Lab #2 Help Document

Lab #2 Help Document

This help document will be structured as a walk-through of the lab. We will include instructions about how to write the report throughout this help document.

This lab will be completed in room 335 CTB.

You will need to partner up for this lab in groups of two.

Just as a reminder, when writing your lab report, remember to state what you did, how you did it, and why you did it. Include equations used and all calculations performed. Also, if the lab says to repeat the steps in a previous part, repeat all of the steps including the making of any calculations, analysis, graphs, or other requirements.

This lab will require the use of the following pieces of equipment:

DC power supply

DMM

Bread Board

3 Resistors:

There are 6 steps in this lab. It is best for this lab to do them in the order that they are written.

1. First measure the resistance of each of your resistors using the DMM. As you did in the previous lab, determine how close each resistor is to its rated value, and whether it is within its rated tolerance.

This part is relatively straight-forward. It’s the same set of steps performed for part 1 of the previous lab except that you use two fewer resistors this time. If you need to review how to make a resistance measurement or how to use a breadboard, pleasesee the following section, else you can skip the next section:

How to measure Resistance using the DMM

To measure the resistance of a resistor, you only use the DMM (never have the DC power supply connected to a circuit when measuring resistance. Doing so can damage the DMM and will give incorrect readings). First configure the DMM for measuring resistance by placing the leads in the correct holes and selecting the resistance setting. Make sure the automatic unit resolution is on.

Connect the red lead to one of the resistor’s leads and the black lead to the other of the resistor’s leads. The reading on the screen is your resistance.

Back to Part 1

First, make and record your resistance values for your 3 resistors. Next, use the % difference equation to calculate how close each resistor is to it’s rated value. The equation is:

[(MV – RV)/RV] * 100 where

MV = Measured Value

RV = Rated Value

Remember to state whether each resistor falls within its rated tolerance. All of the resistors in the lab have gold bands, so they have ±5% tolerances. If the percent difference you found with your calculation is less than 5%, then your resistor is within tolerance.

Lab Report

For your lab report, you will need to record how you configured the DMM to measure resistance, how you measured the resistors using the DMM, the resistances that you measured, the percent difference equation and it’s components, your calculated percent difference values, and whether or not the resistors were within tolerance. Be sure to specify the units with all resistance values. You can use your resistance measurement descriptions from lab one (if they are complete) to describe how you made measurements. Remember that they need to describe completely how you set up the equipment and made the measurement. For example, to describe how you set up the DMM you can say:

“I configured the DMM to make voltage measurements,” and follow that with a picture or diagram of the DMM showing the red and black leads in the proper holes along with the screen on showing the units for resistance.

Then for the actual measurements, you could say something like:

“Measurements were taken as shown in this picture/diagram,” and then have a picture showing a resistor on a breadboard with the red lead of the DMM on one side and the black lead on the other. You could also use a written description, though image descriptions may be simpler to create especially if pictures are taken of the steps during the lab process. Regardless of whether you use images or written descriptions, ensure that the description is complete, meaning another person who hasn’t taken this lab and doesn’t know how to set up the equipment could use your description to make the same measurements. Also, keep digital copies of this and future lab reports in case the same measurement is made during a future lab. You can use the descriptions from this lab for those reports as well as long as they don’t have extra bits that need to be documented.

If measurements are made multiple times during a lab, you only have to describe the first measurement with the detail indicated above. All other measurements of the same type can refer to this description. For example, if you measure resistance in part 1 and part 6, you only have to describe the method of making the measurements and setting up the DMM in part 1. You can refer back to them in part 6. You do still need to include any measured values however for both parts.

As far as recording your data is concerned, include the following information:

Include all used equations. This part of the lab only had one, so include it:

[(MV – RV)/RV] * 100 where

MV = Measured Value

RV = Rated Value

This is an example of how you can record your data. You don’t have to do it this way as long as your recording includes all of the values/statements specified in the table below with their corresponding units.

2. With the DC power supply set to +5V, connect the 330Ω resistor and measure the current.

This part is like part 3 of the previous lab where you measured the DC current through 5 resistors, except that you are only measuring current not calculating expected current and you are only measuring it through the 330 ohm resistor in this case. The +5V setting on the DC power supply is constant throughout this lab so don’t change it after setting it the first time. You’ll also be doing current measurements from now on, so keep the DMM and DC power supply configured for DC current measurements. If you need to refresh on how to make DC current measurements, refer to the following. If not, skip it:

How to measure DC current through a resistor using the DC power supply and DMM

To measure the DC current through a resistor, you will need the DMM and the DC Power supply. First, make sure that the DC power supply is correctly configured and set the voltage to the value indicated in the lab using the rightmost voltage knob. Also configure the DMM for DC current measurement by placing the leads in the correct holes and selecting the DC current (not AC current) setting.

Connect the black leads of the DMM and the DC Power Supply together. Then connect the red lead of the DC Power supply to one lead of the resistor and the red lead from the DMM to the other lead of the resistor. This configuration places the DMM in series with the resistor allowing it to measure current.

Lab Report

Describe how you set up the DMM and DC power supply. Remember to state that you set the voltage to 5v.

Describe how you measured current through the resistor.Remember to make the descriptions complete such that someone else could read your report and replicate your experiment.

Show the current value that you measured. Remember to include the units for the measurement (mA in this case).

3. Now add the 100Ω resistor in parallel with the 330Ω resistor and measure the total current. Did the current go up or down? Calculate the expected current for the parallel combination and compare this value to the measured current. Were the results as expected?

This is where things start being new. For most of you, this is probably the first time you have made a parallel circuit, so I’ll explain a few things about them before moving on.

In a series circuit, current flows through each component in sequence. Say you have 3 resistors: R1, R2, and R3. The current will flow from the power supply, through R1, then through R2, then through R3, and finally back into the power supply. As the current flows through each resistor, a portion of the voltage is lost depending upon the size of the resistance (the bigger the resistance, the larger its share of the voltage drop). The voltage starts out at the voltage set on the power supply and ends as 0V after the current passes through the last resistor and returns to the power supply. The current remains constant throughout a series circuit and does not diminish at all between leaving and returning to the power supply. The total resistance of a series circuit is also equal to the addition of all of the resistances in that circuit. So, Rt would equal R1 + R2 + R3 and would continue in this manner if there were more resistors (Rt = R1 + R2 + R3 + …).

In a parallel circuit, current flows through each branch of the circuit simultaneously. A branch is simply a path that current can follow through. If you have 3 resistors in parallel with each other, then you have 3 different branches for current with a component (the resistor) on each branch.

Now let’s say you have 3 resistors: R1, R2, and R3. The current will flow from the power supply, through the three branches created by R1, R2, and R3 at the same time, and then return to the power supply. As the current flows, the voltage still goes from what it is set at on the power supply at the beginning of the circuit to 0V at the end when it returns to the power supply. This time, however, the voltage drop is the same for each branch and by extension each resistor meaning that the voltage drop across each resistor is 5V (voltage drops across each resistor are not added in a parallel circuit because the three resistors are basically treated like one resistor when they are in parallel). Also, unlike in a series circuit where the current through each component is the same as the current going through the circuit, the current going through parallel resistors gets split up depending on the resistance of each resistor. The amount of current that goes through each resistor is inversely proportional to the size of that resistor. In other words, if R1 and R2 are in parallel and R1 is 100 ohm but R2 is 10,000 ohm, then R2 will have much less current than R1 pass through it. This does not mean that current is lost. The current at the end of a parallel circuit is the same as the current at the beginning before going through the resistors. All that happens is that the current gets divided up when it hits the parallel resistors and added back together when it is through them. Also, unlike a series circuit, the total resistance of the circuit is found using this equation:

Rt = 1/[(1/R1) + (1/R2) + (1/R3) + …]

This has the effect of reducing the total resistance across the circuit. The resistance of the circuit will be less than that of the lowest value resistor.

For part 3 of the lab, the first thing that you do is add a 100 ohm resistor to the 330 ohm resistor in parallel. This is done using a breadboard by placing the 100 ohm resistor and the 330 ohm resistor such that one lead from each is in the same column of a breadboard. We’ve included the section on using the breadboard from the first help document if you need it. Pay attention to the part that shows how to set up a parallel circuit. If you don’t need this, skip down to the next section called MeasuringDC current through a parallel circuit.

How to use a breadboard

A breadboard is a device that allows you to make circuits without soldering the leads. The breadboards used in this lab have 3 sets of connections composed of columns of connection holes. The first set is in columns of two holes. The second and third sets both contain columns of five holes.

The sets do not connect to each other so connections between sets are okay. Columns are not connected to each other through their connections while rows are. All connections within a column are connected together while all connections within a row are independent. Because of this, you NEVER place a component like a resistor such that two or more of its leads are in the same column. This will create a short circuit and can burn out the power supply. You can place leads from different components in the same row to connect them. For example, you can place one lead from a 100 ohm

resistor and one lead from a 330 ohm resistor in the same row in order to connect them. If their other leads connect with different rows, the resistors will be connected in series. If their other leads connect in the same row, the resistors are connected in parallel.

Measuring DC current through a parallel circuit

This is not as complicated as it may sound. After setting up your two resistors in parallel simply connect the red lead from the DC power supply to one lead of the 330 ohm resistor and connect the red lead from the DMM to the other lead of the 330 ohm resistor (your DMM and DC power supply should already be configured for DC current measurement and the two black leads from both of them should be connected unless you changed something during the lab). The reading on your DMM is the current through the two parallel resistors.

Something you should know is that you can take the red leads of the DMM and DC power supply off the 330 ohm resistor and connect them to opposite leads of the 100 ohm resistor instead. You will get the exact same measurement as when the red leads were connected to the 330 ohm resistor. If you added a third resistor in parallel and measured off of both of its sides, you would also get the same reading as if you measured from the 330 ohm or 100 ohm. This is because the DMM is measuring the resistance of the parallel circuit in all cases. In fact, as long as one red lead is on one side and the other is on the other side (of the resistors that is) you can measure between any pair of the resistors’ leads and get the same thing. Left side 330 ohm with Right side 100 ohm, left 33 ohm with right 330 ohm, and any other combination will all yield the same result. Think of it like water flowing through a pipe. In series, if you add a thinner pipe to the chain, water can’t flow through faster than what the thinnest pipe will allow. If you are adding pipes in parallel, however, each added pipe increases the amount of water that can flow through and water will flow through all 3 regardless of where the inputs and outputs are.

Based on this measurement and the measurement from part 2, was this measurement a higher or lower current? Is that what you would have expected? When you’re done measuring and recording the current through these two parallel resistors, move on to the next part.

Calculating expected DC current through a parallel circuit

To calculate the current through a DC circuit, you will need the following equations:

I = V/R which is ohm’s law and

Rt = 1/[(1/R1) + (1/R2) + (1/R3) + …] which is the equation for finding the total resistance in a parallel circuit.

All you have to do is find the total resistance and use that as the value for R in ohm’s law along with the voltage your power supply is set to (which should still be +5V at this point) and you get the current through the circuit.

Lab Report

For this part of the lab report, include how you set up a parallel circuit with the two resistors (can use a picture for this). Also specify how you measured the current through the parallel circuit with the leads of the DMM and Power Supply (can use a picture for this as well). You don’t need to describe how you set up the DMM and DC Power supply for a current measurement since you did this for part 2, but you do need to refer to that setup.

You need to include the measurement for current through the two parallel resistors and it’s units.

You need to state whether the current measurement for the two resistors in parallel went up or down from the current through the single resistor in part 2.

You need to include the equations you used to find the expected current through the parallel circuit. You used:

I = V/R ohm’s law and

Rt = 1/[(1/R1) + (1/R2) + (1/R3) + ] total resistance in a parallel circuit

So you need to include both of these as well as the number that you plugged into their variables. You need to indicate which numbers went where either by showing a calculation or labeling the values you used in the equation according to their parts. Example: