Power Supply Ripple: What It Is, How to Measure It, and When It Matters

A power supply datasheet says ripple is “≤100 mVp-p.” You put a scope on the output and see a waveform that looks nothing like a clean 100 mV sawtooth, and a hobbyist down the hall just measured 1.4 Vp-p at 5 V on his new bench supply and wants to know if it is broken (Reddit r/AskElectronics, 2025). Both of you are looking at the same page of the story: the word “ripple” is doing too much work. Under it hide four different signals with four different causes, four different measurement problems, and four different ideas of “acceptable.” This guide separates them, then gives you a framework for deciding how clean your DC rail actually needs to be.

What Power Supply Ripple Actually Is: The Four Signals Hiding on Your DC Output

Ripple is the residual periodic variation of DC voltage on a power supply output (Wikipedia, “Ripple (electrical)”). That definition is correct but useless by itself, because what you see on a scope is never one signal. Four components ride on that DC rail, and only one of them is what datasheets call ripple:

Why does the split matter? The four components differ in frequency by three to four orders of magnitude, they need different measurement setups, they damage different loads, and a single “≤100 mVp-p” line covers exactly one of them. Ripple factor (the old RMS ratio RF = V~AC / V~DC) describes only the smooth periodic part too, and because it is an RMS ratio it always looks bigger than the mVp-p number people quote today. That is why the two never seem to agree.

One quick detour, because search results keep pulling it in: “alternator ripple,” the AC content on a car’s charging output that technicians check to judge diode health, is a different measurement on a different system. Same word, different discipline; this guide stays on power supplies.

How to Measure Power Supply Ripple: Getting a Number You Can Trust

Most “bad ripple” readings are bad measurements. Three settings decide whether your number means anything:

• Coupling: use AC coupling so the DC level is blocked and only the residue is amplified. A surprising number of guides forget to say this (EEVblog forum, 2021).
• Bandwidth: the 20 MHz limit on many scopes comes from older test specifications. It hides the spiky, high-frequency components of modern switching supplies. If you are comparing against a datasheet, use the same bandwidth the vendor used (often 20 MHz); if you are hunting a real problem, open the full bandwidth. The truth can look frightening.
• Probe connection: the long ground lead on a standard probe is an antenna. Its ground loop picks up magnetic fields from the switching currents and shows you phantom spikes. That is exactly why “I added capacitors and the ripple is still high” is usually a probe problem, not a power supply problem (Electronics StackExchange, 2024). Use a tip-and-barrel contact, a paperclip ground, or a 50 Ω coaxial termination close to the output capacitor (Analog Devices AN-1144).

One instrument deserves its own warning.

It is the one most people reach for first:

The “two capacitors at the end of the cable” trick that circulates on video platforms is a judgment call: it simulates a connected load, which is more realistic than a bare supply, but it also filters out some of what you may actually need to see (EEVblog forum, 2021). Decide first which of the four signals you are chasing. Then pick the setup that lets it through.

How Much Power Supply Ripple Is Acceptable? It Depends on the Load

Ask “what ripple is acceptable” and you will get contradictory answers: absolute millivolts (the ATX spec allows 120 mVp-p on the +12 V rail and 50 mVp-p on 5 V/3.3 V rails, per Tom’s Hardware), percentages (1% is a popular rule of thumb; ARRL handbooks historically recommended better than 5% for a CW transmitter and 3% for linear amplifier plate supplies, per WSJT-X groups.io, 2021), and ripple factor. None of them is wrong; each measures a different thing for a different load. The only framework that survives contact with reality sorts the question by what you are powering.

The four load bands: who cares, and how much

• Actuation and immunity (relay coils, lamps, pumps, contactor cabinets): ripple here is essentially irrelevant. These loads have huge operating margins; a 100 mV wiggle on a 24 V rail is nothing to them.
• Digital logic: most logic boards carry their own local regulation, which absorbs rail ripple before it reaches the silicon. What actually hurts logic is transient droop at the point of load. That is a transient problem, not a ripple-spec problem.
• Analog measurement and data acquisition: 4–20 mA loops, transmitters, ADC front ends. Here ripple walks straight into the error budget. A rule of thumb voiced by practicing engineers is that 30 mV is tolerable for general use but far too much for low-voltage precision logic at 1.8 V (Electronics StackExchange, 2020); the same reasoning scales to any analog front end whose reference and signal share the rail.
• RF and audio: the most demanding band, where ripple and noise become hum and spurs. This is the world of the ARRL percentages above.

Why the three “acceptable” scales never agree

Millivolts, percent, and ripple factor measure different quantities on different rails. A 100 mV ripple is 2% of a 5 V rail and 0.4% of a 24 V rail. The same physical number flips verdicts depending on which scale you read. Ripple factor makes it worse: because it is an RMS ratio, a supply that looks exemplary as “0.07” (7%) in that metric looks alarming next to a 50 mVp-p line. Before comparing any two numbers, put them on the same rail, in the same metric, under the same conditions. Most “conflicting advice” evaporates when you do.

A working matrix for your own rails

The whole framework in one sentence: a rail that is perfectly fine for a relay cabinet can be the reason a 4–20 mA transmitter misbehaves. Classify the load first, then argue about millivolts.

One note on honesty: none of these bands has an official universal threshold, because every load’s datasheet is the real authority, so the matrix gives you where to look, not a number to trust blindly. And remember what the spec never covers: aging. As output capacitors age, ESR rises and filtering degrades, so a supply that passed at year one can drift noisier by year five. Call it the “it used to be fine” failure mode.

Reading the Ripple Spec: What Datasheet Numbers Do and Don’t Tell You

A spec row like “Ripple & noise: 120 mVp-p” looks comparable across suppliers. It is not, until you know the conditions.

The number is a photograph of a test setup

Take a common class of DIN-rail 75 W supplies: the Mean Well NDR-75 series lists 80 mVp-p at 12 V output, 120 mVp-p at 24 V, and 150 mVp-p at 48 V. What the distributor’s table does not show, and what the full datasheet usually states in fine print, is the measurement recipe: bandwidth (typically 20 MHz), load condition (often full load), and termination method (the 50 Ω coaxial technique in Analog Devices’ application note, AN-1144). Two supplies can both honestly claim “120 mVp-p” and behave differently at your load current, because one measured at 50% load and the other at 100%, or one at 20 MHz and the other wide open.

Where cheap and premium supplies actually diverge

Here is the pattern worth knowing: on the base switching ripple, budget and brand-name supplies in the same class land in the same millivolt neighborhood: 80 to 150 mVp-p on 12–48 V rails is typical across the market. The gap appears in the other three signals. Noise spikes are not specified (or are specified away with bandwidth limits); transient response is not in the ripple row; and condition transparency (whether the vendor tells you the recipe at all) varies widely. So when a price difference buys you something, it is usually max-versus-typ discipline, batch consistency, honest test conditions, and a warranty behind the number, not a lower ripple figure.

Five questions to ask any power supply vendor

And the boundary nobody prints: when filtering cannot fix it. If you have already tried output capacitors and the “ripple” stayed high, you were almost certainly chasing noise spikes, not ripple. Capacitors at the output can smooth the periodic component but will not remove spikes whose bandwidth overlaps the switching signal (Electronics StackExchange, 2024). For precision loads, the honest answer is often a post-regulator (an LC filter or an LDO after the switching supply) rather than a more expensive switching supply with a prettier number.

Field Troubleshooting: “Dirty” 24 V Killed My Sensor — Is It Ripple or Not?

When a 4–20 mA transmitter keeps burning out, or an analog reading drifts, the field instinct is right: suspect the power supply first. “First thing I’d check is the power supply — make sure it’s a clean, stable 24 VDC without spikes or ripple; dirty power can toast sensors fast,” as one r/PLC technician put it while diagnosing a pressure transmitter that had fried two replacements in days (Reddit r/PLC, 2025). But notice what the checklist in that thread actually hunted: spikes when contactors and motors kicked in, grounding and bonding, VFD leakage, not the ripple spec. That is the four-signal distinction arriving in the field.

1. Scope the rail across switching events: look at what happens the instant a contactor or motor in the cabinet operates, not just at the steady waveform.
2. Check the 0 V reference: is the supply’s negative tied to a ground that carries current from other equipment? A “0 V” that moves under load is an injection problem, not a ripple problem.
3. Watch for external sources: VFD-driven motors, welding nearby, corroded bonding on a process line. One r/PLC case traced repeated sensor deaths to a ground that had corroded off a VFD-driven pump motor physically connected to the same tank (Reddit r/PLC, 2025).
4. Run the isolation experiment: power the suspect sensor from a separate, dedicated supply (or through a signal isolator). If it survives there, the supply was innocent of the symptom; if it dies there too, look at the sensor and the loop.

The boundary runs the other way too: plenty of dead sensors have nothing to do with the supply. Ground loops, a VFD’s common-mode leakage, welding during a shutdown, and the sensor itself all masquerade as “dirty power.” The four steps exist precisely to keep you from replacing a good supply with a better one and calling the job done. That is the classic expensive non-fix.

For Distributors: Sell Diagnosis, Not the Lowest Millivolts

If you stock power supplies, everything above compresses into one commercial fact: the customer who calls about a burned sensor or a drifting loop is not asking which supply has the lowest ripple number. They are asking whether the supply they have is the problem, and what to buy instead. The components you sell sit on both sides of that question.

So the sales logic follows the technical logic. For immunity-class loads (relays, lamps, contactor cabinets, the bulk of the repair-and-maintenance market), ripple is not a selling point. Compete on price, availability and durability, and stop wasting good spec sheets on customers who will never see the difference. For sensitive loads (4–20 mA loops, transmitters, analog cards, encoders), sell spec transparency instead of the lowest millivolt. Hand over the max value (not typ), the bandwidth it was measured at, and the load condition; explain what the number does not cover (noise, transients). That is the difference between a commodity transaction and a consultative one, and it is the part of the conversation that cannot be undercut by a catalog price.

The same logic protects your after-sales: when a customer reports a failure, walk the field triage before offering a replacement supply. A sensor killed by spikes and transients will kill the replacement too. The honest “this is not the supply, here is what to check” answer is what makes a distributor worth calling back. And keep in mind the whole chain sits on one shelf: the power supply, the sensor, and the relay that switches the load are the same order, the same cabinet, and often the same diagnosis.

If you need to compare DIN-rail switch-mode power supplies across a range from 15 W to 480 W, or want someone to work through a selection with you, OMCH’s switch-mode power supply range and product selection support are a practical place to start.

References

1. Wikipedia. “Ripple (electrical).” https://en.wikipedia.org/wiki/Ripple_(electrical)
2. Analog Devices. “AN-1144: Measuring Output Ripple and Switching Transients in Switching Regulators.” https://www.analog.com/en/resources/app-notes/an-1144.html
3. Tom’s Hardware. “Ripple Voltage — How We Test Power Supply Units.” https://www.tomshardware.com/reviews/how-we-test-psu,4042-5.html
4. EEVblog Forum. “Measure power supply ripple with this technique.” https://www.eevblog.com/forum/beginners/measure-power-supply-ripple-with-this-technique/
5. Electronics StackExchange. “Switching power supply has high ripple voltage no matter how many capacitors are added.” https://electronics.stackexchange.com/questions/707711/
6. Electronics StackExchange. “Output voltage ripple considerations.” https://electronics.stackexchange.com/questions/501187/
7. Reddit r/PLC. “4-20 mA pressure transmitter help.” https://www.reddit.com/r/PLC/comments/1kragst/420_ma_pressure_transmitter_help/
8. Reddit r/AskElectronics. “What is acceptable noise/ripple on cheap switch-mode supplies?” https://www.reddit.com/r/AskElectronics/comments/1gm8h7m/
9. WSJT-X Groups.io. “Acceptable PA Power Supply Ripple?” https://wsjtx.groups.io/g/main/topic/acceptable_pa_power_supply/71775932
10. OMCH. “Switch Mode Power Supply.” https://www.omch.com/switch-mode-power-supply/
11. OMCH. “Product Selection.” https://www.omch.com/product-selection/
12. OMCH. Home. https://www.omch.com/