Monday, July 28, 2014

Experiment 11 Part 1 (Chapter 11) -- Voltage Readings For Op-Amps

Chapter 11 is off to an interesting start. First, this early circuit is done so that I can use a multimeter to read DC voltage, not AC as in earlier experiments. The circuit from Chapter 10 is stripped down... no LED, no electret. Just a trimmer and two pair of matching resistors (2k and 100k). The LM741 chip is still in play, and the schematic in Figure 11.1 is really easy to understand if you flip back to page 64 and really understand the pinouts of the LM741 and what it's doing in this new circuit. I think when I'm done with Chapter 11 I'm going to go back and re-read Chapters 10 and 11 again... this info really needs to sink in and I'm just now starting to figure it out, especially the volt dividers.

Finding a pair of 2.2k resistors with identical values (2.19 on my meter) was easy... found two matches in the first five tests on a string of 100 resistors. Those go in and replace the earlier 100k pair at the top of the breadboard -- the chapter recommends trimming them down (the leads) so that they are less susceptible to electromagnetic interference, so I did just that... and on the 100k pair.

Notice anything missing?
 When I first tested the circuit, I was getting a steady voltage reading. I knew something was wrong, so I rechecked my wiring. Don't do as I did and forget to add a jumper wire between the 2.2k resistors! I had a green jumper going between them to pin 2 on the LM741 but dummy-me forgot to make the final connection from 9V to GND across the breadboard. Oops.

Once that correction was made, I started seeing the results I wanted. Charles wasn't kidding when he says there's a point in the middle where the voltage jumps fast from negative to positive. Turning the 1k trimmer (I couldn't find my 5k) allowed me to watch the voltage go back and forth between positive and negative. now I've just got to figure out why the high end voltage is +4 and the low end is around -2.6. Probably something with the resistor pairs and them still having a slight variation in value... maybe?

 Up next, I'll be dealing with negative feedback as opposed to the earlier experiment that used positive feedback. The circuit is almost identical, but Figure 11-4 has a few extras inserted -- a 10k and 1M resistor.

Video for Experiment 11 Part 1 is below...

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