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After rectification the output is DC with an AC "ripple" riding on top. Two dimensionless numbers describe its quality. The form factor (FF) is the ratio of the RMS value to the average value, FF = Vrms/Vdc. The ripple factor (RF) measures how much AC ripple remains relative to the DC: RF = Vac(rms)/Vdc = √(FF²−1). A perfect DC source has FF = 1 and RF = 0.
| Quantity | Formula |
|---|---|
| Form factor | FF = Vrms / Vdc |
| Ripple factor | RF = √(FF² − 1) |
| AC ripple (RMS) | Vac = √(Vrms² − Vdc²) |
| Rectification efficiency | η = (Vdc/Vrms)² = 1/FF² |
| Rectifier | Form factor | Ripple factor |
|---|---|---|
| 1φ Half-wave | 1.571 | 1.21 (121%) |
| 1φ Full-wave | 1.111 | 0.482 (48.2%) |
| 3φ Half-wave | 1.017 | 0.183 (18.3%) |
| 3φ Full-wave bridge | 1.0009 | 0.042 (4.2%) |
Ripple factor (RF) is the ratio of the RMS value of the AC ripple component to the DC (average) value of a rectified output: RF = Vac(rms)/Vdc = √(FF²−1). A smaller ripple factor means smoother, cleaner DC.
Form factor (FF) is the ratio of the RMS value to the average value of a waveform: FF = Vrms/Vdc. For a pure DC it equals 1; the more ripple present, the larger it becomes.
They are linked by RF = √(FF²−1). Equivalently FF = √(1+RF²). Knowing either one gives the other.
About 1.21 (121%). The half-wave output is very rough because half of each cycle is missing, so it needs substantial filtering to be usable.
About 0.482 (48.2%). Full-wave rectification uses both half-cycles, roughly halving the ripple compared with half-wave and doubling the ripple frequency.
For sensitive electronics you typically want the ripple factor well below 1% after filtering. Power supplies often specify ripple as tens of millivolts on a several-volt rail, which corresponds to a fraction of a percent.
Add filtering: a smoothing capacitor, an LC filter, or a voltage regulator. Using a full-wave instead of half-wave rectifier and increasing the ripple frequency (3-phase, higher pulse number) also reduces it.
It is the fraction of the total output power that is useful DC power: η = (Vdc/Vrms)² = 1/FF². The theoretical maximum is 40.6% for half-wave and 81.1% for full-wave rectifiers.
Yes. A half-wave rectifier has a ripple factor of 1.21, meaning the RMS of the ripple actually exceeds the DC value. This shows how poor unfiltered half-wave rectification is.
Use a DC-coupled meter (or the DC/average mode) for Vdc, and a true-RMS meter (AC+DC mode) for Vrms. An oscilloscope can also read the average and RMS of the waveform directly.
A 6-pulse bridge produces six output humps per cycle, so the output never drops far from the peak. Its ripple factor is only about 4.2%, which is why large drives and chargers use three-phase rectification.
No. Ripple voltage is the actual AC amplitude (in volts, often peak-to-peak). Ripple factor is a dimensionless ratio of that ripple (RMS) to the DC value, so it lets you compare supplies of different voltages.
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