In this lab we will explore some basic circuit features and use of the Digital MultiMeter while examining the characteristics of voltage dividers. This site has some nice tutorials on using various types of lab equipment that we will use this term. We will refresh some ideas about ohmic devices and our intuitive understanding of how circuits behave. I apologize if some of this is a little ragged but I'm still in the process of settling out how best to try to develop your skills in this arena.
Procedure:
1) Before we really dive into the material for this lab you need to take some time to understand how to use the Digital Multi Meter (DMM) to measure DC and AC voltages as well as resistance. We're not going to worry about current for the time being. For the DMM and other equipment that we will be using I recommend downloading the equipment manual from MIT's Circuits and Electronics Lab. The section that is relevant at the moment begins on pp12 of this manual.
2) We are using handheld DMM unlike the benchtop devices described in the MIT manual but the principles are the same. There are several tasks I'd like you to do to verify that so can operate the DMM without significant danger to you or to the DMM.
- Show me that you can measure the AC voltage difference between all three pins on a wall outlet and show how this determines which blade is "hot" and which is neutral. Notice what happens if you repeat the same meaurement with your DMM set to DC volts. You will of course record this info in your lab notebook for future reference including where the measurement leads are connected.
- On the powered project board measure the voltage of each binding post (gotta figure out what that is!) relative to ground (reference node eh?) and set the variable supplies to +8V and -8V respectively and verify that the voltage difference between the two is the proper value. Be sure to note how your measurements change when you reverse the measurement leads. Record in notebook.
- Set your DMM up to measure resistance and measure the resistance of at least 10 resistors from the junk box provided and put them away in the correct location in the supply boxes. The point here is to get you to pay close enough attention to the details of the digital display that you can do this without errors (...and I get some of the stuff put away:). For at least three different resistors verify that the measured value is consistent with the value determined from the color codes. What is the maximum and minimum variation of the measured values from the stated values?
Introduction:
Now you need to learn how to use a prototyping board (also known as breadboard or project boards) to set up the following circuit. A basic description of how a prototyping board is internally connected is on pp 20/21 of the MIT equipment manual. There is also a OSU Engineering Dept video about prototyping boards (may be slow as a dog to download). When you have perused whatever materials you can find you will build the following circuit on an unpowered project board. You will ignore the triangular thingy (this is an op amp which we will explore later) and any dangling wires which don't have resistors. In the end it should be easy to hook your unpowered board up to a 9 V battery. Verify with your instructor that you have accomplished this appropriately and get him to sign off on your circuit drawing that you have made in your notebook. Good laboratory practice is to turn off the power to your circuit before and while making any changes!
Measurements:
3) Before starting any measurements it is important to consider the following feature of resistors. They use energy which means they heat up. Because of this resistors are rated with a maximum power consumption. Most of the small color coded resistors that you see everywhere are rated at 250 mW or 1/4 W. This means you can quickly calculate the maximum voltage such a resistor can "drop" without begining to overheat. Use Ohms Law to show that the power used by an ohmic resistor is given by v2/R and that this means the maximum voltage for a given 250 mW resistor =sqrt(0.25*R). Determine vmax for at least one of the resistive paths to ground in the dotted box above.
4) Construct the portion of the solar tracker circuit (shown above) on your project board. Build the circuit so that the physical circuit looks as much like the schematic drawing as possible. Explain your circuit to your lab instructor so he/she can verify that you have constructed it correctly.
5) Now connect your circuit to a 9V battery. Then, with the battery connected, use your DMM to measure the voltage (relative to ground) at each resistor and at each of the three pins of the variable resistors.
6) Use your knowledge of Kirchoff's Laws to calculate the expected voltage at each of these points. Compare the predicted values with the measured values and comment on any differences.
