We are using Cree MX-6 LED which is recommended to use 350mA current (or lower) for Energy Star qualification. We are running 4 parallels and 8 serial circuits for the lamp. The driver puts out current of 1.3A, and 26 volts. So each LED gets 330mA and stays within the recommended range. In this case, what is the voltage each LED is getting ? (Cree recommends 3.0-3.8V) Thank you for your help.

 

Four paralleled series of 8 leds ?
26V divided by 8 = 3.25 V

 

Have you read this? It is written using low power LEDs but the principles are the same.

LEDs, 555s, Flashers, and Light Chasers

It would help if we had a basic schematic, then we could help more.

 

At 100 lumens each, 3200 Lumens total that is enough for a good work light or reading lamp. Brighter than a 100W incandescent I think.

Just interested enough to look parts and other stuff up.

 

Thank you all for the replies. Great. At 3.25v per LED, it is well within Cree's specs of MX-6. This lamp is good for 70W HPS replacement on small and residential streets.
Thanks again.

 

But wait a minute, all LEDs are not equal, even of the same model number. There are small variations that cause difference in Vf and luminosity. That is why they bin them.

Here is Cree's binning specs on this lamp. (Click link, see page 2 for Vf binning)

I have thought about this a lot in the LED-in-series questions.

They give a range of 2.8 to 3.8 for the bins. That is a wide range.

So if you're running 26V on 8 LEDs in series, and three of them are at 3V at 330 mA then another may be at 3.75V. It is totally possible unless you bought binned lamps.

If you're feeding the whole 1.3A into several strings of LEDs in parallel, then it's also guaranteed that they are *not* receiving the same current. Some strings will be taking more and some less current. The question is really, by how much?


Driving LED lamps

 

I'm not being nit-picky -- LEDs are not created equal, even by Cree.