Calculator KB-19
Rectifier Filter Capacitor Calculator
A classic power supply: transformer, diode bridge, and a bank of capacitors.
Result
In brief: Calculate the filter capacitor after a rectifier: bridge, center tap, or half-wave. Output voltage, ripple, capacitor voltage rating, and diode losses. Example: Capacitor capacitance: 10 000 µF.
Calculation example
| Rectifier circuit | Diode bridge |
|---|---|
| AC voltage | 12 V, RMS |
| Mains frequency | 50 Hz |
| Load current | 2 A |
| Allowable ripple | 2 V, peak-to-peak |
| Diode voltage drop | 0.7 V |
| Capacitor capacitance | 10 000 µF |
|---|---|
| Peak voltage | 15.57 V |
| Average under load | 14.57 V |
| Minimum output voltage | 13.57 V |
| Capacitor voltage rating from | 21.21 V |
| Diode losses | 2.8 W |
How it is calculated
C = I / (k · f · ΔU) (k = 2 for the bridge)
The capacitor charges to the sine wave peak, then supplies current to the load and discharges between peaks. The higher the current and the lower the capacitance, the greater the voltage drop. The formula slightly overestimates the required capacitance, providing some margin.
What to look for
- With no load, the transformer outputs 10–20% more than its rated voltage. Calculate the capacitor's voltage margin from this value.
- Higher capacitance means higher inrush current. Check that the diodes and fuse can handle it.
Frequently asked questions
What capacitor is needed after a diode bridge?
C = I / (2 · f · ΔU). At 2 A, 50 Hz, and 2 V ripple, that’s 10,000 µF.
What voltage will you get after rectifying a 12 V transformer?
The peak voltage minus the drop across two diodes: 12 · √2 − 1.4 ≈ 15.57 V. Under load, the average voltage is lower by half the ripple amplitude.
What voltage rating should I choose for a filter capacitor?
Allow for the no-load peak voltage and mains voltage fluctuations: after a 12 V transformer, 25 V capacitors are usually used; after rectifying 230 V mains with a 325 V peak, use 400 V capacitors.
