How to measure power supply ripple with an oscilloscope

What is power supply ripple and why does it matter

Every modern power supply, especially switching regulators that convert mains electricity efficiently, leaves behind small unwanted residues on its output voltage. This phenomenon is known as ripple, a residual AC fluctuation riding on top of the DC voltage delivered to electronic components. In practice, this means that the voltage expected to be completely flat actually exhibits small periodic oscillations synchronized with the power supply internal switching frequency. If you are designing or repairing a sensitive analog circuit, such as an audio amplifier or an analog-to-digital converter, this residue can cause audible noise, unstable readings, and intermittent failures that are notoriously difficult to track down.

To understand the difference between fundamental ripple and high-frequency noise, it is helpful to examine the waveform on the oscilloscope screen. Fundamental ripple has a lower frequency and directly correlates with how fast the internal transistors switch on and off to transfer energy. High-frequency noise, often referred to as spikes, arises from extremely fast switching transitions and the parasitic inductances of internal components. Grasping this distinction is the first step toward performing reliable measurements and knowing precisely what is interfering with your prototype behavior on the electronics workbench.

Frequently, instability issues in embedded systems or microcontrollers are mistakenly blamed on software bugs when the true culprit is a power rail polluted by excessive voltage ripple. When the supply voltage dips below the chip tolerance threshold during peak current draws, the system suffers spontaneous resets or unexplained freezes. To expand your knowledge on signal behavior on the bench, you can also check out our guide on how to measure motherboard clock with an oscilloscope, where we cover similar fast-signal capture techniques.

The DC coupling trap and the importance of AC setting

The biggest mistake made by beginners when trying to view ripple on an oscilloscope is keeping the channel configured in DC coupling mode. When you measure a 5-volt power supply with DC coupling active, the instrument displays the entire DC voltage right in the center of the screen. Because ripple typically has an amplitude of only tens or hundreds of millivolts, the vertical scale of the scope must be widened to accommodate the full 5 volts, causing the tiny ripple to visually disappear within the thickness of the trace. It is like trying to spot a grain of dust on a mountain using a camera positioned miles away.

The solution to this technical dilemma is to switch the oscilloscope channel coupling from DC to AC via the equipment menu. AC coupling inserts a capacitor in series with the channel input, completely blocking the DC voltage component (the main 5 volts) while allowing only alternating variations (the ripple and noise) to pass through. With this simple adjustment, you can turn the vertical sensitivity knob to much finer values, such as 10 millivolts or 20 millivolts per division, magnifying the noise image on the screen and enabling a detailed analysis of its true amplitude.

Keep in mind that when using AC coupling, the oscilloscope acts essentially as a high-pass filter, attenuating very low frequencies below a few Hertz. For the vast majority of switching power supply analyses, whose operating frequencies range from tens to hundreds of kilohertz, this attenuation does not interfere with practical results. Furthermore, if you are investigating thermal or load problems on other power rails, complementary diagnostic techniques, such as those discussed in how to diagnose power supply noise with an oscilloscope, help map systemic faults on the circuit board.

Eliminating the parasitic antenna from the probe

Another critical detail that invalidates many bench ripple measurements is the incorrect use of the standard oscilloscope probe, specifically that long alligator clip used as a ground lead. In theory, the alligator clip should zero the electrical reference, but in high-frequency practice, it behaves like an inductive magnetic loop. This long wire easily picks up all electromagnetic radiation present in your workbench environment, including 60 Hz mains hum, fluorescent lighting interference, and radio waves. The result on the oscilloscope screen is a signal full of aggressive spikes that seem to come from the power supply, but are actually noise picked up out of the air.

To eliminate this unwanted antenna effect, engineers use a simple mechanical trick: they remove the plastic cap and hook tip from the probe, exposing the central metal needle, and snap a small grounding spring onto the base of the BNC connector. With this spring, the ground reference point is just a few millimeters away from the probe tip. In practice, you touch the needle to the positive output pin of the power supply and rest the grounding spring directly on the nearest ground pin. This closes an extremely short circuit, almost entirely eliminating external interference pickup and revealing the true profile of the power supply noise.

If you do not have the specific spring provided with the oscilloscope, an acceptable DIY alternative is to remove the hook and wrap a short stripped wire around the metal shielding of the probe, forming a small makeshift ground hook. The fundamental principle remains the same: minimize the ground loop area as much as possible to prevent the induction of stray electromagnetic fields. This exact caution regarding external noise is vital when dealing with sensitive control signals, such as when you how to diagnose PWM signal noise with an oscilloscope in automation and motor drive projects.

Step-by-step guide to performing proper bench measurements

Now that you understand the fundamental concepts of coupling and external interference attenuation, it is time to get hands-on and perform the actual bench measurement in a structured way. Carefully follow the procedure below to ensure clean and reliable results:

1. Turn on the oscilloscope and set the probe attenuation correctly, usually adjusted to 1X to maximize sensitivity for low-amplitude signals.
2. Connect the modified probe with the ground spring to the output of the switching power supply, touching the needle to the voltage pin and the spring to the nearest ground.
3. Set the oscilloscope channel to AC coupling mode and limit the channel internal bandwidth to 20 MHz to filter out irrelevant spurious noise.
4. Adjust the time base knob to display a few complete cycles of the power supply switching frequency on the screen.
5. Use the oscilloscope automatic peak-to-peak measurement tools to record the exact value of the generated ripple.

Using the 20 MHz bandwidth limit is a universal recommendation among hardware engineers because it eliminates high-frequency components that lie beyond the range of interest for standard power supplies, preventing alarmist and unreal readings. If after performing this procedure you notice that the ripple level exceeds the specifications of the powered components, you will need to investigate the health of the output filtering capacitors on the board.

Final considerations on power integrity

Measuring ripple correctly with an oscilloscope is an indispensable skill for any technician or engineer working with hardware development and maintenance. Mastering AC coupling, discarding the alligator clip in favor of short ground connections, and applying bandwidth limits transforms a confusing diagnosis into a precise and repeatable analysis. With these precautions, you avoid false positives and can accurately identify when a power supply needs improvements in its filtering design or the replacement of worn components.

Maintaining the quality of the power supplying your circuits is the foundation for ensuring the longevity and operational stability of any electronic system. Whether in adjustable bench supplies or embedded converters on complex boards, attention to detail during measurement makes all the difference between reliable equipment and a chronic source of bench headaches. Continue practicing these techniques in different scenarios to gain increasing agility and precision in your daily diagnostics.