AC ripple is the small amount of alternating current (AC) left over in a direct current (DC) power supply after conversion. When you plug a device into a wall outlet, the power supply converts the AC from the wall into the steady DC your electronics need, but that conversion is never perfect. A residual wave pattern rides on top of the DC output, rising and falling slightly like a tiny wobble in what should be a flat, steady voltage. That wobble is AC ripple.
How Ripple Gets Created
Wall power in the US alternates at 60 Hz, meaning the voltage swings positive and negative 60 times per second. A power supply uses components called rectifiers to flip the negative swings so they all point in the same direction, producing pulsating DC. But this pulsating output still rises and falls rather than staying perfectly flat.
How much it pulses depends on the rectifier design. A half-wave rectifier, which only uses half of the AC cycle, produces ripple at the same frequency as the input: 60 Hz. A full-wave rectifier uses both halves of the cycle, doubling the ripple frequency to 120 Hz. The full-wave version is far more common in real-world power supplies because the faster, smaller pulses are easier to smooth out.
Measuring Ripple
Engineers quantify ripple in two main ways. The simplest is peak-to-peak ripple voltage: the difference between the highest and lowest points of the wobble, usually expressed in millivolts. The other is the ripple factor, which compares the AC component to the DC component as a ratio or percentage. A lower ripple factor means cleaner DC power.
The ripple factor formula is straightforward: divide the AC voltage by the DC voltage. An unfiltered half-wave rectifier has a ripple factor of about 1.21 (121%), meaning the AC component actually exceeds the DC component. A full-wave rectifier starts at roughly 0.48 (48%). Both figures drop dramatically once filtering is added.
For battery charging systems, ripple is sometimes calculated differently. The peak-to-peak method takes the difference between the maximum and minimum voltage, divides by their sum, and multiplies by 100 to get a percentage. Battery chargers generally aim to keep this figure below 15%.
How Filtering Reduces Ripple
The most basic filter is a capacitor placed across the output. The capacitor charges up during each pulse and releases energy between pulses, filling in the valleys of the waveform and producing a much smoother DC output. The ripple voltage that remains after filtering follows a simple relationship: it depends on how much current the connected device draws, the frequency of the ripple, and the size of the capacitor.
For a full-wave rectifier, the remaining ripple voltage equals the load current divided by twice the line frequency times the capacitance. Larger capacitors store more energy and produce less ripple. This is why power supplies for sensitive equipment use physically large capacitors or multiple stages of filtering.
Beyond simple capacitors, voltage regulators provide a second line of defense. These circuits actively monitor the output and correct for fluctuations. Their effectiveness is measured by something called power supply rejection ratio (PSRR), expressed in decibels. A regulator with 60 dB of PSRR reduces ripple by a factor of 1,000. Good regulators maintain high rejection across a wide frequency range, from 10 Hz up to 10 MHz, which matters in wireless and radio-frequency applications where even tiny ripple at certain frequencies can cause interference.
Why Ripple Matters for Electronics
A small amount of ripple is normal and harmless in most circuits. But excessive ripple causes real problems. The most direct victim is the electrolytic capacitor itself. Ripple current flowing through a capacitor generates internal heat due to the component’s internal resistance. That heat accumulates, raising the capacitor’s core temperature. Since electrolytic capacitors degrade faster at higher temperatures, excessive ripple current shortens their lifespan, sometimes dramatically. This is one of the most common failure modes in aging power supplies: the capacitors dry out, ripple increases, and performance spirals downward.
In digital circuits, ripple can introduce noise that causes erratic behavior, data errors, or timing glitches. In audio equipment, ripple manifests as an audible hum, typically at 60 Hz or 120 Hz depending on the rectifier type. In precision measurement instruments or medical devices, even millivolts of ripple can compromise accuracy.
Effects on Batteries
AC ripple also affects batteries during charging. When a charger with significant ripple charges a lithium-ion battery, the alternating component forces the battery’s internal chemistry to repeatedly absorb and release energy at the ripple frequency. Research using artificial AC waveform signals to simulate poor-quality fast chargers found that high-amplitude current ripple can accelerate battery degradation by up to 15%. That study, which tested commercial electric vehicle batteries, linked the ripple effect to increased internal impedance, the same marker associated with 20% capacity loss over a battery’s lifetime.
This is particularly relevant for electric vehicles and large battery storage systems, where charger quality varies. A cheap or failing charger with excessive ripple doesn’t just charge slowly or inefficiently. It actively wears out the battery faster than a clean power source would.
How Much Ripple Is Acceptable
There is no single universal standard, and the acceptable level depends entirely on the application. A general-purpose bench power supply might allow ripple in the tens of millivolts. A high-quality lab power supply typically keeps ripple below 1 millivolt. Sensitive RF and wireless systems require even less.
For practical purposes, the goal is always to keep ripple as low as the application demands without over-engineering the power supply. A circuit powering LED lights can tolerate far more ripple than one running a precision analog-to-digital converter. If you’re troubleshooting a circuit and suspect ripple is a problem, an oscilloscope set to AC coupling on the DC output will show you the ripple waveform directly, letting you measure both its amplitude and frequency.