Ripple voltage is the small AC signal left over on a DC power supply’s output, and you can measure it with either an oscilloscope or a multimeter set to AC volts. An oscilloscope gives you the most complete picture, showing the waveform shape and peak-to-peak amplitude, while a multimeter provides a quick RMS reading. The method you choose depends on how precise you need to be and what equipment you have on hand.
What Ripple Voltage Actually Is
Every DC power supply output is really two components stacked together: a steady DC voltage and a small AC voltage riding on top of it. That AC portion is the ripple. In a well-designed power supply, ripple typically sits between a few millivolts and a few percent of the total output voltage. For context, the ATX specification for computer power supplies caps ripple at 120 mV on the 12V rail and 50 mV on the 5V and 3.3V rails, though modern units usually come in well below those limits.
Ripple matters because sensitive digital circuits expect a clean, stable voltage. Excessive ripple can cause timing errors, audible noise in audio equipment, or unstable behavior in microcontrollers. Measuring it tells you whether your power supply is doing its job or whether additional filtering is needed.
Measuring Ripple With an Oscilloscope
An oscilloscope is the preferred tool because it shows you the actual waveform, letting you read peak-to-peak amplitude, frequency, and shape. Here’s how to set one up for a ripple measurement.
Equipment You Need
Use a standard 10:1 passive probe for most measurements. If you need high accuracy or are working with floating supplies, a differential probe is better. Connect a resistive load to the power supply output to stabilize the regulator, since many supplies behave differently under no-load conditions.
Use AC Coupling
This is the single most important setting. Switch the oscilloscope channel to AC coupling mode. AC coupling inserts a high-pass filter on the input that strips away the DC component entirely, leaving only the AC ripple on screen. Without it, you’d be trying to see a 50 mV ripple sitting on top of a 12V DC level. Your scope would need an extremely sensitive volts-per-division setting combined with a huge vertical offset, and most scopes simply can’t offset far enough at fine voltage scales to make that work. AC coupling solves the problem by removing the DC entirely, so you can zoom in on the ripple as much as you want.
One caution: AC coupling can attenuate very low frequency signals. A 10 Hz component, for example, might drop noticeably in displayed amplitude. For typical switching power supply ripple (tens of kHz to hundreds of kHz), this isn’t a concern. But if you’re measuring ripple from a 50 or 60 Hz rectifier circuit, be aware the reading could be slightly low.
Enable the 20 MHz Bandwidth Limit
Most oscilloscopes have a bandwidth limit option, often labeled “BW Limit” or “20 MHz.” Turn it on. Power supply ripple is a low-frequency phenomenon, typically at the switching frequency of the converter (a few hundred kHz or less). The capacitors in a power distribution network naturally form low-pass filters, so the meaningful ripple content sits well below 20 MHz. Without the bandwidth limit, your scope picks up high-frequency noise from switching edges, ringing, and electromagnetic interference, all of which inflate your reading and obscure the actual ripple waveform.
Adjust the Display
After connecting the probe to the power supply output (tip to the positive rail, ground clip to ground), decrease the volts-per-division setting until the ripple waveform fills a reasonable portion of the screen. Then adjust the time-per-division to show a few complete cycles. If your oscilloscope has automated peak-to-peak measurement, enable it. Otherwise, use the cursor function to mark the highest and lowest points of the waveform manually. The difference between those two points is your peak-to-peak ripple voltage.
Measuring Ripple With a Multimeter
A digital multimeter gives you a faster, simpler reading, but with less detail. You won’t see the waveform shape or the peak-to-peak value directly. Instead, you get an RMS (root mean square) value of the AC component.
Set your meter to AC volts mode. This mode blocks the DC level automatically, similar to how AC coupling works on an oscilloscope. Select the lowest AC voltage range available. On the Fluke 88V, for instance, that’s a 600 mV AC range. Place the red probe on the positive output terminal and the black probe on ground. The displayed reading is the RMS value of whatever AC content is present on the supply output.
The key limitation is accuracy. A standard multimeter assumes incoming AC is a sine wave and calculates RMS based on that assumption. Power supply ripple is almost never a pure sine wave. It’s typically a sawtooth or triangular waveform with harmonics. A “True RMS” meter handles this correctly by computing the actual RMS value regardless of waveform shape. If you’re using a basic meter without True RMS capability, your reading could be off by 10% to 40% depending on the waveform. For anything beyond a rough sanity check, a True RMS meter is worth the investment.
Converting Between Peak-to-Peak and RMS
Oscilloscopes display peak-to-peak ripple voltage, while multimeters display RMS. These are different numbers for the same signal, and you’ll sometimes need to convert between them. For a sine wave, RMS equals the peak-to-peak value divided by about 2.83 (that’s 2 times the square root of 2). For a sawtooth wave, the conversion factor is different. In practice, most engineers just report whichever value their instrument gives them and note which type it is.
The ripple factor expresses ripple as a proportion of the DC output. It’s calculated by dividing the RMS ripple voltage by the DC voltage. Multiply by 100 to get a percentage. A 12V supply with 230 microvolts of RMS ripple, for example, has a ripple factor of about 0.002%, which is exceptionally clean. Most practical circuits are fine with ripple in the range of 0.5% to 2% of the DC output, though precision analog circuits and sensitive digital logic often need much less.
Common Measurement Mistakes
Long ground leads are the most frequent source of bad readings. The wire loop formed by a long ground clip acts as an antenna, picking up electromagnetic interference and adding noise to your measurement. For accurate ripple readings, use the shortest ground connection possible. Many probe manufacturers sell a small spring-tip ground adapter that clips directly to the probe barrel, keeping the ground loop area to a minimum.
Probing at the wrong location also skews results. Measure directly at the power supply’s output terminals or as close to the load’s power pins as possible. Long PCB traces add their own inductance and resistance, which can either increase or mask the ripple you’re trying to measure.
Forgetting to load the supply is another common oversight. Many voltage regulators have higher ripple under load than at idle. If you measure with no load connected, you’ll get an optimistically low number that doesn’t reflect real operating conditions. Use a resistive load that draws a current representative of your actual circuit’s power consumption.
Oscilloscope vs. Multimeter: Which to Use
- Oscilloscope: Best for detailed analysis. Shows waveform shape, peak-to-peak voltage, frequency, and any unusual spikes or ringing. Essential for debugging power supply designs or diagnosing intermittent problems.
- True RMS Multimeter: Best for quick checks. Gives you a single RMS number in seconds, which is enough to confirm a supply is within spec. Practical for field work, automotive alternator testing, or routine maintenance where you just need a pass/fail answer.
If you’re troubleshooting a power supply that’s causing circuit instability, the oscilloscope is worth the setup time. If you’re verifying that a known-good supply still meets its specs, a True RMS multimeter gets the job done faster.