Measuring Resistance with a Multimeter
Resistance measurement seems simple — select Ω, touch the probes to the component, read the value. But the ohmmeter is the function most frequently misused by beginners, leading to mysterious wrong readings and damaged meters. The reason is that the ohmmeter supplies its own small current to make the measurement, and anything else in the circuit that conducts — including charged capacitors and parallel components — will corrupt the reading or even harm the meter. Understanding these constraints turns a potentially confusing function into a reliable and powerful tool.
Power Must Be Off
The ohmmeter works by supplying its own small voltage (from the meter's internal battery) and measuring the resulting current through the unknown resistance. If external voltage is present on the circuit under test — from a power supply, a charged capacitor, or an adjacent circuit — that voltage will add to or subtract from the meter's own reference voltage, corrupting the reading completely. Charged capacitors are particularly dangerous: they can discharge through the ohmmeter's input circuit, potentially destroying the ADC or shunt resistor.
In-Circuit Measurement Errors
Even with the circuit powered off, measuring a component that is still connected to the board will usually give a wrong reading. The reason is parallel paths: other components in the circuit form parallel connections with the component you are trying to measure, and the ohmmeter reads the parallel combination — which is always lower than the component alone.
Left: correct out-of-circuit measurement — one lead lifted, only the component under test in the measurement path. Right: in-circuit measurement — parallel paths through other components give a reading lower than the true resistance.
View LargerYou want to measure a 10 kΩ resistor (R1) that is soldered in parallel with a 4.7 kΩ resistor (R2) on a PCB.
In-circuit reading: 1 / (1/10k + 1/4.7k) = 3.2 kΩ — much lower than R1 alone.
Only by lifting one end of R1 off the board will the meter read the true 10 kΩ.
Situations where in-circuit resistance readings are valid:
- When there genuinely are no other conductive paths — for example, checking a single resistor in an otherwise open circuit branch.
- When the reading matches the expected value — useful as a quick sanity check, though the match could be coincidental.
- When measuring very low resistance (mΩ range) in wiring, where other resistances in the circuit are orders of magnitude higher and contribute negligible parallel loading.
Out-of-Circuit Measurement
The reliable method: lift one end of the component off the board before measuring. This eliminates all parallel paths. For through-hole components, lift one lead with a desoldering pump or wick. For SMD components, use a hot-air station or a soldering iron to lift one pad end. Many technicians prefer to remove the component entirely and measure it in free air, then reinstall if it passes.
When measuring bare components (not soldered to anything), touch the probe tips to the component leads — but do not hold the probe tips and leads together with your bare fingers. The resistance of your skin (typically 10 kΩ–10 MΩ depending on moisture) is in parallel with the component and will reduce the apparent resistance. Hold the component with insulating tweezers or by its body, keeping your skin away from the probe tips.
Range Limits — Too Low and Too High
Ohmmeters have practical limits at both ends of their range:
Very low resistance (under about 5 Ω)
The resistance of the test leads themselves (typically 0.1–0.5 Ω) becomes significant. On a reading of 0.5 Ω, the lead resistance alone is 0.1–0.5 Ω — a very large percentage error. To eliminate this, use the Relative (REL) function: short the probes together, press REL to zero out the lead resistance, then measure the component — the displayed value is only the component's resistance, with lead resistance subtracted. For measurements below about 0.1 Ω (bonding resistance, RF ground connections), a four-wire (Kelvin) measurement method is needed — beyond the capability of a standard DMM.
Very high resistance (above about 10 MΩ)
Most DMMs run out of range above 20–40 MΩ and display OL. The meter's own input circuitry begins to conduct significantly at these levels. Additionally, surface contamination on PCBs (flux residue, finger oils, moisture) can create parallel leakage paths in the megohm range that corrupt readings of high-value resistors. Clean the PCB surface with isopropyl alcohol and allow to dry before measuring components above 1 MΩ.
Ham Radio Resistance Checks
- Verifying resistor values: Pull unknown or unlabelled resistors from the junk box and measure them out of circuit. Compare to nominal value and check whether they are within tolerance.
- Coax braid continuity: Check that the outer conductor (braid) of a coax run is continuous from end to end by measuring resistance from braid to braid. Should be below 1 Ω for short runs. High resistance or OL indicates a break or corroded connector.
- Coax center conductor continuity: Similarly check end to end. Very high or OL resistance means an open center conductor — common at connector crimps that have pulled loose or corroded.
- Coax short check: Measure from center conductor to braid at one end while the other end is open. Should read OL (open circuit). Any resistance below several MΩ indicates a short — water ingress, pinched cable, or a damaged connector.
- Relay coil resistance: A typical 12 V relay coil is 200–400 Ω. Reading OL means an open coil; reading near zero means a shorted coil. Both are faults.
- Transformer winding resistance: Primary winding resistance of a power transformer is typically 10–200 Ω. Secondary winding resistance depends on turns ratio and wire gauge. An open winding reads OL; a shorted winding reads near zero.
Hands-On Experiment
⚖ Experiment: In-Circuit vs Out-of-Circuit Resistance
Measure the same resistor first while it is connected to a simple circuit, then after lifting one lead. This demonstrates directly how parallel paths corrupt in-circuit resistance readings.
- Breadboard
- One 10 kΩ resistor (R1 — the one to measure)
- One 4.7 kΩ resistor (R2 — to create a parallel path)
- Digital multimeter set to Ω
- Connecting wires (no battery needed — power off for all steps)
- Insert both resistors into the breadboard with both ends connected to the same two rows — they are now in parallel. Make no other connections. Ensure no battery is connected.
- Measure the resistance across the two rows using the ohmmeter. Record the reading. You should get approximately 3.2 kΩ — the parallel combination of 10 kΩ and 4.7 kΩ.
- Now lift one end of the 10 kΩ resistor (R1) out of the breadboard row, leaving it dangling in air. Only R1's remaining connected end and the other row are accessible.
- Measure from the connected end of R1 to its free end (in air — the free end of R1 only). You should now read approximately 10 kΩ — the true value of R1 alone, with no parallel path.
- Re-seat R1 and lift one end of R2 instead. Measure R1 again in-circuit with R2 floating — you should again read approximately 10 kΩ, confirming that removing the parallel path restores the accurate reading.
The in-circuit reading (3.2 kΩ) is substantially lower than the true R1 value (10 kΩ) because R2 provides a parallel current path through the ohmmeter. Lifting one lead of either resistor eliminates the parallel path and restores an accurate reading. This is why disconnecting one component lead before measuring resistance is not optional — it is the only way to trust the reading.
Frequently Asked Questions
Why does my ohmmeter give a fluctuating reading on a capacitor?
The ohmmeter charges the capacitor with its internal battery current. As the capacitor charges, the current decreases and the apparent resistance rises. The reading will climb toward OL as the capacitor approaches the battery voltage. This is normal behavior — it is not a fault in the meter or capacitor. A perfect capacitor eventually charges to the battery voltage and the current falls to zero (OL). A partially leaky capacitor stabilizes at a finite resistance value — indicating dielectric leakage.
Can I check if a fuse is blown with the ohmmeter?
Yes — remove the fuse from the circuit, power off, and measure resistance across the fuse. A good fuse reads very low resistance (near zero, limited only by the wire element and lead resistance — typically <1 Ω). A blown fuse reads OL (open circuit). Use the continuity beeper mode for a quicker result — it beeps for a good fuse and stays silent for a blown one.
The resistance reading changes when I hold the probe tips with my fingers. Is the meter broken?
No — your skin resistance (typically 10 kΩ–10 MΩ) is forming a parallel path between the probe tips through your hands. Hold the probes by their insulated barrels, not the metal tips, and keep your fingers away from the probe ends. For very high-resistance measurements above 1 MΩ, even ambient humidity on the PCB surface matters — handle boards by the edges.
Test Your Knowledge
Answer the questions below to check your understanding. Every answer can be found in the lesson above.