The multimeter, oscilloscope and function generator 1.0 Objective:
After performing this experiment, students will able to: 1. Use function generator, DC supply and digital multimeter. 2. xplain the four ma!or function of controls on the oscilloscope. ". Use an oscilloscope to measure ac and dc #oltages
!"#$%&'#T( )!*+* +!!& *+*('#* -&*"% #"'T#T!T%
2.0 Theory:
$here are four ma!or function of control on the oscilloscope %Display control, #ertical control, triggering control and hori&ontal control'.
$he display control includes ()$)*($+,
-CU*, and A/ ()D0. $he #ertical controls include input C-U()3, 4-$5D(4, 4ertical -*($(-) and channel selection %C61, C62, DUA, A$, and C6-'. $he
"/3103 easurement and #nstrumentation 'ystem triggering controls include /-D, *-U0C, trigger C-U()3, trigger 4 and others. $he hori&ontal controls include the *C5D(4, /A3)((0 and hori&ontal -*($(-) controls. 3.0 rocedure:
ab 0eport 1
1. 0e#iew the front panel controls in each of the ma!or groups. $hen turn on the
/ultimeter, unction 3enerator oscilloscope, select C61,-scilloscope set the *C5D(4 toand 7.1 ms5di#, select AU$triggering, and AC obtain a line across the%DC face ofand the C0$.
/easurement'
2. $urn on power supply and use the D// to set the output for 1.74. Use multimeter to measure this dc #oltage from the power supply. ollow the following step: a. lace the #ertical C-U()3 %AC83)D8DC' in the 3)D position. Disconnect the input to the oscilloscope. Use the #ertical -*($(-) control to set the ground reference le#el on a con#enient graticule line near the bottom of the screen. b. *et the C61 for 4-$5D(4 control to 7.245di#. c. lace the oscilloscope on the positi#e side of the power supply. lace the
)ame : *hahrin insupply ;amaru&aman oscilloscope ground on the power common. /o#e the #ertical coupling to (D the DC position.:$he 9721<11=721 line should !ump up on the screen by 9 di#isions. Note that 5 3roup )1=> division times :0.2V per division is equal to 1.74 the (supply voltage). /ultiplication of the number of di#isions of deflection time #olts per di#ision is eual to the #oltage measurement.
". *et the power supply to each #oltage listed in table 1. /easure each #oltage using the abo#e steps as a guide. $he first line of table has been completed as an example. $o obtain accurate readings with the oscilloscope, it is necessary to select the 4-$5D(4 that gi#es se#eral di#isions of change between the ground reference and the #oltage to be measured. $he reading on the oscilloscope and meter should agree with each other within approximately "?. =. efore #iewing ac signals, it is a good idea to chec@ the probe compensation for your oscilloscope. tlo chec@ compensation, set the 4-$5D(4 control to 7.145D(4, the AC83)D8DC coupling control to DC, and the *C5D(4 control 2ms5di#. $ouch the probe tip to the 0- C-/ connecter. +ou should obser#e a suare wa#e with a suare wa#e with a flat top and suare corners. (f necessary, ad!ust the compensation to achie#e a good suare wa#e. 9. *et the function generator for an AC wa#eform with a freuency of 1.7@6&. ad!ust the amplitude of the function generator for 1.74 rms as a read on your D//. *et the *C5D(4 control to 7.2ms5di# and the 4-$*5D(4 to 7.945di#. Connect the scope probe and its ground to the function generator. Ad!ust the #ertical -*($(-) control and trigger 4 control for a stable display near the center of the screen. +ou should obser#e approximately two cycles of an AC wa#eform with pea@8to8pea@ amplitude of 2.<4. this represents 1.74rms as shown in figure 1.1. . Use the D// to set the function generator amplitude to each #alue listed in table 1.2. 0epeat the ac #oltage measurement as outlined in step =. $he first line of the table has been completed as an example.
.0 ata and &esults:
oer 'upply
$O+T#$
"umber of
Oscilloscope
'etting
setting
divisions of
4measured
4easured
7.2 #olt5di# 7.9 #olt5di# 2.7 #olt5di# 2.7 #olt5di#
deflection4div5 9.7 di# =.< di# 2.2 di# =.7 di#
voltage5 1.7 4 2.= 4 =.= 4 <.7 4
voltage5 1.7 4 2.=< 4 =.9< 4 <."" 4
1.0$ 2.6$ .6$ 7.3$
Table 1.1
oer
$O+T#$
"umber of
Oscilloscope
Oscilloscope
generator
setting
divisions of
measured
measured
7.9 #olt5di# 1.7 #olt5di# 2.7 #olt5di# 2.7 #olt5di#
deflection4div5 9. di# B.7 di# .2 di# <.7 di#
4pea89to9pea85 2.<4pp B.74pp 12.=4pp 1.74pp
4rms5 1.74rms 2.=B4rms =."<4rms 9.B4rms
amplitude 1.0$ 2.6$ .6$ .0$
Table 1.2
6.0 ata #nterpretation:
3raph /easured DC 4oltage %4' o#er ower *upply %4' for D// and -scilloscope
9 8 7 6
Measured DC Voltage (V)
5
DMM
4
Linear (DMM) Oscilloscope
3
Linear (Oscilloscope) power supply
2 1 0 0 1 2 3 4 5 6 7 8 9 Power Supply (V)
3raph /easured AC 4oltage %4' o#er ower *upply %4' for -scilloscope 7 6 5 4 Measured AC Voltage (V)
Oscilloscope (RM) 3
Linear (Oscilloscope (RM))
2
!ower supply
1 0 0 1 2 3 4 5 6 7 Power Supply (V)
rom the results obtained, it showed that the DC #oltage measured by digital multimeter is #ery closely to the actual #alues of power supply. /eanwhile, the #oltage measured by oscilloscope is slightly different to the actual #alues of the power supply. $his
data pro#es that the digital multimeter has better accuracy compared to the oscilloscope. $he first graph showed the readings of oscilloscope lower than power supply. Although, oscilloscope is lac@ compared to digital multimeter because of D// are more practical ways to measure close to the input #oltage. $he second graph showed AC #oltage measured by oscilloscope has difference compared to the actual #oltage. $he line graph shows that the higher the #alues of power supply the higher the error measured by the oscilloscope. $hese obser#ations pro#e that the oscilloscope has some lac@ in accuracy in measuring AC #oltage. $his is could be due static error such improper way ta@ing reading or shortcomings of oscilloscope.
.0 onclusion:
y completing this experiment, the students now @now how to use basic instrument which power supply, function generator and oscilloscope. rom the analysed data, using digital multimeter for this experiment has more suitable with #ery good accuracy in measuring the DC #oltage. /eanwhile, the used oscilloscope in this experiment has a lac@ of accuracy in both measuring DC and AC #oltage. (ts measurement accuracy of AC #oltage is much lac@ compared to its measurement accuracy of DC #oltage.
%valuation and &evie ;uestions:
1.
%a'
Compute the percentage different between the D// measurement and
the oscilloscope measurement for each dc #oltage measurement summari&ed in table 1.1.
Calculation: oer
Oscilloscope
*ccuracy 4<5
ercentage
'upply
4measured
4easured
'etting
voltage5
voltage5
Oscilloscope
4<5
1.0$
1.7 4
1.7 4
7.77?
7.77?
7.77?
2.6$
2.= 4
2.=< 4
>.77?
>>.27?
".27?
.6$
=.= 4
=.9< 4
>B.B
171.BB?
".>>?
7.3$
<.7 4
<."" 4
>.">?
177."?
".>B?
ifferent
Table 1.3
%b' hich you thin@ is most accurate xplain why. rom the calculation in $able 1.", the percentage shows that Digital /ultimeter %D//' measurement are more accurate than -scilloscope which compared by the calculated accuracy %?'. $he difference between an oscilloscope and a digital multimeter %D//' #alue is simply stated as Epictures #s. numbers.F $he D// is a de#ice that gi#es a single scalar reading which means used for high8precision chec@s of #oltage, current, resistance and other electrical parameters. $he display will be a " or four digit number. -h, yes, it refreshes se#eral times a second. $he -scilloscope is a #oltage #s. time instrument and can ma@e both uantitati#e and ualitati#e measurements which capable of recording #oltage wa#eforms, and at #ery uic@ speeds.
2. riefly describe the four mains functional group of controls on oscilloscope and the purpose of each group.
Display control
•
An intensity control to ad!ust the brightness of the wa#eform. As you increase the sweep speed of an analog oscilloscope, you need to increase the intensity le#el.
•
A focus control to ad!ust the sharpness of the wa#eform. Digital oscilloscopes may not ha#e a focus control.
•
A trace rotation control to align the wa#eform trace with the screenGs hori&ontal axis. $he position of your oscilloscope in the earthGs magnetic field affects wa#eform alignment. Digital oscilloscopes may not ha#e a trace rotation control.
•
-ther display controls may let you ad!ust the intensity of the graticule lights and turn on or off any on8screen information %such as menus'.
4ertical control •
Used to #ertically scale and position the wa#eform, the #ertical controls can also be used to set the input
•
coupling, as well as to ad!ust other signal conditioning $he #ertical position control also enables the user to
•
mo#e the wa#eform up and down the screen. $he #olts8per8di#ision setting %written as #olts5di#' of the oscilloscope is a scaling factor that changes the si&e of the wa#eform displayed. (f the #olts5di# setting is fi#e #olts and the graticule has eight main di#isions, then the user can expect the whole screen to display =7 #olts from top to bottom since each di#ision of the graticule represents fi#e #olts.
$riggering
control
•
$he
trigger
section
is
de#oted
to stabili=ing and
focusing the oscilloscope. •
$he trigger tells the scope what parts of the signal to EtriggerF on and start measuring. (f
the wa#eform
is periodic, the trigger can be manipulated to @eep the •
display static and unflinching $he trigger section of a scope is usually comprised of a le#el @nob and a set of buttons to select the source and type of the trigger. $he level 8nob can be twisted to set a
•
trigger to a specific #oltage point. $his ad!ustment pro#ides a mechanism for ignoring small signals or low #oltages that are well below the
•
le#el of the signal you are interested in. esides, the rising or falling8edge switch selects whether the oscilloscope will trigger on the positi#e or negati#e edge of the signal. ositi#e is for rising and negati#e is
•
for falling. $hen, the trigger8mode switch will normally be set to
•
AU$- but it can be changed to )-0/. $he hold8off @nob affects the delay associated with
•
triggering. $he trigger8source switches allow us to select which signal the oscilloscope will attempt to loc@ onto.
6ori&ontal control
•
$he hori&ontal section of the scope controls the time scale on the screen. i@e the #ertical system, the
hori&ontal control gi#es you two @nobs: position and seconds5di#. $he coarse and fine8position8@nobs allow the hori&ontal mo#ement of the traces for both rough manner and precise manner. •
$he seconds
per
division 4sdiv5 @nob rotates
to
increase or decrease the hori&ontal scale. (f you rotate the s5di# @nob cloc@wise, the number of seconds each di#ision represents will decrease H youIll be E&ooming
inF on the time scale. 0otate counter8cloc@wise to increase the time scale, and show a longer amount of time on the screen. $he hori&ontal8magnification switch •
allows the hori&ontally magnified trace. $he position @nob can mo#e your wa#eform to the right or left of the display, ad!usting the hori&ontal offset. $he seconds8per8di#ision @nob sets the time base for the hori&ontal scale. (t is mar@ed in seconds, milliseconds, and microseconds.
". (f you ha#ing difficulty obtaining a stable display, which group of controls should you ad!ust
$rigger control. $his control sets an internal #oltage which is compared to the #oltage of the input signal. =.
%a' (f an AC wa#eform has ".= di#isions from pea@8to8pea@ and the 4-$5D(4 control is set to 9.745Di#, what is the pea@8to8pea@ #oltage ".= di# x 9.745di# J 1B4pp b' hat is the rms #oltage $he term E0/*F stands for E0oot8/ean8*uaredF. /ost boo@s define this as the Eamount of AC power that produces the same heating effect as an eui#alent DC powerF, or something similar along these lines, but an 0/* #alue is more than !ust that. $he 0/* #alue is the suare root of the mean %a#erage' #alue of the suared function of the instantaneous #alues. (n other words, the effecti#e #alue is an eui#alent DC #alue which tells you how many #olts or amps of DC that a time8#arying sinusoidal wa#eform is eual to in terms of its ability to produce the same power. or example, the domestic mains supply in the United ;ingdom is 2=74ac. $his #alue is assumed to indicate an effecti#e #alue of E2=7 4olts rmsF. $his means then that the sinusoidal rms #oltage from the wall soc@ets of a U; home is capable of producing the same a#erage positi#e power as 2=7 #olts of steady DC #oltage
9. (f you want to #iew the amplitude of an AC wa#eform that is 27.74rms, what setting of the #olt5di# is best 27.7 4rms J 9.4pp, $herefore the best setting of the #olt5di# is 17 with the number of deflection is because the larger the portion of display, the more accurate the measurement. . $he most accurate way to measure a wa#eform on an oscilloscope is to use a large portion of the display area. hy y using the large area of display, we can set the 4-$5D(4 to the smallest ones which is 7.9#olt5di#. or high #oltage measurement, each di#ision will represent the smallest changes of #oltage in wa#eform.
