Getting to Know Your Multimeter

 

Materials:

Multimeter with 2 probes (one black, the other red)

200 in 1 Electronic Project Lab circuit board

Genco Hand Crank DC Generator

Battery(s) (at least 3 AA’s and  one 9Volt)

 

Key Formulas  and Ideas to be aware of:

 

V = IR

V = Voltage measured in Volts

I = Current (amount of charge (Q) passing a point in time (t)).  Measured in Amps = Q/t

R = Resistance measured in Ohms (symbol: W)

 

If you hook up a battery to a system, the battery acts as a pump and continuously pumps the voltage rated on the side of the battery.  Eg.  A 12 V battery will always produce 12 Volts

 

A resistor will continue to resist the current at the same rate as well.  Except, as it gets hotter, many resistors will INCREASE their resistance.  But, if we say that something obeys "Ohm’s Law”, we are saying that the resistor is maintaining a constant value.

 

Warning:  Never ever hook up a multimeter which has its dial set to the resistance setting (W) and a LIVE circuit (one with a battery hooked up to it).  WHY?  Because this setting has your multimeter sending out a current from the 9.0 Volt battery from inside the multimeter to test the resistance of a resistor.  You will wreck the multimeter if you also send a current through the circuit from another source (such as another battery outside of the multimeter).  This setting is to test a resistor which has no current flowing through it.

 

Pre-Lab Questions:  Type answers in space provided in RED type.

 

Look at the equation V = IR. 

 

1.  What happens to I if V is increased and R stays the same?  ____________

2.  What happens to I if R is increased and V stays the same? ____________

3.  What happens to V if R is increased and I stays the same? ____________

4.  What happens to V if I is increased and R stays the same? ____________

5.  What happens to R if V is increased but I stays the same? ____________

6.  What happens to R if  I is increased but V stays the same? ____________

 

7.  What is the slope of a V vs. R Graph equal to? ____________

8.  What is the slope of a V vs. I Graph equal to? ____________

 

9.  What should you never ever do with your multimeter (see above warning)

 

Instructions:  Fill in the blanks with numerical values.  Be sure to include Positive and Negative signs. Type answers in space provided in RED type.

 

Voltage:

-Turn Multimeter Dial to 20V

-Connect Black probe to COM (COM stands for “common” or ground)

-Connect Red probe to W/V

 

1.  Touch the black probe to the – side of the battery and the red probe to the + side of the battery.  What does the multimeter read?  (include positive or negative)

_______________________V

 

2.  Switch the probes so the red is touching negative, and the black is touching positive.

What does the multimeter read?  (include positive or negative)

_______________________V

 

3.  Hold 2 or 3 batteries in SERIES (or if you have a battery “pack” where you can put two or more batteries in there, use that). 

How many batteries (and what type, AA, D, C?…etc) did you put in series _______

What did the multimeter read?  __________

Add up the voltages on the sides of the batteries and put that value here _________

 

4.  Predict the voltage of 4 AA batteries in series (each AA has 1.5V)  ___________

 

Voltage Part II:

 

-Hook up the multimeter probes to the hand-crank Genco Generator.  It doesn’t matter which probe gets which alligator clip from the Genco Generator. 

-Set the multimeter for 20 volts

 

1.  Crank the generator clockwise.  Are the voltage readings you are getting positive or negative?  ________________

 

2.  Crank the generator counter-clockwise.  Are the voltage readings you are getting positive or negative?  ___________________________

 

3.  Switch the alligator clips so that the one on the red goes to the black (and vice-versa).  Now crank the generator clockwise.  Are the voltage readings positive or negative?  ________________

 

4.  Based on your observations from the previous 3 questions, would you say that the generator provides AC or DC?  ________________________

 

5.  How do you change the direction of current flow with the hand-crank generator?  ________________

 

Resistance (ohms or W):

 

-Keep red and black probes connected to multimeter as you did for Voltage

-Turn multimeter dial to 20MW

-Make a dark line of PENCIL on a sheet of paper.  Thick dark line, about 2 cm wide and maybe 5-6 cm long.

 

1.  Hold one probe on one extreme of the pencil mark and the other probe on the other extreme.  What does the multimeter read?  ________________

 

2.  Bring the probes closer together along the pencil line.  What does it read?  ________

 

3.  Bring them closer still  ________________

 

4.  Touch the probes together, what does it read? ______________

 

WHY?  Because:  The multimeter sends out a voltage (9V because it is a 9V battery in its battery pack).  The carbon in the pencil “lead” resists the flow of electrons being pushed by the 9V battery.  The greater the distance between your probes, the greater the amount of carbon the electrons can be resisted.  Touch the two probes together and there is no resistance – the multimeter should register ZERO!

     NEVER EVER HAVE A RESISTOR POWERED BY A BATTERY AT THE SAME TIME YOU ARE USING A MULTIMETER TO TEST IT.  WHY?  BECAUSE THE MULTIMETER IS GIVING IT SOME VOLTAGE AND SO WOULD YOUR BATTERY.  NOT ONLY WOULD THIS GIVE YOU A FALSE READING ON YOUR MULTIMETER, BUT IT MIGHT ALSO CAUSE THE RESISTOR TO BURN OUT IF IT GETS TOO HOT

 

Resistance Part II:

 

-Set the multimeter to 200W

-Get out the 200 in 1 Project Kit

-Touch the probes to the springs across a 100W resistor (Spring #1 and Spring #2)

 

1.  What does the multimeter read (include + or – sign)  _____________

 

2.  Switch the black and the red probes so that they are now touching the other spring.  What does the multimeter read (include + or – sign)  _____________

 

3.  Use V = IR and the fact that the multimeter has a 9V battery in it to help you figure out what current (I) must be flowing through that resistor ____________

 

4.  Set the multimeter to 2K and touch the springs across a 1KW resistor (springs #13 and #14)   What does it read?___________________

 

Resistance Part III:

 

-Set your multimeter dial to 2KW

-Wire your 200 in 1 kit as follows:  Wire spring #1 to spring #3.  In future, I will write a wiring instruction like this as:  1-3.  That means put a wire in #1 and connect it to #3.

 

1.  Put the red probe into #2 and the black probe into #4.  What does the multimeter read? _____________

 

2.  Did you wire these two resistors in SERIES or PARALLEL? ___________

 

3.  Use the equation:  Rseries = R1 + R2….etc to determine the total resistance __________ I hope this matches your answer to #1.

 

-Set your multimeter dial to 200W

-Wire your 200 in 1 kit as follows:  1-3 and 2-4

 

1.  Put the red probe into #1 and the black probe into #2.  What does the multimeter read? _____________

 

2.  Did you wire these two resistors in SERIES or PARALLEL? ___________

 

3.  Use the equation Rparallel = 1/(1/R1 + 1/R2) to determine total Resistance ___________   I hope this matches your answer to #1.

 

4.  ON A SEPARATE SHEET OF PAPER – NOT HERE ON THE COMPUTER:  Draw a schematic diagram of your resistor set-up.

 

5.  Put Red probe in #3 and the Black probe in #4.  What does it read?  _________

 

6.  Try Red in #3 and Black in #2.  What does it read?   ________

 

7.  Try Red in #1 and Black in #4  ________________

 

8.  Try Red in #2 and Black in #4  _______________

 

9.  Try Red in #1 and Black in #3  _______________

 

10.  Why do you get a lower reading in Question #8 and #9? ___________________________________________________________________

 

11.  Hold Red in #3 and Black in #1.  Turn the dial to 2KW.  Pull out the wire which connects 1-3.  Does the multimeter now read the same as when these two resistors were in SERIES?  Yes or no  _______________

 

12.  Why is it not in SERIES with the wire still connected to 1-3?  Refer to the drawing you made on your paper to help you explain this.

 

 

Current (amps or Charge/time or Q/t):

 

-Plug the red probe into the 10A part of the multimeter

-Keep the black probe in the COM

-Set the dial on the multimeter to 10A

-Hook up your 200 in 1 kit as follows:  Use a wire to connect the following numbers

 60-1

 58-into the air (don’t hook 58 up to anything; just leave its wire hanging out of the box)

 

1.  Hold the red probe tip into #2 on the 200 in 1 kit board.  Hold the black probe tip to the wire leading out of #58.  What does the multimeter read?  _____________

 

2.  Now connect 58-1 and leave 60 hanging out of the box.  Again, hold Red in #2 and hold black on the wire hanging from #60.  What does the multimeter read?  ________

 

3.  Use V = IR to confirm what you just saw:  The batteries you hooked up had a total voltage ot 4.5V and the resistor is 100W.  Your Current should be 4.5V/100W = 0.045 amps.  Is that what you got for questions 1 and 2?  ____________

 

Why?  Because:  Current flows through a circuit and in order for the multimeter to read the amount of current, it must be in SERIES with the circuit.  Can you see where the circuit is broken so that you could insert the red and the black probes?

    

-Plug the red probe into the W/V part of the multimeter again.

-Hook up the 200 in 1 kit as follows:  60-1 and 58-2

-Turn the dial of the multimeter to 20V

 

1.  Hold the red probe in #1 and the black probe in #2.  What does the multimeter read?  ____________VOLTS

 

2.  In this arrangement, are your probes in SERIES or PARALLEL with the circuit?  _________

 

3.  Fill in the blanks with SERIES or PARALLEL:  Ammeters must be in ____________ with the circuit, while Voltmeters must be in ______________

 

Current part II:

 

-Hook up your 200 in 1 kit as follows:  1-3, 60-4 and 58 – air.

-Make sure your multimeter is set to 10A on the dial and the red probe is back in the 10A hole on the face of the multimeter.

 

1.  What is the total resistance of this series?  You can look at your answer from Resistors III if you want to or you can re-do the math (Rseries = R1 + R2) ______________

 

2.  Use V = IR to predict the I for this one.  The V = 4.5 volts – can you see why?  What is the Current you should read on the multimeter? _________________

 

3.  Hook the red probe into #2 and the black probe to the bare wire coming out of 58.  What does it read?  ______________(was your prediction correct?)

 

4.  Now wire it up like you did for the PARALLEL resistors in Resistance part III.    What was the total resistance of the PARALLEL resistors?  You can look at your answer from resistors part III if you want to  _________________

 

5. Use V = IR to predict the I for this one.  The V = 4.5 volts – can you see why?  What is the Current you should read on the multimeter? _________________

 

6. Hook the red probe into #1 and the black probe to the bare wire coming out of 58.  What does it read?  ______________(was your prediction correct?)

 

7.  Hook Red probe into #2.  What does it read?_________________

 

8.  Hook Red probe into #3.  What does it read?  _________

 

9.  Hook Red probe into #4.  What does it read?  ____________

 

10.  Why is this happening?  ON A SEPARATE PAPER, DRAW A SCHEMATIC OF THIS AND EXPLAIN IT HERE  _________________________________________________________________

 

 

Current part III:

-Plug the red probe into the 10A part of the multimeter

-Keep the black probe in the COM

-Set the dial on the multimeter to 10A

-Have 3 batteries at the ready.

 

1.  Hook up the red probe to the positive terminal of one battery and the black to the negative terminal.  What does it read?______________ (call this battery #1)

 

2.  Do this for the second battery (by itself).  What does it read?  ____________ (call this battery #2)

 

3.  Do this for the third battery (by itself).  What does it read?  _________ (call this battery #3)

 

4.  Now put battery #1 and battery #2 in series and test the current from the positive terminal of the top battery to the negative terminal of the second battery.  What does it read?___________amps

 

5.  Now put battery #2 in series with battery #3  _________amps

 

6.  Now put battery #1 in series with battery #3  ________amps

 

7.  Now put battery #1, #2 and #3 in series _______amps.

 

Essentially, what you are doing when you do this is you are creating a “dead short” in the battery so that the battery is forced to give you all of its possible current outflow at once.  Remember that if you have a low resistance, current is high.  All you have in the way between the positive and negative terminal of the battery is a wire (very low resistance). 

     With this information you can determine the minimum resistance you could hook up to these batteries and get the current that resistor needs (V/I = Rminimum)

 

8.  What is the lowest resistor you could hook up to battery #1?__________ohms.

9.  What is the lowest resistor you could hook up to battery #2?__________ohms.

10.  What is the lowest resistor you could hook up to battery #3?__________ohms.

 

What generalization can be made about hooking up batteries in series which have different maximum current outputs?  Look at your answers to 1-7.  Is the current output from a series of #1, #2 and #3 the sum of their individual currents in a dead short?  Is it the average?  Is it the lowest one?

 

____________________________________________

Show and Tell Time

 

Now I want you to create some circuits and show me that you know what is going on.  Make your numerical predictions first, then set the whole thing up, then call me over to show me that you are a genius!

 

1.  Create a circuit with at least two resistors in series and put those two in parallel with another two resistors in series (in other words, 4 resistors in all.  2 in series with each other, and another 2 in series with each other, but the two sets are PARALLEL to each other.

 

Answer the following:

a.  Predict the Voltage across each of the 4 resistors

Resistor 1 ______________

Resistor 2  _____________

Resistor 3  _____________

Resistor 4 ______________

 

b.  Test the Voltage across each of the 4 resistors and show Mr. Young that it matches your predictions

 

c.  Predict the Current (I) flowing through each of the 4 resistors and the total resistance.

Resistor 1 ______________

Resistor 2  _____________

Resistor 3  _____________

Resistor 4 ______________

Resistancetotal____________________

 

d.  Test the Current across each of the 4 resistors and show Mr. Young that it matches your predictions

 

e.  ON A SEPARATE PAPER, DRAW A SCHEMATIC OF YOUR SET-UP.

 

 

 

What is due for this lab?

 

1.  Answer everything you see from the PRE-LAB on down to the bottom of this lab wherever you see a blank line.  Be sure your answers are in RED.  Turn in this whole document as a file with your initials followed by the word “multimeter”. (gymultimeter.doc)  Be sure to put the names of all of your group members at the top of the lab!

 

2.  On a separate sheet of paper, with each one labeled as to which one it is, draw the schematics for:  (or, if you want to impress me – scan them into your document!!!!!)

·        Resistance III #4

·        Current II #10

·        Show and Tell Time #1e

You can write any explanations for these schematics on your electronic version you turn in (you know, in RED)