Yesterday I posted I vs V plots for the orange LED WP710A10ND and commented that “one can see the crude quantization of the bad DAC in the FG085 function generator, as banding in the line.”
Today I tried to improve the plots by using a 470µF capacitor across the function generator outputs. Since the FG085 has an approximately 47Ω output impedance (nominally 50Ω, but see FG085 function generator output impedance), a 470µF capacitor would give a low-pass filter with a time constant of about 22ms, or a corner frequency of 7.3Hz. I was doing a triangle wave sweep with a period of 9 s, so the 256-step DAC makes for about 57 steps/s or 17.6ms/step. Having a time constant about the same duration means that the steps will be converted into ramps and we’ll avoid most of the quantization artifacts.
I had two conjectures about the source of the hysteresis:
- The 32× averaging takes some time, so the voltage and current measurements are not precisely synchronized, and the current-before-voltage measurement would give different systematic errors depending whether the voltage and current were rising or falling.
- The properties of the LED change because of heating and cooling effects. The upward leg has a cooler LED than the downward leg.
Note that these two hypotheses are distinguishable by experiment. If I slow down the ramps, then the time discrepancy results in smaller voltage and current discrepancies, and the effect should diminish. But the LED gets hotter if it is run at maximum current for longer, so the thermal effect should be greater.
I tried using a 33s period instead of a 9s period, and the two curves moved further apart (from about 0.4mA to 0.6mA at near the max separation), consistent with the thermal theory, but not the time-bias theory. (I leaned toward the thermal theory initially anyway, since the two curves are further apart than one step of the DAC in the FG085.)
The hysteresis makes it clear why manufacturers don’t try to specify the I-vs-V characteristics very precisely—not only can there be variation from component to component, but the parameters are temperature dependent.