- Learn what power supply ripple is and why it matters.
- Discover the tools needed for accurate ripple testing.
- Follow step-by-step oscilloscope measurement procedures.
- Understand RMS, peak-to-peak, ripple factor, and PARD.
- Identify low-frequency and high-frequency ripple patterns.
- Improve power supply reliability through proper testing.
Ripple voltage is a crucial parameter that determines power supply performance, especially when you need stable voltage to power sensitive equipment. In this article, you’ll explore how to measure industrial power supply ripple and how to interpret results. Understanding this can help keep your power supplies reliable and functional.
What Is Power Supply Ripple?
Power supply ripples are those variations that occur in the power supply output voltage after rectification. They usually occur as a result of the AC power supply before conversion into DC.
Power supply ripples are calculated in terms of AC voltages that are placed atop the DC voltage supply. These have the capacity to influence the performance of some electronic circuits.
Key Ripple Metrics You Need to Know
- Ripple voltage (mV peak-to-peak, mV RMS):Peak-to-peak is the entire swing from top to bottom; RMS is the effective value and is always lower. They are not the same thing, so when comparing power supplies, make sure that what the data sheet states is the same thing.
- Ripple factor (RF):RF = Vac / Vdc. The ripple factor is what normalizes ripple to the output voltage, The ripple factor expresses the RMS value of the AC component relative to the DC output level. The lower the ripple factor, the better and cleaner the power supply.
- Ripple and noise (PARD):The ripple and noise measurement accounts for any noise peaks that a simple ripple value does not catch. Compare test conditions (bandwidth and load) before comparing two PARD values.
- Typical ripple ranges for industrial power supplies:As a rough starting point, 24V industrial supplies often fall somewhere around 1-2% ripple and noise, which can work out to roughly a few hundred millivolts Vpp depending on the specific unit and how it’s tested. Although this is a good rough guide, it is not an absolute rule.
Tools You'll Need to Measure Ripple
- Oscilloscope:An oscilloscope allows viewing the ripple waveform directly in the time domain: the waveform’s shape, amplitude, and rough oscillation rate. Choose a scope that offers good vertical resolution and a bandwidth of at least 100 MHz, even though you will be reducing the bandwidth in practice (as mentioned below).
- Spectrum analyzer:As opposed to an oscilloscope, which plots amplitude over time, the spectrum analyzer plots amplitude over frequency. In case you have to determine where the noise comes from, this device is extremely helpful, especially when considering electromagnetic interference from the neighboring equipment (such as contactors, VFDs, and welding machines). If you see something suspicious in your ripple results, the spectrum analyzer will help you distinguish the true switching ripple from the external noise interference.
- Differential probes.If you’re measuring on a high-voltage rail, or in a system with significant common-mode noise (which describes a lot of industrial panels), a standard single-ended scope probe can introduce errors or, worse, create a ground loop that damages equipment. A differential probe measures the voltage difference between two points directly, without referencing your scope’s chassis ground, which makes it a safer and more accurate choice for these situations.
How to Set Up an Industrial Power Supply for Ripple Measurement
Connect the power supply to a controlled load
Ripple changes with load. A supply that looks perfectly clean at no load can show significantly more ripple once you put it under real current draw, especially near its rated maximum. Use an electronic load or a resistive load bank that lets you dial in a specific current, ideally matching the actual operating current your application will draw. If you do not measure at a load, you’ll not get accurate results to know what the real condition is.
Connect the oscilloscope across the DC output
Attach your probe directly across the positive and negative output terminals, or better yet, as close to the output capacitor as physically possible. The closer you probe to the source of the ripple, the less you’re picking up noise or voltage drop from cabling in between.
Verify the test environment
Industrial panels are noisy places, electrically speaking. Before you trust your reading, take a moment to consider what else is running nearby. VFDs, contactors, relay coils, and welding equipment can all radiate EMI that gets picked up by your probe and shows up looking like ripple. If possible, route your probe cable away from high-current wiring, and be skeptical of any reading that looks unusually spiky or erratic compared to what the datasheet promises.
How to Measure Power Supply Ripple With an Oscilloscope
Step 1: Verify the DC output voltage
Before chasing ripple, confirm the supply is actually outputting the correct nominal voltage under your test load. If the DC level itself is off, fix that first. If there is a voltage regulation problem, it can mask or mimic what looks like a ripple issue.
Step 2: Set the oscilloscope to AC coupling
This is the most important aspect to get right when measuring ripple. AC coupling filters out the DC component of the signal and allows you to use the full vertical resolution of the oscilloscope for measuring the AC ripple present in the signal. Without AC coupling, the ripple would appear as an invisible line below the mostly flat line of DC. This would affect readings, making it possible to find out what the cause of the ripple is.
Step 3: Select an appropriate vertical scale
Start with a coarse measurement of 500mV/div and then go down to 10-50mV/div after seeing the waveform. If the measurement is too coarse, it would make the ripple look like a flat line. On the other hand, if the measurement is too fine, it would clip off the top, giving you an incorrect measurement. The correct vertical scale will help you detect the problem and give a proper solution.
Step 4: Set the time base
Set it fast enough to resolve several full ripple cycles; microseconds-per-division for switching ripple, a slower setting for 100/120Hz line ripple. If you do not set the time base promptly, you’ll either miss the switching ripple entirely or misread its frequency, which makes it harder to trace the noise back to its actual source.
Step 5: Apply an appropriate bandwidth limit
A 20MHz bandwidth limit is a common measurement condition in the industry, not a way to strip out “all noise that isn’t ripple.” It simply sets the upper frequency range your scope will capture, matching how many manufacturers define their ripple and noise test setup.
Failing to do so results in measuring all the noise present in the environment and adding this to the signal, thus getting an exaggerated value and making a perfectly fine supply appear to be below spec. Limiting to 20MHz makes sure that the measurement is equivalent to the specification provided.
Step 6: Capture and measure the waveform
Take measurements using the built-in features on the oscilloscope for both the peak-to-peak and RMS values and make sure to take a few samples at different loads rather than taking a single sample at a single load. A single reading taken at one particular load may miss any ripple present in the load swing.
How to Calculate Power Supply Ripple
RMS ripple voltage can be considered as an effective continuous equivalent of the ripple wave, or in other words, as a value of “real” AC riding on your DC output signal. This value is much more representative when we are interested in heating aspects or the reaction of some analog downstream circuitry.
RMS and peak-to-peak values aren’t interchangeable. They are well defined and predictable. So don’t try to eyeball a conversion. Hence, if your specification or test procedure mentions RMS values, then you should take those values. On the contrary, if peak-to-peak is required, then this parameter should be taken.
Always use the value specified by the test standard you’re working against. Otherwise, you might fail a spec check that your supply actually meets.
Ripple as a Percentage of DC Output
Once you have your ripple voltage, expressing it as a percentage of the DC output makes it easier to judge at a glance and easier to compare across different voltage rails. The formula is straightforward:
Ripple (%) = (Ripple Voltage / DC Output Voltage) × 100
For example, a 24V supply with 200mV of peak-to-peak ripple works out to roughly 0.83% ripple. This percentage view is especially useful when you’re evaluating a supply against a general industrial guideline (often somewhere around 1-2%).
How to Interpret the Ripple Waveform
Low-Frequency Line Ripple
This shows up as a smooth, rounded waveform cycling at the line frequency or its double (100Hz or 120Hz depending on region). This could result from filtering capacitors that are undersized, aging, or simply not doing their job anymore. If you see this pattern growing worse over time on a unit that’s been in service for years, it means the capacitors are drying out. If this problem isn’t fixed, sensitive equipment won’t receive a clean power supply that would prolong their lifespan.
High-Frequency Switching Ripple
This waveform cycles at the supply’s switching frequency, often tens or hundreds of kHz, and tends to have a sharper, more angular shape than line ripple, sometimes with a visible spike right at the switching transition. Some amount of this is completely normal and expected in any switching supply. The question is magnitude: if it’s within spec and stable across load conditions, you’re fine.
If it’s climbing or unusually large, it may point to an aging output capacitor or a control loop that isn’t in full function. If this is not addressed, the damage will spread from the capacitor to the connected device.
Conclusion
Ripple is easy to overlook and expensive to ignore. However, measuring an industrial power supply ripple can save you hours of repair and ensure the system continues to function efficiently. At Anssin Electric, we design and test our industrial power supplies for your equipment. Reach out to us, and we can help you find the right power supply for it.
FAQs
The best way to test the ripple of a power supply is by having a 20 MHz limit of the bandwidth of the oscilloscope. The reason behind this is that it filters out high-frequency broadband and switching noise and gives the essential low-frequency ripple.
An acceptable amount of ripple voltage for an industrial power supply is 1% or less of the output voltage of the supply (for example, 24V DC). Sensitive devices such as sensors, analog circuits, and microprocessors require 0.1% or less (24 mV or less) of ripple voltage to operate correctly.
Ripple voltage of a power supply must be tested in peak-to-peak for worst-case voltage and component stresses, and RMS for thermal and total energy AC. Peak-to-peak is mostly mentioned in the data sheets of components.