HOW-TO GUIDE
How to Measure Phase Shift Between Voltage and Current on an Oscilloscope
By the OhmSavvy Research Team ·
Safety first: mains voltage and current need the right probes
- The ground clip of a bench scope probe is connected to mains earth. Clipping it onto a live or neutral conductor, or onto a shunt resistor in a mains circuit, creates a short through the scope. Never float the scope by removing its earth.
- For mains measurements, read the voltage with a rated differential probe and the current with a current probe or clamp, or power the equipment under test from an isolation transformer. Keep within the CAT and voltage ratings printed on every probe and input.
- If you are learning the method, start with the low-voltage RC circuit below, driven from a signal generator, where nothing on the bench can hurt you.
Why current and voltage shift apart
In a resistor, current follows voltage exactly, so the phase shift is zero. A capacitor draws current to change its charge, so its current peaks while the voltage is still rising: current leads. An inductor resists changes in current, so its current peaks after the voltage: current lags. Real loads mix resistance with one or both, giving an angle somewhere between 0° and 90° either way.
The two formulas
1Turn the current into a voltage the scope can read
A scope input measures voltage only. To see current you need a sensor that produces a voltage proportional to it, with as little phase shift of its own as possible. The usual choices:
| Current sensor | How it works | Watch out for |
|---|---|---|
| Shunt resistor | Voltage across a small resistor = I × R, in phase with the current | Must be non-inductive; on the high side it is not at ground potential |
| Grounded sense resistor | Shunt placed in the return leg so one end is at circuit ground | Only safe on circuits where that point really is ground |
| Current probe (clamp) | Clamps around the wire and outputs a voltage per amp | Rated bandwidth and DC capability vary; check the probe's specs |
| Current transformer | Isolated AC-only sensor with a burden resistor | Adds its own phase error at low and high frequencies |
A shunt reads in volts, so convert with I = V ÷ R: 100 mV across a 0.1 Ω shunt is 1 A. A current probe gives a scale such as 100 mV/A, which some scopes let you enter in the channel menu so the trace reads in amps directly. For phase, though, only the timing matters, so the scale can be anything readable. Most clamp meters, like those in measuring AC current with a clamp meter, only display a number and have no waveform output, so they cannot be used for this.
2Connect the voltage to CH1 and the current to CH2
Put the voltage across the load on CH1 and the current signal on CH2. On a low-voltage circuit, place the sense resistor in the ground return so both probes share one ground point and nothing is shorted through the scope. If the shunt has to sit on the high side, neither end is at ground, so measure it with a differential probe or with two channels and CH1 − CH2 math, which has accuracy limits explained in measuring a differential signal with two channels. Use 10x probes on both channels and compensate them first so neither probe adds its own shift.
3Trigger on the voltage and scale both traces
Trigger on CH1, the voltage, so it becomes the fixed reference and the current trace moves relative to it. Set the timebase to show two or three cycles, then adjust each channel's V/div so both waveforms are several divisions tall. They do not need to be the same height; only the timing of the zero crossings matters. Center both traces on the middle line, or use AC coupling if one sits on a DC offset, because a shifted baseline moves the apparent zero crossing.
4Measure the delay and the period
Place time cursors on a rising zero crossing of the voltage and the nearest rising zero crossing of the current. That gap is Δt. Then measure one full period T, or read the frequency and take T = 1 ÷ f. If the current crossing comes first, current leads; if it comes after, it lags. Many scopes offer an automatic phase or delay measurement between two channels; it does the same arithmetic, so check it against the cursors once.
Measuring across several cycles improves the accuracy of T, and turning on averaging steadies a noisy current trace. For general phase methods such as XY mode, see how to measure phase difference.
5Convert the delay to degrees and power factor
Divide Δt by the period and multiply by 360. For sine waves, take the cosine to get the power factor, and note whether the current leads (capacitive) or lags (inductive).
| Frequency | Period T | Delay Δt | Phase φ | Power factor cos φ |
|---|---|---|---|---|
| 1 kHz (figure) | 1 ms | 125 µs | 45° | 0.707 |
| 50 Hz | 20 ms | 1 ms | 18° | 0.951 |
| 50 Hz | 20 ms | 2 ms | 36° | 0.809 |
| 60 Hz | 16.67 ms | 0.5 ms | 10.8° | 0.982 |
| 60 Hz | 16.67 ms | 1.5 ms | 32.4° | 0.844 |
| 50 Hz | 20 ms | 5 ms | 90° | 0 (pure reactance) |
The cos φ result is only the full power factor when both waveforms are clean sines. Loads with rectifiers, such as most switching power supplies, draw current in short pulses near the voltage peaks. The zero crossings can line up while the power factor is still poor, because the distortion lowers it too. Look at the current shape before you trust cos φ.
Worked example: a low-voltage RC circuit
This bench setup reproduces the figure above. A signal generator drives a 100 nF capacitor in series with a 1.6 kΩ resistor, with the resistor's far end at ground. CH1 goes on the generator output (the voltage across the whole RC), CH2 on the junction of C and R. The voltage across the resistor is the current times 1.6 kΩ, so CH2 shows the current's shape and timing.
01
Sense current
Shunt, grounded resistor or current probe
02
Trigger on V
CH1 voltage is the reference
03
Read Δt and T
Matching zero crossings, one period
04
φ and PF
Δt ÷ T × 360°, then cos φ
At 1 kHz the capacitor's reactance is 1 ÷ (2π × 1000 × 100 nF) ≈ 1,592 Ω, almost equal to the resistor, so theory predicts the current leads by arctan(1592 ÷ 1600) ≈ 44.8°. With 2 V peak from the generator, the total impedance is about 2,257 Ω, the current peaks at about 0.89 mA, and the resistor voltage peaks at about 1.42 V, the CH2 height in the figure. Reading 125 µs out of a 1 ms period gives 45°, within cursor resolution of the calculation. Swap the capacitor for an inductor and the current trace moves to the right of the voltage instead.
Errors that skew the angle
- Sensor phase error. Current probes and current transformers have their own delay, and wirewound shunts add inductance. At 50 or 60 Hz this is usually small; at higher frequencies check the sensor's specs.
- Channel skew. Different probe types or cable lengths delay one channel. Some scopes offer a deskew setting; feed the same signal to both channels to check for a zero reading.
- Inverted sensor. A reversed shunt or clamp adds 180°. A reading near 180° on an ordinary load almost always means this.
- Offsets and noise. A DC offset or hum shifts the zero crossings. Use AC coupling, averaging, or the bandwidth limit on slow mains signals.
Which scope you need for voltage and current phase
Two channels are the minimum, and bandwidth is rarely the limit at mains or audio frequencies. A built-in signal generator is handy for the RC practice circuit. Compare models in our best beginner oscilloscopes roundup.
Two-channel-and-up scopes in our reviews
50MHz, 4 channels, 1 GSa/s; the default beginner recommendation for years.
100MHz, 2 channels, 1 GSa/s; an oscilloscope and waveform generator in one unit.
100MHz, 4 channels, 1 GSa/s; more bandwidth and a better UI than the Rigol.
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Frequently asked questions
Does current lead or lag voltage in a capacitor?
In a capacitive circuit the current leads the voltage, reaching its peak up to a quarter cycle (90°) earlier in a pure capacitor. In an inductive circuit, such as a motor or transformer winding, the current lags the voltage. On the scope, a leading current trace sits to the left of the voltage trace when you trigger on the voltage.
Can I measure power factor with an oscilloscope?
For sine-wave voltage and current, yes: power factor equals the cosine of the phase angle you measure. Many loads, such as switching power supplies and LED drivers, draw distorted, spiky current, and then cos φ only gives the displacement part. A power analyzer or a scope with a power analysis option handles those loads properly.
What value shunt resistor should I use?
Small enough that it barely changes the circuit, large enough to give a readable signal. A common target is a shunt voltage of tens to a few hundred millivolts at the expected current, with the shunt well under 1% of the load impedance. Use a non-inductive resistor, because a wirewound shunt adds its own phase shift at higher frequencies.
Why does my phase reading jump by 180 degrees?
The current signal is inverted. That happens when the shunt or current probe is connected the opposite way round, for example a clamp facing backwards on the wire. Flip the probe, swap the shunt connections, or use the channel's invert setting, then check that the reading lands in a sensible range for the load.
Can I use a current probe instead of a shunt resistor?
Yes. A scope current probe clamps around the wire, so nothing is added to the circuit and the probe output is isolated from it. The catch is that the probe adds its own delay, and AC-only transformer types also shift phase at low frequencies, so deskew it against the voltage channel first. A clamp meter without an analog output cannot feed a scope at all.
