Phy 3311 Electronics Lab #2: DC Circuits

bahmotherElectronics - Devices

Oct 7, 2013 (3 years and 10 months ago)

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Phy 3311 Electronics Lab #2:

DC Circuits

For this experiment you

ll need
:


two multimeters (a Fluke and an Extech),

a breadboard,

some hookup wires

(which can be found in plastic cases on the electronics cart)

multimeter leads
,

BK Precision DC power supply
,

and three unique resistors with resistances between 1
-
10 k

.


This is a bit of a long lab; th
e idea is to get very comfortable

setting up circuits on your breadboard, and
taking measurements with your multimeter, of both voltage (in which you hook the meter
across

the
component
to be measured) and current (in which you must

break in


to the circuit being measured).


P
art 1: Know thy Multimeter


Connect one multimeter to another, with one operating in voltmeter mode, and the other operating in
ohmmeter mode.


What is the input resistance of the voltmeter? How does
that compare

to what you expect
?


What is the output voltage of the ohmmeter? How does that compare

to what you expect
?


Now, switch the settings so the first meter is in ohmmeter mode, and the second in voltmeter mode.
Are the results any different?


Meas
ure the input resistance of your meters

when operating in ammeter mode. How does that
compare

to what you expect
?



For the Extech meter, measure the in
put resistance in the
“A,” “mA” and “

A” modes, keeping in mind
that you have to use the “left”
terminal in “A” mode, but the “right” terminal in the “mA” and “

A”
modes. What do you find?


I would ask you to measure the resistance of an ohmmeter, but you cannot do this. Why? (Think about
what’s inside an ohmmeter, and what kinds of things you a
re allowed to measure with an ohmmeter.)


Finally, measure your own resistance, by taking one lead of the ohmmeter in each hand. How much
resistance does a human being have? Try holding both leads in one hand


does this change the value?
How about if y
our hands are wet?




Part 2
: Voltage Divider & Series combination of resistors


For this part of the experiment you will use a breadboard,
three
uniquely
-
sized
r
esistors (R between 1
-
10

k

), the BK
Precision power supply, two multimeters
, and lots
of wires
.


Remember that the Extech meters (in mA setting) are better
for measuring small amounts of current!


Set up the
circuit
at right
using all three of your resistors
.
According to what we studied in class, the “equivalent
resistance”

R of
three
series
resistors is given as


R = R
1

+ R
2

+ R
3



and the total current drawn is



I = V/R = V

/

(R
1
+R
2
+R
3
)


Note
: y
ou

should

use the measured
(ohmmeter)
values of R
1
, R
2

and R
3
, rather than the nomin
al values

from the color bands
!

Even for gold
-
banded resistors, the difference can be up to 5%!


Apply a vol
tage V = 10V
to the circuit. (U
se your second mu
ltimeter to determine

the terminal voltage).


Use the ammeter to measure the current I. Does it match with the value you calculated?


Next, measure
and record
the voltages V
1
, V
2

and V
3

across each of the three resist
ors

(how should you
connect the voltmeter to do this?)
. According to Ohm’s Law, the voltages should be given by:



V
1

= I R
1


V
2

= I R
2


V
3

= I R
3


Do these values match with what you measure?


A
s a check of your results, see whether the three vo
ltages sum
up to the total applied voltage:

does V
1

+
V
2

+ V
3

= V?


Finally, compute the power dissipated by each resistor

(e.g. P
1

= V
1
I)
. Which resistor dissipates the most
power? Does that make sense to you?





Part 3
:
Current Divider &
Parallel
combination of resistors


Set up the
circuit

below
, using the same components from the previous section
.


According to what we learned in class, the “equivalent resist
ance” R

is
1/R = 1/R
1

+ 1/R
2

+ 1/R
3
.
Compute R
, and n
ote
that R is <

the s
mallest of t
he three resistances

(this will always be true)!


Apply
about 10 V to the circuit
, and record the accurate value by connecting the voltmeter across the
supply’s terminals
.
We expect that all three resistors will

have the same voltage drop V;
verify
this
by
measuring the voltage drop across each resistor
.



C
onnect an ammeter in series with R
1
:

(
T
hink carefully about how to do this!)



Measure
and record
the current I
1

going through the R
1

branch. Does I
1

= V / R
1
, as you would expect?

Do the same for
the
R
2

and R
3

branches, to find I
2

and I
3
.


N
ext
, hook the ammeter in series with the battery, as in the following circuit:






Y
ou are now measuring the total current I being drawn from the power supply. Does I = V / R, as you
would expect? Does I = I
1

+ I
2

+ I
3
, as you would expect?


Finally, compute the power dissipated by each resistor

in this circuit (e.g. P
1

= I
1
V)
. Which of the three
resistors dissipates the most power? Does this make sense to you? (And how does this result compare
with what y
ou found in the series circuit?)