Example Data Table
| Supply Type | C1 | L | C2 | Load | Expected Use |
|---|---|---|---|---|---|
| Small audio preamp | 1000 uF | 220 mH | 470 uF | 100 mA | Low hum rail |
| General 12 V DC rail | 2200 uF | 100 mH | 1000 uF | 500 mA | Cleaner supply output |
| Tube bias supply | 47 uF | 10 H | 47 uF | 20 mA | High voltage smoothing |
Formula Used
Ripple frequency: Half wave uses f. Full wave and bridge use 2f.
Capacitive reactance: Xc = 1 / (2 × π × f × C)
Inductive reactance: XL = 2 × π × f × L
Load resistance: Rload = Vdc / Iload
First capacitor ripple estimate: Vripple = Iload / (fripple × C1)
Approximate cutoff: fc = 1 / (2 × π × √(L × Ceq))
Equivalent capacitance: Ceq = C1 × C2 / (C1 + C2)
Output ripple: Vout ripple = Vin ripple × attenuation ratio
The calculator also uses complex impedance to estimate the divider effect of C1, L, C2, source resistance, ESR, DCR, and load.
How to Use This Calculator
- Enter the expected ripple before the filter. Use zero to estimate it.
- Enter DC voltage and load current.
- Select the rectifier type.
- Add C1, inductor, and C2 values.
- Add ESR, DCR, and source resistance for better estimates.
- Press the calculate button.
- Review output ripple, attenuation, cutoff, and voltage drop.
- Download the result as CSV or PDF.
Power Supply Pi Filter Design Guide
Overview
A power supply pi filter smooths rectified DC with two capacitors and one series inductor. The layout looks like the Greek letter pi. The first capacitor stores charge near each rectifier peak. The inductor resists current change. The second capacitor shunts remaining ripple to ground. This calculator helps estimate ripple, reactance, cutoff frequency, impedance loading, voltage drop, and stored energy.
Why Pi Filters Matter
Unfiltered rectifier output contains pulsating voltage. Sensitive circuits can hum, reset, heat, or measure badly when ripple is high. A pi filter improves the rail before regulators, audio stages, sensor circuits, and bias supplies. It is most useful where load current is steady and the supply has enough voltage margin for inductor resistance loss.
Key Design Ideas
Choose capacitor values for ripple storage and low capacitive reactance. Choose an inductor with enough current rating, low winding resistance, and safe saturation margin. Larger inductance gives stronger ripple opposition at line ripple frequency. Larger output capacitance lowers the remaining ripple at the load. Very large values can increase inrush current, diode stress, and transformer heating.
Reading the Results
The calculator models the pi network at the selected ripple frequency. It compares capacitor reactance, inductor reactance, load resistance, and losses. The attenuation value shows how much of the input ripple reaches the load. Lower values are better. The ripple percentage relates final ripple to DC output voltage. This makes comparison easier across different supplies.
Practical Notes
Real supplies include diode recovery, transformer resistance, capacitor tolerance, equivalent series resistance, temperature drift, and load steps. Always add design margin. Check capacitor voltage ratings. Check ripple current ratings. Verify inductor saturation current. Use a fuse and safe discharge resistor when working with high voltage supplies. Measure the finished circuit with proper tools. Simulation and bench tests should confirm any critical design.
Common Limits
No online calculator can replace careful hardware testing. Ripple waveforms may not be perfect sine waves. Loads can pulse, and regulators can draw sharp current. Layout also changes noise. Keep high current loops short. Place capacitors close to the load. Use rated parts from trusted suppliers. Recheck heat after long operation. Increase capacitance or inductance when margin is small. Then confirm startup behavior under expected worst case load.
FAQs
What is a pi filter?
A pi filter is a power supply smoothing network. It uses a capacitor, an inductor, and another capacitor. The shape resembles the Greek letter pi.
Why is ripple frequency important?
Ripple frequency sets capacitor and inductor reactance. Full wave rectifiers usually create ripple at twice the line frequency. Half wave rectifiers use the line frequency.
Does a larger capacitor always help?
A larger capacitor can reduce ripple. It can also increase inrush current and stress the rectifier. Always check current ratings and startup behavior.
Why include inductor DCR?
DCR is the inductor winding resistance. It causes DC voltage drop and heat. It also affects the final loaded output voltage.
What does ESR mean?
ESR means equivalent series resistance. Real capacitors have ESR. It affects ripple current, heating, and filter performance.
Can this calculator design high voltage supplies?
It can estimate values for high voltage supplies. Use extra care. Confirm voltage ratings, discharge paths, insulation, fuses, and safe working methods.
What ripple percentage is good?
It depends on the circuit. Audio, sensor, and reference circuits need low ripple. Relays and lamps can often tolerate more ripple.
Should I test the final circuit?
Yes. This calculator gives estimates. Real parts have tolerances. Test the supply under load with proper instruments and safe procedures.