Donald R. Getzin The Newark College of Engineering Newark, N e w Jersey 07102
An Inexpensive DC Ohmmeter
It is generally too expensive to supply each laboratory student with his own ohmmeter, especially in freshman chemistry laboratory. The ohmmeter shown in Figure 1 avoids this problem for it can be easily assembled for well under ten dollars' and may be used to determine resistances in the range of 25-500 ohms with an average accuracy of better than 2%.% This ohmmeter is specifically designed for use in an experiment on semiconductors3 but may find wider use in electrochemistry experiments in freshman chemistry laboratory. In using the ohmmeter it is necessary, however, that one portable multimeter be available for calibration purposes. The schematic wiring diagram of the ohmmeter is shown in Figure 2. S represents the test sample whose resistance, R,, is to be determined. An inexpensive 0-10 V panel meter with an internal resistance, Re of about 880 ohms is used.4 The ballast resistor has a resistance, Rg, of 1200 ohms and is rated at 10yo tolerance and 2 W. The voltage source is a 45-V battery. A tap switch or MOMENTARY ON button should he used in the circuit to minimize battery drain and sample heating. If the true voltage, V , across the sample is known, the sample resistance may be calculated from eqn. (1).
(1) The resistance of the ballast resistor should be measured with a multimeter or other suitable instrument. This may he done in advance by the instructor and the value written on the ohmmeter. To avoid this step, a. 1% tolerance resistor may be used instead of the specified 10% resistor, hut this will increase the cost a little. (2) The internal resistance of the panel meter should he cheeked. This also may be done in advance. (3) The voltage output of the battery under load should he measured once by the stndent during the laboratory session using the ~ o r t a h l emultimeter. Since the hatterv is usuallv under load a f r a ininu~r.;dl~ring111 r&perlnwnr,thew lor a coca1 rime of i., a i.egligiblp rhancr i t , this VHIW during rlw rxpenmrrt. TIII-
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Figure 1. An inexpensive dc ohmmeter. A shunt is shown in position between the binding pods which ore used to attach the sample of unknown relirtonce. (Photograph by Charles Flemm.1
E is the voltage output of the battery under losd. Derivation of this equation may be left to the student since it only involves a knowledge of Ohm's Law and how to calculate the total resistance of resistors in series and parallel. To achieve an accuracy of better than 2% it is necessary to calibrate the apparatus in four ways.
' \\'hen the r.tmpnnelrt.~are purvhured I I L a q t ~ s n t i ~sulli~iwt v to ron.t I urt fifty ultro>mtten, esch dmlnetrr w l l n s t ntxmt .37. All estimates of accuracy were made by using the ohmmeter to measure the resistance of a decade box of 1% resistors. a GETZIN,D. R., J. CHEM. EDUC., 48, 541 (1971). 'The panel meter found most suitable is a Sburite, model 81072. I t has a list price around $3.
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Figure 2.
Ohmmeter schematic wiring diogrom.
measurement may he made a t the same time the fourth calibration is performed. There is about a I-V drop in battery voltage when s current is drawn. (4) The panel meter itself should he calibrated. This may be done indirectly and in an approximate manner. One 500-ohm, 2-W, ly0resistor should he available in the laboratory. A t some point during the laboratory session, the student should observe the voltage reading, V ' , on the panel meter when this resistor is used in place of the test sample. The panel meter ought to read V as calculated from eqn. (2). I n general, V' will not equal V
Calibrating the panel meter in this manner assumes that any errors are linear. While this is generally not the ease, this method is sufficient to decrease the errors in R, to under 2y0. Without this edihration, the errom will average around 87' with some being as large as 15% when higher resistances are measured. Two precautions should he observed when using this apparatus. First, the apparatus should not he activated unless a sample is in place. This will overload the voltmeter and possibly damage it. To prevent inadvertant activation, a shunt should he used when the samole is not in dace. Second. the meter should be
A correction
It is possible to change the range of measurable resistances by selecting a different ballast resistance and 8. voltmeter with a different range. The internal resistance of the voltmeter chosen must he greater than the resistance of the test sample in order to maintain adequate sensitivity o i the apparatus.
because of errors primarily within the panel meter. factor, C, may be calculated from eqn. (3).
v'=CV'
(3)
Typically, the panel meter reads 5% too high and C is 0.95. The student may then calculate the voltage, V , when using the ohmmeter to measure an unknown resistance by multiplying the panel meter reading, V', by his value of the correction factor, C.
6 Successful results are obtained with a Burgess XX30. a list price around 52.25.
Volume 49, Number 6, June 1972
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