs hooking things up incorrectly. Get someone who has experience to help you out in person. Without more info on the exact specs of the LEDs/drivers you intend to use, it's near impossible to provide useful advice on how you need to proceed.

LED lighting is complex. Different colour LEDs require different Vf (forward voltage) as well as varying maximum current based on their power rating in watts. For example, the typical Vf for a 10 watt multi chip red LED is 6 volts, whereas a blue 10 watt multi chip needs 9 volts & a white 10 volts. Each of these 10 watt LEDs are designed to run at a maximum current of approximately 1,000 mA or 1 amp, but the Vf for each is different. Using a single constant current driver & attempting to run different colour LEDs can lead to disaster. Typically each colour requires it's own constant current driver. One can get around that hurdle by under driving the colours that require more current in order not to burn out the ones requiring less current. The relationship between voltage & current with regards to an LED is not linear, a small rise in voltage, less than 0.5 of a volt above the LED's maximum Vf will cause a sharp rise in current. The LED will burn brightly for a few seconds, then smoke! Throw in the added complexity of hooking them up in series, parallel or a combination thereof & things get even more funky. Of course the constant current driver will attempt to keep current constant, but mixing colours on the same driver is not generally a good idea due to the varied Vf.

There are plenty of off the shelf options out there these days for most applications that won't break the bank. Might be worthwhile having a look around.

[510christie]

Lifesci

The short answer is: all leds in one string see the same current, adding another led to the series does not change the current flow just the total forward voltage.

As Kamloops-reefer said, LED groupbuy is a good resource - without having to wade through a whole bunch of info.