Calculation
Umin = Upeak − Iload/(2·f·C)
Between recharge pulses the reservoir capacitor discharges approximately linearly with load current; for bridge rectification the discharge interval is about 1/(2f).
Busch 2006, Abschnitt 7.4.2.1.1 Zweipulsgleichrichter B2; Restwelligkeit nach Näherung linearer Kondensatorentladung
Bridge rectifier ripple with reservoir capacitor
Between recharge pulses the reservoir capacitor discharges approximately linearly with load current; for bridge rectification the discharge interval is about 1/(2f).
MINTSI
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Result
Select a target and calculate.
Understand the inputs
- Minimum voltage Umin — Lowest value of the smoothed DC voltage just before the next recharge pulse.
- Peak voltage Upeak — Peak value of the rectified voltage after the bridge, approximately √2 times mains RMS voltage minus diode threshold drops.
- Load current Iload — Average, assumed-constant DC current the reservoir capacitor supplies between recharge pulses.
- Mains frequency f — Frequency of the AC supply feeding the bridge; recharge pulses occur at 2f for full-wave/bridge rectification.
- Reservoir capacitance C — Effective capacitance of the smoothing/reservoir capacitor directly after the bridge.
Example
Upeak=325 V, Iload=0.5 A, f=50 Hz and C=1,000 µF give ripple Vpp=Iload/(2fC)=5 V and Umin=320 V.
Assumptions and limits
Approximately linear capacitor discharge with constant load current and negligible recharge duration; real diode thresholds, capacitor ESR and strongly nonlinear load currents are excluded.
Understand Bridge rectifier ripple with reservoir capacitor
This calculator estimates how much the smoothed DC voltage after a bridge rectifier still fluctuates under load. It extends the basic B2 bridge topology found in introductory electronics literature with the residual ripple on the reservoir capacitor that matters for power-supply sizing.
What does this quantity describe?
After a full-wave bridge rectifier (B2 topology), each of the two half-wave pulses per mains cycle recharges the reservoir capacitor to approximately the peak voltage Upeak. Between pulses the capacitor discharges approximately linearly with the assumed-constant load current Iload over time 1/(2f), since bridge rectification produces two recharge pulses per mains cycle. This gives peak-to-peak ripple Vpp=Iload/(2fC) and minimum voltage Umin=Upeak−Vpp.
Formula and variables
Umin = Upeak − Iload/(2·f·C)
| Symbol / input | Meaning |
|---|---|
| Minimum voltage Umin | Lowest value of the smoothed DC voltage just before the next recharge pulse. |
| Peak voltage Upeak | Peak value of the rectified voltage after the bridge, approximately √2 times mains RMS voltage minus diode threshold drops. |
| Load current Iload | Average, assumed-constant DC current the reservoir capacitor supplies between recharge pulses. |
| Mains frequency f | Frequency of the AC supply feeding the bridge; recharge pulses occur at 2f for full-wave/bridge rectification. |
| Reservoir capacitance C | Effective capacitance of the smoothing/reservoir capacitor directly after the bridge. |
Choose the inputs correctly
Upeak is the peak voltage right after the bridge, approximately √2 times mains RMS voltage minus two diode threshold drops. Iload is the average DC current the downstream circuit draws from the capacitor. f is the mains frequency feeding the bridge, C the reservoir capacitor's capacitance.
How to use the calculator
Estimate or measure Upeak from the known mains voltage. Compute Iload from the powered circuit's consumption at the expected operating voltage. Use f and C as planned for the supply; if ripple is too high, increase C or add a regulator stage.
Worked example
Upeak=325 V, Iload=0.5 A, f=50 Hz and C=1,000 µF give ripple Vpp=Iload/(2fC)=5 V and minimum voltage Umin=320 V.
Understand the result and units
Small ripple relative to Upeak indicates an adequately sized reservoir capacitor. Ripple grows proportionally with Iload and inversely with C; doubling C halves it for otherwise unchanged values.
Upeak, Iload and C are entered in the offered units and converted internally to coherent SI values; f is a frequency.
Where the topology and approximation come from
The bridge topology (B2, full-wave rectifier) is described in Busch, Elektrotechnik und Elektronik für Maschinenbauer, Abschnitt 7.4.2.1.1; that chapter explicitly defers to downstream smoothing elements at this point without deriving reservoir-capacitor ripple. The linear approximation Vpp=Iload/(2fC) used here is a widely documented standard estimate in introductory electronics literature for full-wave/bridge rectifiers with capacitive load.
Typical applications
Initial sizing of the reservoir capacitor in unregulated or linear-regulated small power supplies, and checking whether a downstream regulator or filter stage can still adequately suppress the remaining ripple.
Assumptions, limits and common mistakes
Approximation with linear capacitor discharge, constant load current and negligible diode recharge duration. Diode threshold drops, capacitor ESR and ESL, mains voltage fluctuation and strongly pulsed or capacitive load currents are excluded.
Common mistake: Do not enter mains RMS voltage instead of the already-rectified peak voltage Upeak. Half-wave rectification uses a different discharge interval (1/f instead of 1/(2f)); this model applies only to full-wave/bridge rectification.
Frequently asked questions
Is this a complete power-supply verification?
No. Diode threshold drops, capacitor ESR, mains voltage tolerance and downstream regulator or filter stages are outside the model; it only gives an approximate reservoir-capacitor sizing.
Where do the formula and validity limits come from?
The B2 bridge topology comes from Busch, Abschnitt 7.4.2.1.1; the linear ripple approximation is a publicly documented standard estimate for full-wave rectifiers with a reservoir capacitor.
Does the formula also apply to half-wave rectifiers?
No. Half-wave rectification produces only one recharge pulse per mains cycle, so the discharge interval is 1/f instead of 1/(2f), doubling ripple for otherwise identical values.