|Measuring Base Current|
Besides regulated power, you'll also want to grab a 1M potentiometer, a 470 ohm resistor, and a 2N2222 transistor. If you do it my way (versus Charles' way), you'll also want a large mix of jumper wire, tiny gator clips, and a few other items I'll try and point out.
First, I didn't have the tiny little potentiometer shown in Figure 2-3 on page 9. Oh, how I wish I had. Instead, I had one that I *thought* was 1M but turned out to be about 1/4 of what I needed. Rather than drive to Radio Shack for a single component (although I have done it in the past), I recalled that I had a 1M hand-dialed potentiometer in my component collection... from way back in Book 1. It works, but as you can see from some careful examinations of my photos, it did require a number of jumper wires and tiny gator clips to keep everything together. A few times a wire would pop out, so trust me... grab a tiny potentiometer designed for insertion in your breadboard.
|The initial 5 microamps dialed in.|
Once I got the experiment wired up and figure out the right pattern of shifting jumper wires between the various locations necessary to get the proper readings for the chart, everything went smooth. But it did take about 5-10 minutes for me to debug my breadboard. I don't care how comfortable you are at dealing with breadboards, you're still going to make errors in wiring. I did. A lot.
|The initial milliamps reading settled at 0.82.|
For example, look carefully at the two above photos for initial 5 microamps and initial 5 milliamps, and notice the very tiny red jumper wire in the first photo that connects from the Power column on the breadboard to the Collector of the transistor. See it? Took me 10 minutes to figure out it needed to be removed so the circuit matched up to the one shown in Figure 2.5. Once that wire came out, the readings began to flow.
|My data -- yes, my handwriting isn't great.|
It's explained in the chapter, but in case you missed it... there are a number of factors involved here that include the quality of my meter, the variation in that single resistor value (after the experiment, I measured it and found it to be 460 ohms, not the 470 ohms specified... Charles' resistor was very likely a completely different value), and the variation of voltage being provided by my 5V regulated power. Let's not forget variations in the transistor chemistry as well as the conductivity of all those jumper wires I'm using. The point is... my values may be different, but I *understand* what the chapter is trying to teach me. Make sure you do, too... before moving on to the next experiment.
If you read the chapter in its entirety before actually attempting the experiment (and I read it twice!) you'll know that the last column in the table on page 10 asks you to measure the voltage between the Emitter and ground. You can set this up and take this measurement in parallel to taking a reading of the current into the collector (starting in Step 3) and save having to go back and recalibrating the base current for each step of 5 microamps... but it does require a second multimeter. Use some jumper wires inserted into the proper places on the breadboard and when you're measuring milliamps but before you shut it all down and reset for microamps, take a voltage reading.
|Don't forget to measure your voltage.|
Oh, and you'll notice I skipped setting up for 35 and 40 microamp experiments... mainly because I had a 7 year old complaining I was wasting his summer vacation by not spending every waking minute with him. Feel free to go all the way to 40 microamps, but I was able to get enough valid readings to see the linear pattern in my data. I even plotted it in Excel for a nifty line chart. (I also plotted a graph with Base current versus Voltage between Emitter and GND.)
|Hey, look! A (somewhat) straight line!|
|Another straight line! (Base current vs Voltage b/w Emitter and GND)|
That's it for right now. Again, if you've got a copy of Charles' book, Encyclopedia of Electronic Components Volume 1, now would be a great time to read through the chapter on transistors.
And last but not least, my summary video of Experiment 2 below...