I was interested in looking at the forward voltages for the various LEDs that I have on hand, to see if there was an easy way to convert the single numbers given on the spec sheets into typical curves.
I used a very simple setup:
- LED with series resistor (resistor size changed for different current ranges)
- PteroDAQ (using Teensy 3.1) to record the voltage across the LED (A11) and across the resistor (A10-A11), 32x averaging 1kHz sampling.
- JYEtech FG085 function generator to drive LED+ resistor (3.2Vpp, +1.6V offset, 1Hz)
I recorded a several seconds of data for each resistor size I used (47Ω, 150Ω, 1kΩ, 12kΩ, 220kΩ). For a couple of the LEDs, I also tried 3.3MΩ, but the noise in the ADC for that high an impedance source was too much—I would have needed to add unity-gain buffers to be able to read the signal reliably.
I found that the differential inputs were rather noisy below about 100mV, so I only used the larger signals. I should figure out how to use the programmable gain amplifiers on the Teensy 3.1 and integrate that into PteroDAQ, so that I could read small voltages more reliably. Here are the I-vs-V curves for the identifiable LEDs I had on hand (I have some other “grab-bag” LEDs that I didn’t bother measuring):
The low-end of the I-vs-V curve for each resistor is noticeably noisier than the high end—even after I trimmed off the results with differential inputs lower than 100mV. With the 220kΩ series resistor, the single-ended voltage measurement was poor—the high-impedance source causes problems removing the noise injected by the sampling circuitry. This is not such a problem with the differential measurement, since the differential amplifier provides a low-impedance source to the sampling circuitry. There is a second differential channel on the K20 chip, but one of the pins for it is A13, which is only available as a surface-mount connection on the back of the Teensy board—not accessible for PteroDAQ.
The IR emitters have a much lower forward voltage, and a much higher maximum current (50mA instead of 25mA). The IR LED has a 1.6V max and 1.2V typical specified forward voltage at 20mA—I got 1.199V at 20mA, which is closer that I have any right to get to the typical value, since I didn’t even measure the 47Ω nominal resistor.
Each of the non-IR LEDs has a slightly different forward voltage spec, but they all have a max forward voltage spec of 2.5V at 20mA, and typical values ranging from 2V to 2.25V. The green and red curves (the two on the right) were indeed the two LEDs with the highest typical values. None of the non-IR LEDs got up to high enough voltages in this test to see whether the forward voltage at 20mA was measured correctly. I should be able to do that with a smaller series resistor.
Bottom-line: the typical forward voltage at 20mA does give a fair idea of the I-vs-V curves, though the max current is also important for getting a good shape to the curve—the curve switches from straight-line exponential behavior to the gradual saturation at about 1/5th of the max current.