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Figure 1. Basic 4-Wire Ohmmeter Technique

Figure 2. Schematic Diagram - Low Resistance Measurement Adapter

Practical Low Resistance Measurements

Bob NuckollsSr. Engineer/SME

Raytheon Aircraft CompanyWichita, Kansas

Modern digital multimeters have two importantlimitations:

(1) When taking resistance readings on switch or relaycontacts, the common multimeter often produces high,almost meaningless resistance readings due to very low testcurrents generated within the instrument: A sampling ofresistance measurement currents for five different digitalinstruments showed no current exceeding 1.5 milliamperes.Most measurements were are made atcurrents on the order of 250 microamperes.

(2) Typical off-the-shelf multimeters offera low resistance measurement on the orderof 200 ohms full scale with a resolution of0.1 ohms. For qualitative measurements ofcomponents like switches and relaycontacts the technician needs to resolveresistance values of 1 ohm or less withresolution in the milliohms. Contactresistance due to film or corrosion build-uprequires a substantial current flow topenetrate contact film.

A “4-wire” measurement technique for lowresistance is illustrated in Figure 1. Acurrent of known value is caused to run

through the resistor of interest. A voltmeter is used tomeasure drop across the resistor INSIDE the currentconnections. Ohms law is used to calculate the resistancevalue. This technique eliminates effects of test equipmentlead and probe resistance.A practical implementation of the 4-wire measurementtechnique illustrated in Figure 2. can be assembled in anhour or so with a bill of materials under $10. The outerhousing is a plastic project box from Radio Shack. Dual

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Figure 3. Interior View Component Location

Figure 4. LM317 Connections.

banana plugs to fit mostmultimeters along withcomponents to implement aconstant current source arereadily available from suppliersof commercial components suchas Radio Shack, Digikey,Hosfelt, Jameco and others

Construction Notes

A constant current generator isfabricated from a popularadjustable voltage regulatorcombined with a 12-ohmresistor. The LM317 (Figure 4)is readily available in the TO-220 plastic package from mostparts suppliers. The LM317series, 3-terminal regulatorscome in a variety of packagesand environmental ratings. Anyversion is suited to thisapplication.

Power to bias the test comesfrom three AA sized alkalinecells in series. Spring loadedbattery holders have a potential for poor connection to thecells with age and environmental effects. Solderedconnections are easy to make and very reliable

Brighten an area on thebattery to be solderedwith a file, knife edgeor edge of grindingwheel. The DremelMoto-Tool with a cut-off wheel attachedworks really nice. Use ahot iron to get on andoff quickly each timethe cell is touched.First, tin the area wherea wire will attach to thecell. Tin the wire andthen tack it into placeon the cell.

With a little practice,the two times you touchthe cell with thesoldering iron should be

less than two or three seconds total. Eliminating the batteryholder allows construction to be contained in a smallerenclosure and eliminates future problems that arise frompoor contact with cells.

Calibration

The finished fixture can be tested for calibration byplugging it into the multimeter while it is set up to measurecurrent. The fixture fabricated for this article produces aconstant current of 104 mA. This is expected; the LM317voltage regulator is a 1.25 volt device. A 12.0 ohm resistorwould be expected to produce a current slightly higher than100 mA. For most purposes, this variability is insignificant.

Using the Fixture

The adapter illustrated herein generates 100 milliamperesof test current. When used in conjunction with a multimeterhaving readout in tenths of a millivolt, the resolution of alow resistance measurement will be 1 milliohm.

This device will yield useful resistance measurements up tobetter than 10 ohms (1.000 volts displayed on multimeter)If your multimeter has a 200 or 300 millivolt range, a 100milliampere test adapter will display 2 or 3 ohms full scale.

The low resistance measurement adapter is useful indiagnosing system malfunctions that could be attributed toincreased contact resistance in switches or relays. Switchesoutside the pressure vessel on turbojet aircraft areparticularly good candidates for environmental effects,

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Figure 6. Adapter Shows 2 Milliohms Parasitic Resistance.

especially those under the nacelles or onlanding gear struts.

When investigating the condition of a switchor relay, it’s useful to exercise the part andobserve variations in contact resistance fromone measurement to the next. Unless you areoffered baseline resistance values as part of apublished test procedure, you’ll have to makecomparative measurements between asuspected bad and a known good component.

Storage

Current flows from these cells only while ameasurement is being made. Shelf life of analkaline cell is 5 years or more. Cells in thisfixture should not require frequentreplacement. To take advantage of the verylong shelf life of alkaline cells, be sure to storethe adapter in a way that protects the leadwires and/or exposed banana plugs frombecoming shorted together.