# Current too low at forward voltage of power led

I bought some RGB power leds, o.a. a Cree XML. The spec/description at the (Chinese) site mentioned forward voltage of 3.3-3.9V. But when driving the led at that voltage I measure much less current than expected. ~0.150mA @ 3.9V instead of ~1A for the Cree led, and something similar for other leds with a max of 700mA. I need to go above 5V to get the full max current.

Do I misunderstand the meaning of forward voltage? Or is it happening more often that leds are supplied with higher voltage drops (than described).

I planned to use a 5V power supply and a current regulator (mosfet, resistor and transistor feedback) which requires and extra voltage of ~1.2V. This does not fit if forward voltage is that high.

• My answer to electronics.stackexchange.com/questions/353955/… discusses binning of LEDs to achieve consistency between samples. It won't explain the very high Vf but may help in some way. You should really test the LEDs using a current source rather than a voltage source. – Transistor Feb 19 '18 at 23:15
• Watch out for high-powered LED's. The 10 mm dome type on a ceramic plate often has 2 LED's in series, and another pair parallel to the first. Turn-on voltage is about 6 to 7 volts. They can do the same with RGB LED's except they have six connections instead of two. At full power they MUST be mounted to a large heat-sink with thermal epoxy. – Sparky256 Feb 19 '18 at 23:21
• How did you run these at the right voltage? With a variable power supply? Or did you use a current limiting resistor and measure the voltage across the LED? – jonk Feb 19 '18 at 23:31
• If it's a chinese site then there's a decent chance the specs are simply wrong. There's also a good chance the part is not the part you think it is. – user253751 Feb 20 '18 at 1:04
• I measured with voltage supply, but using limiting resistor, measuring voltage across the led and across the resistor (to get the current). I think I suffer from wrong specs at aliexpress. – pfp.meijers Feb 21 '18 at 6:54

## 1 Answer

The Cree spec shows 2.9 - 3.5 V for 700 mA

You should be able to engineer a current controller that works at 5 V without any problems.
Perhaps something simple like this, though it will have a thermal variation of current which can be reduce by putting the NPN close the LED.

simulate this circuit – Schematic created using CircuitLab

• Any suggestion for a current controller - must be very cheap (<$0.20) because I need to make many? – pfp.meijers Feb 21 '18 at 6:56 • Added to the answer ....but <$0.20 seems a bit aggressive. – Jack Creasey Feb 21 '18 at 15:54
• Yes it is ;-) Found a more-or-less similar schema on a blog. But that did not use a resistor R3. R3 will increase the voltage drop over R1, isn't it? – pfp.meijers Feb 22 '18 at 17:49
• R3 has no real effect but is simply a safety limit on base current. The base current is only uA. – Jack Creasey Feb 22 '18 at 18:29