How to measure ripple in bench power supplies with oscilloscope

Why Measuring Ripple is Crucial in Bench Power Supplies?

Bench power supplies are the heart of any electronics workbench, providing the stable power our circuits need. But not all direct current (DC) is perfectly “direct.” In practice, a small alternating current (AC) voltage ripple can overlay the ideal DC signal, and this ripple is what we call ripple. For a technician or enthusiast dealing with sensitive boards, ignoring ripple is like ignoring a leak in a hose: sooner or later, it will cause problems.

Accurate ripple measurement is a fundamental step in diagnosing the health of your bench power supply or even validating the operation of a new design. Excessive noise can lead to erratic behavior in microcontrollers, distortion in audio or RF signals, and failures in sensitive digital components. Understanding how to use an oscilloscope to identify and quantify this noise is an indispensable skill to ensure your projects receive the cleanest power possible.

What Exactly is Ripple in Power Supplies?

Ripple, in simple terms, is the residual AC voltage variation present at the output of a power supply that should be purely DC. Imagine a hose spraying water: the pressure should be constant, but if the pump were slightly irregular, there would be small fluctuations. In electronics, these fluctuations are primarily caused by the process of rectifying AC mains voltage to DC and the limited ability of filter capacitors to completely smooth out this pulsating voltage.

Power supplies typically convert AC voltage from the outlet to DC. First, a transformer reduces the voltage; then, a rectifier (usually diodes) converts it into a pulsating unidirectional voltage. To smooth out this pulsation and approach pure DC, filter capacitors are added. However, these capacitors cannot eliminate 100% of the variation, resulting in ripple. The higher the ripple, the "dirtier" the power, which can be a serious issue for components like voltage regulators, which need a clean input to function properly, as we discussed in our article on how to test the motherboard voltage regulator on the bench.

Why Measuring Ripple is More Important Than It Seems?

Measuring ripple is not just an academic exercise; it's an essential practice for the longevity and performance of any electronic circuit. Sensitive boards, such as those for high-fidelity audio, RF equipment, or high-speed digital systems, are extremely susceptible to noise on the power supply line. High ripple can manifest as audible hums, sensor reading errors, intermittent microcontroller failures, or even performance degradation in analog-to-digital converters (ADCs).

Furthermore, ripple can be an early indicator of failures in power supply components, such as dried-out filter capacitors or faulty rectifier diodes. Regular ripple measurement can save an entire project by identifying a problem before it causes irreversible damage to more expensive components. In repair environments, diagnosing a mysterious problem often starts by checking the power quality. If a circuit's clock, for example, is unstable, this could be a symptom of excessive ripple affecting the oscillator circuit's power, as we explored when testing SMD crystal oscillator with a digital oscilloscope.

Essential Tools for the Job

To measure ripple effectively, you will need some specific tools. The main one is the oscilloscope, which is essentially an "electronic eye" that allows us to visualize how voltage varies over time. Unlike a multimeter, which only measures average or RMS values, the oscilloscope displays the detailed waveform of the signal, revealing the ripple.

You will also need an oscilloscope probe. These probes usually come with an attenuation switch (1x/10x). For ripple measurements, 10x attenuation is preferred, as it offers higher input impedance, reducing circuit loading, and also better rejection of external noise. Finally, a dummy load (power resistors or an electronic load) is essential to simulate the actual current consumption of your circuit, as ripple can vary significantly with the power supply's output current.

Setting Up the Oscilloscope to Capture Ripple

Correctly setting up the oscilloscope is the most critical step to obtaining an accurate ripple measurement. The goal is to isolate the small AC component of the ripple from the large base DC voltage. Start by turning on the oscilloscope and connecting the probe.

AC Coupling: Isolating the Ripple

The first crucial adjustment is the channel coupling. Select "AC Coupling" for the channel you are using. This will cause the oscilloscope to block the DC component of the signal and display only the AC variation (the ripple) relative to the zero reference level. If you used "DC Coupling," the ripple would just be a small ripple on top of a large DC voltage, making it difficult to visualize.

Probe Attenuation and Bandwidth

Set the probe to 10x and adjust the channel's attenuation setting on the oscilloscope to match 10x. This ensures that the voltage reading displayed on the screen is correct. Many oscilloscopes also have a 20 MHz bandwidth filter. Activate it for ripple measurements, as it helps remove high-frequency noise that is not part of the fundamental power supply ripple, but rather electromagnetic interference (EMI) or switching noise that can obscure the actual ripple reading.

Time Base and Vertical Scale

Adjust the time base (Time/Div) to a scale that allows you to view several ripple cycles. For 50/60 Hz power supplies, something around 5 ms/Div or 10 ms/Div is a good starting point. Adjust the vertical scale (Volts/Div) for appropriate sensitivity. Start with 100 mV/Div or 50 mV/Div and adjust downwards as needed to fill the screen with the ripple signal.

Connecting the Probe Safely and Accurately

How you connect the probe makes all the difference. The probe's ground should be as short as possible to minimize noise pickup. Many probes come with a long ground lead; replace it with a spring adapter or a short wire with an alligator clip for ripple measurements. This helps reduce the antenna effect, which could introduce additional noise into your measurement.

Connect the probe's ground to the negative terminal of your power supply. Connect the active probe tip to the positive terminal of the power supply. If you are measuring ripple at a specific point in a circuit, ensure that the probe's ground is connected to the circuit's ground at that point. It is crucial that the power supply is under load during measurement; without a load, ripple tends to be much smaller and does not reflect the power supply's actual performance.

Step-by-Step: Measuring Ripple in Practice

With the oscilloscope configured and connections made, we are ready for measurement. Follow these steps to obtain an accurate ripple reading:

1. Connect the Load: Apply an appropriate load to your bench power supply. This can be a power resistor, a resistor bank, or an electronic load, simulating the actual current consumption that the circuit will use. Without a load, the measurement will not be representative.

2. Adjust Channel Coupling: On your oscilloscope, select the channel you are using and set it to "AC Coupling." Ensure that the probe attenuation (usually 10x) is correctly configured on the oscilloscope.

3. Adjust Vertical Scale (Volts/Div): Start with a sensitive scale, such as 50 mV/Div or 100 mV/Div. As the ripple appears, adjust so that the waveform fills most of the screen vertically, but without clipping at the edges.

4. Adjust Time Base (Time/Div): For 50/60 Hz power supplies, set the time base to around 5 ms/Div or 10 ms/Div. This will allow you to see a few cycles of the ripple waveform. For switching power supplies, you may need a faster time base, such as microseconds.

5. Activate Bandwidth Filter (if available): If your oscilloscope has a 20 MHz filter, activate it. This will eliminate unwanted high-frequency noise that is not part of the power line ripple.

6. Move the Trigger: Adjust the trigger level to stabilize the waveform on the screen. This will facilitate reading and analyzing the signal.

7. Measure Peak-to-Peak Ripple: Use the oscilloscope's measurement cursors to measure the peak-to-peak voltage (Vpp) of the ripple waveform. This is the most important value for characterizing ripple. Also note the frequency and waveform shape.

Analyzing the Waveform and Identifying Problems

The ripple waveform you observe on the oscilloscope screen can tell you a lot about the health of your power supply. Ideally, ripple resembles a sine or sawtooth wave, but variations can indicate specific problems.

Typical Ripple Waveforms

• Sawtooth: Common in half-wave or full-wave rectifier power supplies with capacitive filtering. The rising ramp is the capacitor charging, and the falling ramp is the discharge. A very steep sawtooth can indicate undersized or damaged filter capacitors.
• Sinusoidal: May appear in power supplies with more robust filtering or audio components, but a purely sinusoidal signal with significant amplitude is still undesirable.
• Spikes and High-Frequency Noise: If you observe abrupt peaks or high-frequency noise, especially in switching power supplies, this may indicate problems with diodes, switching transistors, inductors, or board layout. In these cases, the 20 MHz filter is essential to focus on low-frequency ripple.

The ripple frequency is also an indicator. In mains-rectified power supplies (50/60 Hz), full-wave ripple will have twice the mains frequency (100/120 Hz). If you see a different frequency, there may be a more serious problem or a switching power supply with a higher operating frequency.

Common Pitfalls and Best Practices to Avoid

Ripple measurement can be misleading if proper care is not taken. A common mistake is to use the long ground lead of the probe, turning it into an antenna that picks up external noise, masking the actual ripple. Always use the shortest possible ground lead.

Another mistake is not using an adequate load. Measuring a power supply without a load or with a very low load will result in artificially low ripple, giving a false sense of security. Make sure the load simulates the maximum or typical current draw of the circuit to which the power supply will be connected.

Probe compensation is also important, although less critical for low-frequency ripple. An uncompensated probe can introduce waveform distortions. Check compensation on a square test signal, adjusting it so that the wave looks like a clean square. Finally, avoid touching the probe or circuit terminals during measurement, as the human body can introduce significant noise. To explore these architectural concepts further in production, see our related technical analysis on Slow server? How to spot disk bottlenecks with iostat.

Conclusion

Mastering ripple measurement with an oscilloscope is a powerful skill for anyone working with electronics. It allows you to go beyond a simple voltage reading, revealing the quality of the power your circuits are receiving and identifying potential problems before they become catastrophes. From diagnosing dry capacitors to validating power supply designs, the oscilloscope proves to be an indispensable tool.

By following the techniques of AC coupling, correct probe attenuation, load usage, and careful waveform analysis, you will be well-equipped to ensure your devices receive clean and stable power. Remember: a well-behaved power supply is the foundation for a reliable and high-performance electronic circuit.