Tuesday, June 3, 2014

Experiment 2, Part 1 (Chapter 2) -- Learning Can Be Frustrating

Let's get this right out -- Experiment 2 is a bit tedious, but it's definitely going to cement in your mind some basics about not just transistors but voltage and current in general. For Experiment 2, you're dealing with a fixed voltage -- 5V DC regulated. So if you haven't already got your 5V regulated power via a breadboard, you'll have to get that up and running before attempting Experiment 2.
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.
Also, dialing in the larger pot was less than fun. It's extremely finicky. That said, once I would get the proper base current dialed in, I'd let it sit for 5-10 seconds to make sure there wasn't any fluctuation. And don't MOVE the potentiometer after dialing it in... even a subtle shift in its location would occasionally cause a 0.1 fluctuation. Once I figured this out, I used more jumper wires to extend the reach of the wires connected to the pot -- wires directly connected to the pot were wired into the breadboard so I didn't have to pull them out again... and then another jumper wire would go from that row (on the breadboard) to wherever I needed it. This eliminated my jostling of the potentiometer once I got into the pattern of shifting the various wires to measure current in microamps, milliamps, and also to measure the voltage. (More on measuring the voltage in a second.)

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.
As you'll see in the next photo, my readings for current into Base, current into Collector, and the Beta Value and Voltage between Emitter and Ground differed greatly from the book's values.

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.
Let's see... any more thoughts? Be sure to kill the power (5V) to the entire breadboard as you move back and forth between microamp and milliamp configuration. I forgot a few times, and thankfully I didn't burn out any components. Don't take that risk.

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)
Back in the Experiment 1, I saved that transistor (the one that was possibly damaged) for use in Experiment 2. I'm not using it here, but I am saving it for my writeup for Experiment 2 Part 2 -- for that, I'm still waiting for two little gauges to arrive. They should be here by end of week, but if not I may jump ahead to Experiment 3. But I will come back for a revisit of Experiment 2.

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...

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