I need some help with Diode Mode Measurement.
I got a Crossfit-Clock here for repair. It has six large (122 x 90 mm) 7-Segment LED. Four red color LED (CPS40011DR8) and two blue color LED (CPS40011DSB).
During testing in Diode Mode, i found the red LED elements in working condition.
My default setting for Diode Mode is 10V, 1mA. The red element lights up when testing.
The blue LED elements looks like dead. No color, no reading, no sign of life. Because i couldn't find blue LED as spare part, i ordered red ones.
After desoldering the blue elements, i tought, what, if i use some higher voltage from Lab-PSU for testing. I tested with 0,01A, increasing at 7V. To my surprise, after reaching 10V, the LED came to life and showed up in blue color.
I then played with the Diode Mode settings.
The blue LED seems in working condition. But The DMM6500 has a weird behavior at 1mA in Diode Mode.
At 100µA, the LED light was more bright than at 1mA. Of course, at 10mA it was a strong blue color. I didn't expect that a DMM setting could lure me into a wrong judgment of my faulty DUT.
I attach some pictures with my test settings. I added a shadow for better view. The last two pictures represent my default light situation. As you can see, at 1mA, the blue color is invisible. I would probably have been able to recognize the blue color at 100µA.
Any idea why that happens at 1mA? Why does it works at 100µA?