7/28/2019 An Ohmmeter Potpourri

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Ohmmeters have many commonly known uses on theamateur's test bench. In addition to these, John Schultzdiscusses others which are not so well-known.

An Ohmmeter PotpourriBY JOHN SCHULrz-, W4FA

-Box L, FPO, NY 09544

0 -lmAFig. 1 - This unique ohmmeter has a l inear reading scale,requires no calibration and requires no zero adjustment. Itmaybemademulti-range by switching in different "standardresistors. "

T he standard ohmmeter scales on the usual YOM or VTVMsatisfy most needs. But. they do have limitations. Forinstance. that V4 inch separation between the 1kilohm a nd 5 0kiloh m markers on the upper scale ort he SOk range can proveto be mighty frustrating if one is trying to measure anyresistance between 1k and SOk with any degree of accuracy .Also, the low end of the ohms scale usual ly leaves a lot to bedesired. A tvpfcallowohm rangeon a YOM may havemarkersdown to 2 ohm s but it is totally impossible to evaulate reallylow resistance s on the order of V2 oh m or less. The measurement of such low resistances can be quite useful if one isIry ing to measure the effectiveness of a ground or the metallicconnection in a cable. Lastly, the usual YOM or VTVM (even ad igital mcltnneter ) requ ires that o ne g la nce away f rom thepo int where one is placing the test prob es to check the meterreading (or d igital c nsptay). On a complex p .c . board itdoesn't req u i re in many cases more than that glance away tolose one 's orientation on the p .c . board ,The "po tpourri" of ohmmeter c ircuits described in thisartic le includes circuits that will solve inexpensively andeasily each of the lunuations described above for the usualohms scale on a YOM or VTVM. Not every amateurwill requireall the circuits described in this article bu t surely one or moreof them wil l be found useful in the average case to eliminatesome of the usual ohmmeter frustrations .

The usuatrcrusheo" scaleat the upper end of a resistancerange of a YOM or VTVM results because a linear relationshipdoes not exist between the meter oetlection as caused by thecurrent passing through it an d the unknow n value 01 the

resistance be ing measured , The reasons for this lie in thecurrent equation s for the usual ohmmeter circuit. A battery inthe YOM or VTVM is used to pass a current through anunknown resistance and the meter is used to measure thedivision of current through ,t and the unknown resistance orthe voltage across the unknown resistance in a form of bridgecircuit. So the YOM or VTVM manufacturer has to reach acompromise between the number of ohmmeter ranges on agiven instrument and the price of providing those rangeswhi le still cove ring all commonty encountered resistancevalues with reasonable accuracy .The soiunon to th is problem IS. of course, a linear d irectread ing ohmmeter. That is, a n o hm me te r utiliZ ing . fo rinstance, a basic 0-1 rnA meter used in a sp ec ial ci rcuit SOthat an unknown resistance of zero to , k oh ms prov ide alinea! deflection on the meter f rom 0-' mA oA similar resistance range of 0-100 ki lohms could also be displayed linearlyon such a meier. A c ircuit that will do this is shown in fig . 1.The circuit has a number 01 interest ing features besides itssimplicity , which is apparent. It o per at es by making acomparison between an unknown resistor and a standardresistor in the circuit. The unknown resistance value is simplythe arithmetic product of the value of the standard resistort imes the current reading in mil liamperes. So, for the ci rcuitshown , since a 1k standard resistor is used, a " t ' reading onthe meter indicates 1kilohm, a "OS ' reading is 500ohms, etc.The ohmmeter requi res no calibrat ion, as such, if a 1%tolerance resistor is used as the standard resistor . However, ifit is not convenient to obta in such a resis tor a multiturn tr impotentiometer can be used instead, Then , of course, it will benecessary to do a one-time calibration of the ohmmeter us inga known resistance . Various standard res istors can beswitched In the cucurt SO one can have multiple ranges suchas 1k, 10k, lOOk, etc. There is also no need to zero-set thecircuit as with the usua l ohmmeter . When the test leads areshorted together the meter will a lways read zero . Varioussupply voltages from about +1B volts on up ( two or three9-volt transistor radio batteries in series) can be used.The circuit, in spite of its many useful features, still doesnot provide good enough resolut ion for measurementsbelow 1 oh m . A circuit that wil l provide such a capability isshown in f ig . 2. It ca n be assembled as a complete , portableinstrument or it can be used as an accessory with an existingVOM by utilizing the microampere range on a VOM in placeofthe meter shown.The c i rcuit is Just a simple bridge arrangement with thecomponent values chosen SO it is useful for low ohms measurement down to a fract ion of an ohm. R1 ba lance s the bridgeand should be a mu lti-turn potentiometer , if possible, for easein setnnq the meter reading to zero when the R. terminal (ortest leads) are shorted . A regu lar potentiometer ca n also be

2.71(

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1NS4HEP Rl34

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1Kbt.ndard resistor.

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32 CO Jun . , 1978

7/28/2019 An Ohmmeter Potpourri

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resistance change range, the circuit can be used as an alarmcircuit. A moisture detector can be formed using two wire tipsfor Ax. With a suitable photocell as A., the circuit will "soundoff" when a l ight beam hits the photocell . But, probably, themost useful ancilla ry use forthe circuit isas a battery checker.Two fully charged nf-cad cells in series will not cause thecircuit to "sound-all" when connected at Rx. If the cel ls aredischarged. they will cause the circui t to "sound-of f. " Onecan confirm the circui t action by substituting a variablevo ltage for A. and varying the voltage over the 1.8 to 2,4 voltrange, The point at which the circuit turns on is quite sharplydefined and itis independent of the exact voltage of the a-voltbattery powering the circuit. Ifone wanted to tailor the turn-onvoltage for a particular value, this can be done by theadjustment of Al and R2.

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used if one doesn't mind the "touchiness " of the zeroadjustment. R2 is used to calibrate the meter and this can beeither a comparative type of calibration or a full, absolutecalibration. For instance, if one were checking low resistancetransformer windings and trying to sort out various windings.a comparative sort of calibration could be made . That is, onewinding is chosen as H, and R2 adjusted so the meter readssome value such as half -scale. Then . other windings arechecked and if their resistances are lower, the rneter will readlower. If their res is tances are higher, the meier will readhigher. A simple one-point calibration can be made usmg aknown, low resistance and selling the meter to some convenient sca le marking via adjustment of R2 . Then , one can tell ifunknown resistances are above or below a known value. Afuller calibration can be made if one has a series of knownresistances. R2 is adjusted only once with a known resistanceof mid-range value to approximately half-scale meterreading , Then other known resistors are used for Rx and acalibration curve made up for the meter scale. The meterreadings are not l inear with respect to resistance and so thisis the only way the scale can be fully calibrated.There are two handy sources of known resistance values.One is meter shunts and power resistors. Often surplushouses will have these items in low resistance ranges at verylow prices due to their having been designed for specif icuses and having little general applicability. One surplushouse, for instance, was selling power resistors in the .06 to 5ohm range for anaverage price of 20 cents. Another sourceofknown, low resistance values is small gauge annealedcopper wire. At room temperature , it requires 49 feet of AWG#23 to produce one ohm or 9.7 feet of # 30 to produce oneohm.Note that the battery is inserted in the circuit using apush-button type switch. This is to prevent cont inual meterpegging if the R. points happen to be open. Many audibleohmmeter circuits have appeared over the years butprobably none can beat the simpl ic ity and versatility of thecircuit shown in fig. 3. The circuit is built around a 555 timerand can be powered by a a-von transistor radio battery. Theresistance detection range is from zero ohms to to-plusmegohms. At the high resistance value the output is a seriesof slow spaced c licks from the speaker. At zero ohms, thecircuit oscillates with a high p itched tone. Intermediate resistance values produce different tones. It is quite easy afterone uses the device a bit to distinguish between broadresistance ranges as one goes through the checking of acircui t. The current that the Rx terminals pass through theunknown resistance isvery low (in the microampe re range) sothe device can be used to check semiconductor junctionswithout damage . R2 sets the frequency of oscillation for zeroohms and one could make this resistor variable, if desired, fordifferent tone ranges.The circuit of fig, 3 has a few other interesting uses whichfallout ofthe ohmmeter function but still are worth mentioning.With the H, terminals shorted, one can key the ground lead ofpin 1 and have a code prac tice oscillator with a good , clean