Computing stuff tied to the physical world

LED discharge – more measurements

In Hardware on Jan 6, 2011 at 00:01

Following earlier posts about the LED discharge circuit (here, here, and here), let’s do a few more measurements. Here is the schematic again:

I’m going to apply a 1-second charge pulse and then disconnect for 5 seconds, as before. This time, I’m going to disable the pull-up during those 5 seconds, and improve the timing of the reporting so the time to send out characters over the serial port does not affect the whole measurement cycle:

Screen Shot 2011 01 05 at 13.09.50

The reason the timing is so much better in this version, is that it is now based on MilliTimer, which continues to count while the serial port deals with its outgoing data. Note that 20 chars @ 57600 baud takes a few milliseconds, which can quickly add up when reporting values 10x per second.

The values are sent out in a format which can be used directly in a spreadsheet – here are the first few lines of output:

Screen Shot 2011 01 05 at 13.10.55

The values are:

  • voltage measured over the capacitor, i.e. LED + 1 kΩ resistor (Volts)
  • voltage measured over the 1 kΩ resistor (Volts)
  • difference, i.e. voltage measured over the LED (Volts)

Here’s a graph of the resulting measurements:

Screen Shot 2011 01 05 at 12.42.29

Due to Ohm’s law (E = I x R, i.e. I = E / R), that yellow line is also the amount of current going through the resistor in milliamps. Since there is no other path for the current than through the LED, by deduction this is also the current through the LED.

What you can see (blue curve), is that the capacitor charges up within a fraction of a second, and then discharges to under 2V in the next 5 seconds.

What you can also see (yellow curve) is that the current through the LED is proportional to the voltage on the capacitor. As the capacitor charges up, the current increases, and from the shape of the curves is should be clear that the relationship is essentially linear.

This is not surprising, since linear voltage-vs-current curves are inherent to resistive loads, i.e. the 1 kΩ resistor in this case. What is special is the LED placed in series with that resistor. What it does is “eat up” a fixed voltage, i.e. 1.8..1.9 V of the voltage present on the capacitor “stays” in the LED. The rest is passed on to the resistor, which does its usual linear current-to-heat conversion thing (only milliwats of heat in this case – we’re dealing with very small currents and very low voltages here).

But LEDs are not perfect devices. If they were acting like a perfect diode, they would have a fixed voltage drop, regardless of how much current goes through them. You can see from the green line that there is a slight change in voltage drop.

Does that mean a LED has something like a “hidden resistor” inside it? Not really. Here’s a plot over that same data, as current versus voltage:

Screen Shot 2011 01 05 at 12.50.33

That’s not very linear. Remember that resistors just do what Ohm’s law tells them to (or was it the other way around?): twice as much voltage = twice as much current, etc. Which in a graph would show up as a straight line.

In this case, the graph appears to be tapering off. In other words, as more and more current goes through the LED, its forward voltage seems to stabilize around a certain value. Probably around 1.9V, but this will be different for each type of LED.

Note that I placed the current on the horizontal axis, because that’s the variable we can control. The LED “decides” what voltage drop it wants to add.

What seems to be happening in this circuit, is that between 1.75 and 1.80 V there is a small amount of leakage. This is consistent with the video, showing that the LED continues to light up very dimly as the charge leaks away, even though the current is less than 0.25 mA, i.e. 250 µA, at these voltage levels!

Ok. Can we find out more about this forward voltage behavior? Sure. Let’s put 3.3V on VIN and 0V on VLOW, and then measure the voltage on VHIGH:

Screen Shot 2011 01 05 at 13.45.46

(Isn’t it fun to do this stuff under computer control?)

Sample output:

Screen Shot 2011 01 05 at 13.47.05

Whoa, looks like I need to revise my conclusions a bit. The LED forward voltage does go up quite a bit when more current is applied!

Let’s try to figure out how much current is flowing:

  • the cap is fully charged after a few seconds, so we can ignore it
  • therefore the cap voltage will be 2.229 V, as measured by the ATmega
  • and the voltage over the 100 Ω resistor will be 3.3 – 2.229 = 1.071 V
  • the resistor is 100 Ω so with “I = E / R”, we get 1.071 / 100 = 10.7 mA

IOW, at roughly 11 mA, the foward voltage drop of this particular LED is about 2.23V, quite a bit more than the 1.87 V we measured at 1 mA. But still not anywhere near a linear relationship.

Tomorrow, I’ll examine these results a bit more closely … because they aren’t quite right!