When you connect an LED driver, an amplifier, or any sensitive circuit to a DC power supply built from a transformer and rectifier, the voltage it receives is not perfectly flat. It rises and falls in sync with the AC mains frequency, producing what engineers call ripple. A filter capacitor—almost always a large aluminum electrolytic—is placed directly across the DC output to absorb these voltage swings and deliver something much closer to true DC. This post explains why ripple occurs, how the filter capacitor removes it, and how to choose the right capacitor value for your supply.
Where Ripple Comes From
A linear power supply converts AC mains voltage to DC in two stages. First, a transformer steps the voltage down to a workable level. Second, a rectifier (usually a bridge rectifier—four diodes arranged in a diamond) converts the alternating waveform to a unidirectional one. The problem is that “unidirectional” is not the same as “constant.” The output of a bridge rectifier is a series of humps—the absolute value of the AC sine wave. In a 60 Hz country, those humps repeat at 120 Hz (twice per AC cycle, since both half-cycles are rectified). The voltage surges to a peak, then falls back nearly to zero, then surges again.
The amplitude of this ripple is the difference between the peak voltage and the trough voltage. Without any filtering, that ripple can be as large as the full peak voltage—completely unusable for most circuits. The filter capacitor’s job is to store energy on each voltage peak and release it during the troughs, keeping the output voltage approximately constant.
Circuit Diagram
The diagram shows the complete signal path from AC input to the load. Notice how the voltage waveform changes character at each stage: a sine wave going into the bridge rectifier, a bumpy pulsed-DC waveform coming out, and a nearly flat DC line after the filter capacitor. The residual small ripple on the output is unavoidable with passive filtering—the capacitor can’t be infinite—but it is small enough for most applications.
How the Capacitor Smooths the Ripple
The electrolytic capacitor is connected in parallel with the load across the DC output rails. On each voltage peak from the rectifier, the capacitor charges up to approximately the peak voltage. As the rectifier output falls away from the peak, the capacitor begins to discharge into the load. It’s acting like a small reservoir of charge that tops up the supply between peaks.
The rate at which the capacitor discharges depends on the load current and the capacitance value. A higher load current drains the capacitor faster, causing more ripple. A larger capacitor stores more charge, so it maintains the voltage longer before the next peak arrives. The relationship is captured in the ripple voltage formula:
- Vripple ≈ Iload ÷ (f × C)
Where Iload is the DC load current in amps, f is the ripple frequency in Hz (120 Hz for a full-wave rectifier on 60 Hz mains, 100 Hz for 50 Hz mains), and C is the capacitance in farads. The result is peak-to-peak ripple voltage. Lower is better.
Choosing the Capacitor Value
Rearranging the formula to solve for C: C ≥ Iload ÷ (f × Vripple,max). This gives you the minimum capacitance to stay within a ripple budget. For example, a supply powering a 500 mA LED driver, with a ripple target of 0.5V peak-to-peak, on 60 Hz mains (120 Hz ripple frequency): C ≥ 0.5 ÷ (120 × 0.5) = 0.00833 F = 8,330µF. In practice, you’d choose the next standard value up—10,000µF.
The table below shows practical capacitor values for common supply scenarios:
| Load Current | Target Ripple | Min. Capacitance (120 Hz) | Choose |
|---|---|---|---|
| 50 mA | 0.5V | 833µF | 1,000µF |
| 100 mA | 0.5V | 1,667µF | 2,200µF |
| 200 mA | 0.5V | 3,333µF | 4,700µF |
| 500 mA | 0.5V | 8,333µF | 10,000µF |
| 1 A | 1V | 8,333µF | 10,000µF |
| 2 A | 1V | 16,667µF | 22,000µF |
A widely quoted rule of thumb in audio and power supply design is 1,000µF per amp of load current for a reasonably smooth supply. This is conservative—it typically achieves about 0.8V ripple on a 120 Hz supply—but it gives you a quick starting point without pulling out a calculator.
Voltage Rating
The capacitor’s voltage rating must exceed the peak DC voltage it will see—not the nominal DC output. For a supply nominally rated 12V DC, the actual peak voltage at the capacitor is approximately Vpeak = VRMS × √2 minus two diode drops: for a 9V RMS transformer winding, Vpeak ≈ 9 × 1.414 − 1.4 ≈ 11.3V. Use a capacitor rated at least 25V in this case, giving a comfortable 2× safety margin. Standard voltage ratings for electrolytics are 6.3V, 10V, 16V, 25V, 35V, 50V, 63V, 100V—always round up to the next available rating.
For low-voltage breadboard supplies (5V or 12V regulated), an aluminum electrolytic capacitor rated 50V gives you ample headroom across a wide range of designs. A 50V rating is conservative enough that you can use the same capacitor whether your supply is 5V, 12V, or 24V.
ESR and Ripple Current Rating
Two specifications matter beyond capacitance and voltage rating. ESR (Equivalent Series Resistance) is the internal resistance of the capacitor. At high ripple currents, ESR causes internal heating and creates additional voltage drops that appear on the output as residual ripple. For power supply filter capacitors, choose low-ESR types. Look for electrolytics labeled “low ESR,” “high ripple current,” or “105°C”—the 105°C temperature rating usually indicates a premium grade with lower ESR.
Ripple current rating specifies how much AC current the capacitor can pass continuously without overheating. In a power supply filter, significant AC ripple current flows through the capacitor as it charges and discharges. Exceeding the ripple current rating degrades the capacitor’s life rapidly. For moderate loads (under 500 mA), standard electrolytics are usually fine. For supplies above 1 A, check the ripple current rating in the datasheet and derate by at least 20%.
Polarity and Physical Orientation
Electrolytic capacitors are polarized—one terminal must connect to the more positive voltage. The negative lead is marked with a stripe on the body, and the positive lead is longer. In a power supply filter, the positive terminal connects to the DC+ rail (the output of the rectifier’s positive terminal) and the negative terminal connects to the DC− rail (GND). Installing an electrolytic backwards in a power supply filter is dangerous: under DC voltage, a reverse-biased electrolytic can fail catastrophically, sometimes rupturing the vent at the top of the capacitor. Always double-check polarity before applying power.
Bleeder Resistor
Large filter capacitors store a meaningful amount of charge. A 4,700µF capacitor charged to 24V stores 24 mJ—enough to give a noticeable shock and potentially damage components. When the load is disconnected, the capacitor holds this charge indefinitely. A bleeder resistor—a high-value resistor (typically 10kΩ–100kΩ) wired permanently across the output—slowly discharges the capacitor after power-off, making the supply safer to work on. The bleed resistor slightly reduces efficiency under no-load conditions but is good practice in any supply you service regularly.
Related Guides
- Bridge Rectifier + LED: AC and DCC Wiring Guide — complete wiring diagrams for the bridge rectifier circuit that produces the pulsed DC this filter capacitor smooths
- Decoupling Capacitors: Eliminating Power Supply Noise on IC Circuits — after the bulk filter cap, each IC still needs a local bypass capacitor for high-frequency noise
- 555 Timer LED Blinker: RC Timing Network and Blink Rate — electrolytic capacitors in the RC timing network, with component value tables