8/10/2019 Make Your Own Multimeter
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ake your own multimeter
PARTS AND ATERIALS
Sensitive meter movement (Radio Shack catalo ! ""#$%&'
Selector switch sinle#)ole multi#throw *reak#*e+ore#make
(Radio Shack catalo ! ",-#%./0 is a "#)ole 0#)osition unit
thatworks well'
Multi#turn )otentiometers P12 mount (Radio Shack catalo !
",%#
.$" and ",%#.$. are %-#turn % k3 and %& k3 4trimmer4
unitsres)ectively'
Assorted resistors )re+era*ly hih#)recision metal +ilm or
wire#
wound ty)es (Radio Shack catalo ! ",%#.&5 is an assortment
o+ metal#+ilm resistors 67# %8 tolerance'
Plastic or metal mountin *o9 Three 4*anana4 :ack style *indin
)osts or other terminal
hardware +or connection to )otentiometer circuit (Radio
Shackcatalo ! ",$#00" or e;uivalent'
The most im)ortant and e9)ensive com)onent in a meter is the
movementih#;uality metermovements are e9)ensive *ut Radio Shack
carries some o+ acce)ta*le;uality that are reasona*ly )riced= The
model recommended in the )artslist is sold as a voltmeter with a %-
volt rane *ut is actually amilliammeter with a rane (4multi)lier4'
resistor included se)arately=
It may *e chea)er to )urchase an ine9)ensive analo meter
anddisassem*le it +or the meter movement alone= Althouh the thouht
o+destroyin a workin multimeter in order to have )arts to make your
ownmay sound counter#)roductive the oal here is learnin not
meter+unction=
I cannot s)eci+y resistor values +or this e9)eriment as these
de)end onthe )articular meter movement and measurement ranes
chosen= 2e sure touse hih#)recision +i9ed#value resistors rather
than car*on#com)ositionresistors= Even i+ you ha))en to +ind
car*on#com)osition resistors o+:ust the riht value(s' those values
will chane or 4dri+t4 over timedue to ain and tem)erature
+luctuations= ?+ course i+ you don@t carea*out the lon#term
sta*ility o+ this meter *ut are *uildin it :ust +orthe learnin
e9)erience resistor )recision matters little=
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care+ul not to a))ly too much current to the meter movement
asmovements are very sensitive devices and easily damaed *y
overcurrent=
Most meter movements have +ull#scale de+lection current ratins
o+ % mAor less so choose a )otentiometer value hih enouh to limit
currenta))ro)riately and *ein testin with the )otentiometer turned
toma9imum resistance= The lower the +ull#scale current ratin o+
amovement the more sensitive it is=
A+ter determinin the +ull#scale current ratin o+ your meter
movementyou must accurately measure its internal resistance= To do
thisdisconnect all com)onents +rom the )revious testin circuit and
connectyour diital ohmmeter across the meter movement terminals=
Record thisresistance +iure alon with the +ull#scale current +iure
o*tained inthe last )rocedure=
Perha)s the most challenin )ortion o+ this )ro:ect is determinin
the)ro)er rane resistance values and im)lementin those values in
the +ormo+ rheostat networks= The calculations are outlined in
cha)ter / o+volume % (4Meterin 1ircuits4' *ut an e9am)le is iven
here= Su))oseyour meter movement had a +ull#scale ratin o+ % mA and
an internalresistance o+ $&& 3= I+ we wanted to determine
the necessary raneresistance (4Rmulti)lier4' to ive this movement a
rane o+ & to %- voltswe would have to divide %- volts (total
a))lied voltae' *y % mA (+ull#
