How to Measure Current with an Oscilloscope: Step-by-Step Guide

Introduction

Oscilloscopes are designed to measure voltage over time, but understanding current is just as important for analyzing circuit behaviour, especially in power systems, motor control, and embedded applications. Measuring current with an oscilloscope reveals dynamic changes, helps diagnose power issues, and ensures components operate safely. In this guide, you’ll learn how to use your oscilloscope for current measurement through two main methods: using a current probe or a shunt resistor. We’ll walk through setup, math channel configuration, waveform interpretation, and essential safety practices.

Methods of Measuring Current

Since an oscilloscope can only measure voltage directly, we must use a component that converts current into a proportional voltage. The two most common ways to do this are with a dedicated current probe or a simple shunt resistor.

Using a Current Probe

A current probe is a specialized device that clamps around a conductor and measures the magnetic field created by the current flowing through it. It then outputs a voltage signal that your oscilloscope can read. This is a non-invasive method, as you don’t need to cut or modify the circuit to insert the probe.

Current probes come in two main types:

• AC current probes – These work like transformers and can only measure alternating current.
• AC/DC current probes – These use a Hall effect sensor to measure both AC and DC currents, making them more versatile.

Benefits of using a current probe:

• Non-invasive and easy to use – Simply clamp it around the wire.
• Galvanic isolation –The probe is electrically isolated from the circuit under test, which is a major safety advantage.
• Wide dynamic range – They are suitable for measuring a wide range of AC and DC currents.

Before taking a measurement, it’s critical to degauss and zero the probe. Degaussing removes any residual magnetism, and zeroing compensates for any DC offset, ensuring your measurement starts from a true zero baseline.

Using a Shunt Resistor

The shunt resistor method is a classic technique that relies on Ohm’s Law (V = I × R). By placing a resistor with a very small, known resistance (the “shunt”) in the path of the current, you can measure the small voltage drop across it. From this voltage, you can calculate the current using the derived formula:

                                                 I = V / R

Choosing the right shunt resistor is critical:

• Resistance value – The value must be low enough that it doesn’t significantly alter the circuit’s behavior (a phenomenon called burden voltage—the undesirable voltage drop introduced by the measurement device), but high enough to produce a voltage drop that your oscilloscope can accurately measure.
• Power rating – The resistor must be able to handle the power it will dissipate, calculated with P = I²R. An underrated resistor will overheat and fail.
• Tolerance and stability – Use a precision resistor with low tolerance (e.g., 1% or better) and a low temperature coefficient (tempco) so its resistance doesn’t change as it heats up. Four-wire Kelvin-connected resistors are ideal for minimizing contact resistance errors, as the measurement leads only sense the voltage drop across the shunt and not the current leads.

You can place the shunt in two positions:

1. Low-side measurement – The shunt is placed between the load and ground. This is the simplest setup, as the voltage across the shunt is referenced to ground. However, it also raises the load’s ground reference, which can be an issue in some circuits.
2. High-side measurement – The shunt is placed between the power source and the load. This doesn’t disrupt the ground reference, but it creates a common-mode voltage problem. Since neither side of the shunt is at ground, NEVER use a standard, ground-referenced oscilloscope probe. You must use a differential probe or an oscilloscope with isolated inputs to measure the voltage across it safely.

Step-by-Step Measurement Guide

Let’s walk through the process of setting up your oscilloscope to measure current using both methods.

Setup

Method 1: Current Probe

1. Connect the current probe to one of the oscilloscope’s input channels.
2. Degauss and zero the probe according to the manufacturer’s instructions.
3. Clamp the probe around the wire through which the current you want to measure is flowing. Pay attention to the direction arrow on the probe to ensure correct polarity.

Method 2: Shunt Resistor

1. De-energize the circuit.
2. Insert the shunt resistor into the circuit path (either high-side or low-side).
3. For a low-side measurement, connect a standard oscilloscope probe across the resistor, with the probe tip on the load side and the ground clip on the circuit ground side.
4. For a high-side measurement, connect a differential probe across the resistor.

Acquire Signal

1. Power on your circuit.
2. Adjust the Volts/Div and Time/Div knobs on your oscilloscope to get a clear and stable waveform on the display.
3. Set the trigger correctly to stabilize the waveform. Note that some current probes may invert the signal, so you might need to use the falling edge for your trigger or use the oscilloscope’s invert function.

Interpret Results

How you read the current depends on your method.

• With a current probe – If you’ve configured the probe scaling correctly (more on this below), the vertical axis will read directly in Amps. The Volts/Div control becomes an Amps/Div control.
• With a shunt resistor – The oscilloscope displays the voltage across the shunt. You must use Ohm’s law to convert this back to current. For example, if you measure a 100 mV peak voltage across a 0.1 Ω shunt resistor, the peak current is:

                                 I = V / R = 0.1 V / 0.1 Ω = 1 A

Scaling & Math on the Oscilloscope

Modern oscilloscopes offer powerful features to simplify current measurements. Instead of doing manual calculations, you can configure the scope to do the work for you.

• Probe scaling – When using a current probe, you can enter its probe attenuation/preamplifier factor (e.g., 100 mV/A) into the channel settings. The oscilloscope will automatically scale the vertical axis to display Amps instead of Volts. Your measurements and readouts will all be in the correct units.
• Shunt math channel – When using a shunt resistor, you can use the Math function to display current directly. Set up a math channel that divides the voltage from your input channel by the resistance of your shunt (e.g., CH1 / 0.1). This creates a new waveform that represents current in Amps, provided the resistance is entered in Ohms (Ω) and the input channel is in Volts (V).
• RMS and average current – For analyzing AC signals or noisy DC signals, use the automatic measurement functions to find the RMS (Root Mean Square) or Average current. This is invaluable for power analysis.
• Power measurement – You can take it a step further by measuring both voltage and current on separate channels. Then, create another math channel that multiplies voltage (V on CH1) and current (I on CH2) to display an instantaneous power waveform (P) (in Watts).

For a closer look at these features, see our post on measuring current with the Analog Discovery 2. Also read our article on how to read an oscilloscope.

Safety Considerations

Measuring current can be dangerous if done incorrectly, especially when working with high voltages or mains power.

• Isolation and grounding – The ground clip of a standard oscilloscope probe is connected to earth ground. If you connect this clip to any point in your circuit that is not at earth ground potential, you will create a short circuit, often resulting in sparks, blown fuses/components, and a severe safety hazard. Always use a differential probe or an isolated oscilloscope for floating (high-side) measurements, where neither measurement point is referenced to earth ground.
• Probe and shunt ratings – Never exceed the maximum current, voltage, or bandwidth ratings of your probes or shunt resistors. Overloading a current probe can cause it to saturate, giving incorrect readings, while overloading a shunt can cause it to fail catastrophically (e.g., exploding).
• Power down rule – Always de-energize the circuit and verify it is safe before physically inserting a shunt resistor or making any direct contact connections.

Note on accuracy: Always degauss and zero your current probe before each set of measurements to ensure accuracy and prevent offset errors.

Troubleshooting & Common Mistakes

• Noisy signal – Current measurements are often prone to noise. To fix this, use a short ground lead on your probe, twist the probe leads together, or enable bandwidth limiting on the input channel or waveform averaging on your oscilloscope.
• Incorrect reading – Double-check your setup. Is the probe scaling or math channel formula correct? Did you remember to zero the probe? Is your shunt resistor’s value accurate? Is the probe securely clamped around only one conductor?
• Probe saturation – If you are measuring a large DC current with an AC component, the DC portion can saturate the probe’s core, limiting the probe’s range for the AC signal and distorting the AC measurement. Degauss the probe and ensure it is rated for the DC current you are measuring.
• Aliasing – If the waveform looks distorted or has an unexpectedly low frequency, your sample rate might be too low. Adjust the Time/Div knob to a larger value to increase the total capture time, which may force the scope to increase its sample rate or use a different acquisition mode.

Practice Exercise

The best way to get comfortable with measuring current is to practice.

1. Build a simple DC circuit: a 5V source connected to a 100 Ω resistor. The expected current is 50 mA.
2. Measure the current using the shunt resistor method. Try a 1 Ω shunt. When the shunt is added, the total circuit resistance is 101 Ω. The actual current should be I = 5V / 101 Ω ≈ 49/5 mA (due to burden voltage).

• You should measure approximately 49.5 mV across the shunt. 
• Use your oscilloscope’s Math functions to display the current and RMS value.

1. If you have a current probe, measure the same current by clamping the probe around the 1 Ω shunt lead.
2. Compare the result from the probe with your shunt measurement. They should be very close! 

FAQs

Can I measure AC current with the same setup?
Yes. Both the shunt resistor method and AC/DC current probes work for AC current. The oscilloscope will show you the current waveform, allowing you to measure its peak, RMS value, and frequency. Note that AC-only current probes cannot measure the DC offset component of the signal.

Do I need a differential probe for high-side current measurement?
Yes, if you are using a standard, earth-grounded oscilloscope. A differential probe measures the voltage difference between two points without reference to ground, making it safe for high-side measurements. The alternative is to use an oscilloscope with fully isolated channels.

How do oscilloscope current measurements compare to a DMM?
A digital multimeter (DMM) is great for measuring steady-state DC or RMS AC current. An oscilloscope’s advantage is its ability to show you how the current changes over time, revealing inrush currents, transients, and dynamic power consumption that a DMM cannot see.

Conclusion

While oscilloscopes are fundamentally voltmeters, they are incredibly versatile tools for current analysis. By using a current probe for non-invasive, isolated measurements or a shunt resistor for a simple, cost-effective solution, you can unlock a new level of insight into your circuits. Mastering these techniques and the oscilloscope’s math functions will empower you to diagnose complex issues and optimize the performance and safety of your designs.

Take the next step in circuit analysis. Explore Digilent’s powerful mixed signal oscilloscopes, oscilloscope probes, cables, and clips for your projects.

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