LED stars have a "+" and "-" marking where the red and black cable are soldered onto. Typically a black cable is soldered to "-" and a red cable is soldered to "+":
the DMM be in diode measuring mode:
After removing a cells from the battery compartment, when i touch the "-" solder blob with my black probe (DMM) and the "+" solder bolb with my red probe (DMM), the LED would flash faintly and the DMM would not show/measure any number (voltage). When i reverse the probes, i.e. "-" solder blob with red lead and "+" solder blob with black lead, the LED would not light up and the DMM would, again, not show/measure anything.
Basically i can light up LED's (Cree XP-G, Cree XP-E, Photon, ..) simply by touching their "-" and "+" ends with my DMM.
But what does the following mean? -- "+" solder blob with black lead, and "-" solder blob with red lead, and the DMM measures a number!! (e.g. 0.539V); and reversing the probes: "-" solder blob with black lead, and "+" solder blob with red lead, and the DMM measures nothing and the LED doesnt flash!
LED's are diodes (arent they?) .. so how do we explain the DMM's behavior in the many possible scenarios (probes on blobs, good LED, blown LED, ..)? Maybe this is too technical for us budgeteers ;)
Unfortunately, most reasonably priced DMMs can’t measure the forward voltage of an LEDiode because that voltage is too high for them. If you perform the same experiment on a ‘regular’ diode, you’ll get a reading in one direction of .5 to .7 volts and no reading in the other direction. This tells you that the diode conducts with a .5 to .7 volt drop in one direction (which it should) and that it has not been damaged in its ability to block reverse voltage and thus doesn’t conduct in the opposite direction.
Most (white) LED’s don’t conduct in their forward direction until about 2.3 volts, and that’s more than the average DMM can drive into them. I’m actually a bit surprised that you’re even getting a faint blink/flicker as that means that your DMM is VERY close to getting over the Vf of the LED.
If you have a red or green LED laying around anywhere, try doing the same with it. You’ll get about 1.4 volts in the forward direction and you should get no reading in the opposite direction. Red and green LEDs have much lower Vf’s than Blue LEDs (which is what a white LED really is).
PPtk
Edit:
Also, further working against you is the fact that although you pulled the battery, the LED is still in circuit with the driver. This is confusing the crap out of your DMM even more. Since your DMM is obviously so close to getting over the Vf hump of that LED - you might try de-soldering one of the wires going to the star and seeing what it gives you then. Who knows, maybe it will manage to give you a reading. If it works, a functional white led should read about 2.3-2.4volts in one direction and should give no reading in the other direction.
