ELEC 241 Lab

Experiment 1.1

DC Measurements: the DMM

Equipment

Battery Pack and Batteries
Lightbulb Socket Board
Digital Multimeter
DC Power Supply
Banana Plug Patch Cords

Part 1: Measuring Voltage with the DMM

Step 1:

Turn on the DMM and select the DC voltage measurement function by rotating the selector switch to the DC Volts position (a "V" with solid and dashed straight bars above it). All of the display segments will be activated and after a few seconds, if all is well, it will beep.

Step 2:

Make sure the negative (black) lead is plugged into the lower right ("COM") terminal and the positive (red) lead into the terminal directly above it.

Step 3:

Measure the voltage of each battery by holding the positive probe against the top of the battery and the negative probe against the bottom.

Note In measuring voltage, the meter is always connected across the two nodes whose voltage difference is being measured.

Step 4:

Place the two batteries into the holder in the orientation indicated. Measure the voltage of the battery pack. It should be equal to the sum of the two batteries. Is it?

Caution

Be careful not to short the two leads of the battery pack together once the batteries are installed. To be safe, remove at least one of the batteries when the pack is not in use.

Step 5:

Wire the following circuit by screwing the leads from the battery pack to the center and right hand terminals of the lamp board. $\includegraphics[scale=0.650000]{flashlight1.ps}$

The bulb should light (though rather dimly). Measure the battery voltage again. Is it the same as before?

Part 2: Measuring Current with the DMM

To measure current, we must connect the meter in series with the circuit we're measuring, as in the following figure:

$\includegraphics[scale=0.650000]{flashlight4.ps}$

This is because current flows through a conductor, whereas voltage appears across pairs of conductors.

Step 1:

With the meter disconnected from the circuit, set the function switch to DC current ("A" with straight solid and dashed lines above it). Move the red meter lead to the 300 mA terminal.

Step 2:

Disconnect the battery lead from the center terminal of the lamp board and connect it to the left hand terminal.

Step 3:

Connect the DMM between the left and center terminals. The bulb should light and the value of the current flowing should be displayed on the DMM. Be sure to write it down.

Warning An ammeter must always be connected in series. NEVER connect an ammeter in parallel with a circuit element. This mistake can have the effect of passing very large currents through the ammeter, blowing its internal fuse or damaging it. It's a good idea always to reset the switch on the DMM to "Volts" and return the positive lead to the Volts/Ohms terminal as soon as you have finished making a current measurement.

Part 3: Measuring Resistance with the DMM

Step 1:

Set the function switch on the DMM to Ohms ( $\Omega$ ) return the positive meter lead to the Volts/Ohms terminal. Touch the two probes together. The meter should read zero resistance. If it reads more than a few tenths of an ohm, check for poor connections or have your meter serviced.

Step 2:

Select several resistors at random from your parts kit. For each resistor, determine its nominal value from the color code, then measure its resistance by touching one probe to each lead of the resistor. Do the nominal and measured values agree?

The most accurate way to do this (especially for large value resistors) is to lay the resistor on the bench and pinch the leads between the benchtop and the probe tip. Like this:

What's wrong with holding the leads and probes between your fingers?

Diversion:

Components that we will be using are marked so as to show the tolerance or accuracy of their manufacturing process. A resistor marked red-red-red-gold, for example, is a 2.2 k $\Omega$ resistor with a 5% tolerance. This means that the actual value of this resistor is ``guaranteed'' to be between 2090 and 2310 ohms. This guarantee is enforced by quality control procedures at the factory. Often, the resistance is much closer than that specified.

Question 1:

Formally, the actual resistance

of a resistor having nominal value

and tolerance $\delta$ lies in the range $R_0(1 \pm \delta)$ . Assuming common nominal and tolerance values, what is the tolerance of a series connection of two such resistors? of a parallel connection?

Step 3:

Obtain ten resistors with the same marked value (your parts kit should have 10 1k $\Omega$ resistors). Measure the resistance of each resistor. Note each resistor's value and compare it to its nominal value. Within the accuracy of the ohmmeter, does your "batch" have the stated accuracy?

Question 2:

Calculate the average resistance ( $\bar{R} = \frac{1}{N}\sum_n R_n$ ). What is the greatest measured excursion from the mean? Does it lie within the specified tolerance?

Question 3:

How do we know which is more nearly correct: the DMM or the labels on the resistors?

Step 4:

Holding the ohmmeter's leads, one in each hand, measure your own resistance. How stable is the ohmmeter's reading? If the reading varies somewhat, the resolution of your measurement is limited to the digit that changes least often; what is the resolution of your resistance?

Step 5:

Does your resistance change when you wet your fingers? If so, speculate why. Calculate what voltage would be necessary to produce a 5 mA current through you. (Why 5 mA? See Lab 0.)

Step 6:

Using the DMM, measure the resistance of the light bulb. Does this correspond too the value you would expect from Ohm's Law given the values of voltage and current you measured in Parts 1 and 2?

Caution

The DMM can only measure the resistance when the element being measured is disconnected from the circuit. Attempting to measure a resistor which is part of a circuit will give erroneous results.

Part 4: Measuring the I-V Characteristics of the Lightbulb

An ideal resistor obeys Ohm's law: I=V/R, i.e. the current through the element is proportional to the voltage across it. But for most real materials, the resistance changes as the temperature changes, and clearly, the temperature of the light bulb's filament increases as more current flows through it. Let's find out how the current and voltage of our light bulb are related.

For this measurement, we will need to vary the voltage applied to the bulb, so we will need a variable voltage source. This is provided by the DC Power Supply The DC power supply actually contains three variable voltage sources, but we will be using only one of them, the 0-6V supply.

Step 1:
Set up power supply: Remove all connections from the output terminals, turn both voltage controls to zero (fully counterclockwise), set the meter selector to the 6V supply.

Step 2:
Set the DMM to the DC Volts function. Connect the black (-) probe to the black 0-6V output terminal and the red (+) probe to the red terminal (you may need your lab partner for this).

Step 3:
Turn on the power supply. Slowly increase the output voltage by turning the 0-6V voltage control clockwise. Both the Voltage meter on the power supply and the DMM should show increasing voltage values. For several different values, note both the power supply meter and the DMM reading. How do the two compare?

Step 4:
Return the voltage output to zero.

Note:
For the rest of this part you will need to borrow a second DMM from another lab group. If you like, you can pool resources and both groups make these measurements together.

Step 5:
Wire the circuit below. The easiest way to do this without running out of hands is to use banana plug patch cords (they fit in the DMM if you unplug the probes). $\includegraphics[scale=0.650000]{flashlight3.ps}$

Step 6:
Measure the current for voltages between 0 V and 1 V, in steps of about 0.2 V. and between 1 V and 5 V in steps of about 0.5 V. It is not necessary to have V exactly equal to 1.000, 1.500, etc. Just get it close and write down the numbers accurately.

Step 7:
Plot I as a function of V. How?

Question 4:
To what point on this curve does the value of resistance you measured with the ohmmeter correspond?

Step 8:
Repeat the previous two steps using a 1000 ohm (brown-black-red) resistor in place of the light bulb. (Use the center and left hand binding posts to hold the resistor as shown in the figure.) Measure and plot I vs. V for a 1000 ohm resistor. Is our assumption that I=V/R for all V a valid one?

Step 9:
When finished, turn off the DMM to prevent running down its battery. Return the borrowed DMM.


Step 1:		Set up power supply: Remove all connections from the output terminals, turn both voltage controls to zero (fully counterclockwise), set the meter selector to the 6V supply.
Step 2:		Set the DMM to the DC Volts function. Connect the black (-) probe to the black 0-6V output terminal and the red (+) probe to the red terminal (you may need your lab partner for this).
Step 3:		Turn on the power supply. Slowly increase the output voltage by turning the 0-6V voltage control clockwise. Both the Voltage meter on the power supply and the DMM should show increasing voltage values. For several different values, note both the power supply meter and the DMM reading. How do the two compare?
Step 4:		Return the voltage output to zero.
Note:		For the rest of this part you will need to borrow a second DMM from another lab group. If you like, you can pool resources and both groups make these measurements together.
Step 5:		Wire the circuit below. The easiest way to do this without running out of hands is to use banana plug patch cords (they fit in the DMM if you unplug the probes). $\includegraphics[scale=0.650000]{flashlight3.ps}$
Step 6:		Measure the current for voltages between 0 V and 1 V, in steps of about 0.2 V. and between 1 V and 5 V in steps of about 0.5 V. It is not necessary to have V exactly equal to 1.000, 1.500, etc. Just get it close and write down the numbers accurately.
Step 7:		Plot I as a function of V. How?
Question 4:		To what point on this curve does the value of resistance you measured with the ohmmeter correspond?
Step 8:		Repeat the previous two steps using a 1000 ohm (brown-black-red) resistor in place of the light bulb. (Use the center and left hand binding posts to hold the resistor as shown in the figure.) Measure and plot I vs. V for a 1000 ohm resistor. Is our assumption that I=V/R for all V a valid one?
Step 9:		When finished, turn off the DMM to prevent running down its battery. Return the borrowed DMM.