Doug S
Flashlight Enthusiast
Sometimes a bit of anal retentive behavior allows you to see things that you would otherwise miss. I have been maintaining traceability on Luxeon emitters that I purchase. I do a receipt inspection which consists of measuring the Vf at 90 and 400mA [which allows calculation of dynamic resistance] and measuring the relative brightness with a test fixture that I have built that fits on my photometer sensor. I recently mounted a 1W white emitter on a heatsink along with a NX-05 optic and holder. Wires were soldered to the emitter leads. The emitter was mounted with Arctic Alumina under the slug and JB Weld epoxy under the plastic portion of the emitter. I powered the assembly for testing purposes using a Li-ion cell and a resistor for current limiting. The assembly was powered several times for a minute or two to check out optic centering and general performance. It seemed a bit brighter than expected for the battery voltage, choice of ballast resistor, and the previously measured Vf. On a hunch, I remeasured the Vf at 90 and 400mA. They had shifted significantly! The receipt inspection values had been 3.30 and 3.69V at 90 and 400mA. The post mounting values were 3.13 and 3.52V. Interestingly, these two data sets give exactly the same dynamic resistance of 1.26 ohm. What caused the shift? I don't know. I do know that this is not a case of loss of traceability or inconsistancy in testing methodology. The only "stresses" that the emitter was exposed to are: heat of soldering the leads, several thermal cycles of being energized/deenergized, and pressure applied to the plastic outer portion of the emitter during mounting to spread the heatsink compound and epoxy. While I would really like to know if the relative output has also changed, the way the emitter is mounted precludes retesting it.
Has anyone else observed a similar shift in Vf in an emitter that has not been abused? One of the implications of this phenominon it that setting current by voltage control only is even more iffy than already widely recognized. Up until now, I have been willing to consider voltage only control in applications where the emitters are not heavily driven and ambient temperature ranges are not extremely large. Now I am less confident about that design approach.
Has anyone else observed a similar shift in Vf in an emitter that has not been abused? One of the implications of this phenominon it that setting current by voltage control only is even more iffy than already widely recognized. Up until now, I have been willing to consider voltage only control in applications where the emitters are not heavily driven and ambient temperature ranges are not extremely large. Now I am less confident about that design approach.