Resistance Measurement Problems - What Can Go Wrong?
Today’s digital multimeters (DMMs) are inexpensive, accurate, and easy to use, but sometimes a problem with the circuit or the measurement can cause incorrect readings. Learn to interpret and fix these problems.
Sometimes you need to grab your multimeter and go make some resistance measurements in industrial and control systems. You might be checking for shorts, opens, or continuity. Maybe you suspect some defective components such as switches or relays. You might need to verify proper system connections or proper operation of a sensor such as a potentiometer or resistance temperature sensor. Usually, it’s simple: touch the probes to the measurement points and take a reading.
Figure 1. Measuring ohms with a multimeter. The negative reading is explained below. Image provided by author
Sometimes something goes wrong—the reading makes no sense. This article was prompted by a forum discussion of negative resistance readings. That’s not the only possible problem—there are others. Let’s dive into them.
How Do Multimeters Measure Resistance?
The basics are simple: the meter supplies a fixed current to the resistance and measures the voltage across it. See Figure 2. Ohm's law: voltage equals current times resistance (V = I x R). Switching the range changes the current—higher for low resistances. The meter scales the reading to show ohms. In some meters the details are different, but the basic concept still applies.
Figure 2. How a multimeter calculates resistance using Ohm’s Law with a fixed current and measured voltage drop.
What Can Go Wrong With Measuring Resistance?
As mentioned, you might get a negative reading. This makes no sense, because resistance is always positive. Other possibilities: the reading changes when you reverse the leads, the reading changes when you change the range switch, or the reading drifts or is unstable.
Causes and Cures
Negative Ohms on a Multimeter Display
Cause: Assuming your meter is ok, this almost certainly is due to a voltage or power supply in the circuit you’re measuring. The negative reading shown in Figure 1 was made by adding a battery to the circuit. This changes the resistor’s current: it no longer is simply I x R. The meter measures a higher voltage than the fixed current supply should have allowed, which would be impossible unless the resistor was adding current, rather than decreasing it. The reading will be wrong. Figure 3(a) shows a simple example, a battery connected in series with the resistor.
Figure 3. Active voltage sources in the circuit will cause wrong readings. Sometimes a charged capacitor can be the culprit.
Figure 3(b) has a charged capacitor, no battery. In this case the capacitor’s charge probably will change when the meter is connected: the reading will drift. If you wait a while, the capacitor charge will stabilize, the drifting will stop, and the reading should be correct.
Voltages in the circuit can cause problems other than negative readings. Reversing the DC voltage will make it read positive but too high, maybe off scale. If the voltage is AC, who knows?
Diagnosis: Change the meter to read voltage. It should be zero. If not, there is an unwanted voltage somewhere.
Cure: Always make sure all power and voltage sources are disconnected and capacitors, if any, are discharged. See below for more about capacitors.
Reading Changes if the Leads are Reversed
Cause #1: Unwanted voltages in the circuit—see above.
Cause #2: The circuit includes semiconductors: diodes, transistors, or ICs. These usually conduct better in one direction than the other, so the voltage drop is different for positive or negative currents.
Diagnosis: Review the circuit to see if it includes semiconductors.
Cure?: No real cure, unless you can remove the semiconductors from the circuit. Hopefully, you can analyze the circuit and see if the readings make sense.
Reading Changes when the Range Switch is Changed
The likely causes and cures are the same as those for lead reversal: unwanted voltages or semiconductors. Voltages will affect the reading differently on different ranges. Semiconductors conduct differently when their voltage or current is changed.
Figure 4. Circuits constructed with semiconductor devices have high resistances, which must be measured with the proper polarity, and with high resistance measurement ranges selected. Image used courtesy of Canva
Drifting or Unstable Resistance Readings
Cause #1: Capacitance in the circuit. Even if the capacitors are discharged, the meter’s current may charge them a bit. If their capacitance is large it may take a while for them to reach their final charge. When they do, the ohms reading should be correct. Small capacitors probably will not noticeably affect the reading.
Diagnosis: When the reading settles, reverse the meter leads. If capacitance is the problem the reading will at first be different—perhaps even negative—but then will drift back to the same final reading.
Cure: Just hold the leads on the circuit until the reading stabilizes.
Cause #2: Loose or intermittent connection. The moment-by-moment reading jumps around because the actual ohms are changing. This especially can happen in industrial or extended wiring situations. I once was called to a customer’s plant because they thought our flowmeters were defective. It turned out that their electricians had used crimp-on connectors meant for large power wiring to splice smaller gauge signal wires.
Cure?: If no other cause is diagnosed, investigate the wiring and connections. Find and fix the problem.
Cause #3: Using an auto-ranging ohmmeter with a large inductance in the circuit, such as a motor or power transformer. This is unusual, but it did happen to me once.
When a DC voltage is applied, an inductor’s current starts at zero and then increases. The ohmmeter at first sees this as a high resistance (low current) and switches to a high range. Then, as the current increases, the autorange switches lower. This in turn changes the current supplied by the meter. It takes a while for the inductor to react so, in some cases, the meter may again change its range and its current. With a large inductance the changes and range switching may never settle down. This probably will not happen with small inductances.
Cure: Turn off autoranging and choose a fixed ohms range. The reading should settle quickly.
Today’s digital multimeters are accurate and easy to use, but sometimes circuit or application problems can cause incorrect readings. We’ve reviewed several possibilities and presented cures.