Capacitive Transformerless Power Supply Calculator

Calculate output voltage and input current for capacitive dropper power supply circuits

Circuit Parameters

RMS Voltage (V_RMS)

AC mains voltage (110V, 220V, 230V)

Zener Voltage (V_z)

Voltage regulation by Zener diode

Forward Bias Voltage (V_d)

Typically 0.6V for silicon diode

Frequency (f)

AC mains frequency (50Hz, 60Hz)

Dropper Capacitance (C₁)

Voltage dropping capacitor value

Resistance (R₁)

Discharge resistor for safety

Calculation Results

Output Voltage (V_out)

0.00 V

Input Current (I_in)

0.00 mA

Capacitive Reactance (X_C)

0.0 Ω

Output Power (approx.)

0.0 mW

Resistor Power Loss

0.0 mW

Ripple Voltage (est.)

0.00 V

Formulas used:

V_out = V_z - V_d

X_C = 1 / (2πfC₁)

I_in = √2(V_RMS - V_z) / √(X_C² + (2R₁)²)

Safety Warnings

⚠️ Very small capacitor may cause poor regulation.

Circuit Analysis

Example Calculations

5V Power Supply Example

Given: V_RMS = 110V, V_z = 5.1V, V_d = 0.6V

Circuit: f = 60Hz, C₁ = 0.38µF, R₁ = 517Ω

Output Voltage: V_out = 5.1V - 0.6V = 4.5V

Capacitive Reactance: X_C = 1/(2π×60×0.38×10⁻⁶) = 6982Ω

Input Current: I_in = √2(110-5.1)/√(6982² + (2×517)²) ≈ 9.9mA

12V Power Supply Example

Given: V_RMS = 230V, V_z = 12.6V, V_d = 0.6V

Circuit: f = 50Hz, C₁ = 1.0µF, R₁ = 1000Ω

Output Voltage: V_out = 12.6V - 0.6V = 12.0V

Result: Suitable for LED drivers and small DC loads

Application: Night lights, indicator circuits, timer circuits

Circuit Components

Dropper Capacitor (C₁)

Reduces AC voltage via reactance

Typically 0.1µF to 2.2µF, 400V rated

Zener Diode (D₁)

Voltage regulation

3.3V, 5.1V, 12V common values

Rectifier Diode (D₂)

AC to DC conversion

Silicon diode, 0.6-0.7V drop

Smoothing Capacitor (C₂)

Reduces ripple voltage

Electrolytic, 10µF to 470µF

Safety Guidelines

⚠️

No galvanic isolation from mains

⚠️

Use discharge resistor for safety

⚠️

Proper PCB isolation required

⚠️

Use proper enclosure

⚠️

Limited current capability

Applications

• LED lighting circuits

• Night light indicators

• Timer circuits

• Switch mode control

• Standby power supplies

• Small appliance controls

• Sensor power supplies

Advantages

✓ Compact size

✓ Low cost

✓ No transformer needed

✓ Low power dissipation

Understanding Capacitive Transformerless Power Supplies

What is a Capacitive Dropper?

A capacitive transformerless power supply, also known as a capacitive dropper, is a circuit that converts high AC voltage to low DC voltage without using a transformer. It uses a capacitor's reactance to limit current and drop voltage.

How It Works

•Voltage Drop: Capacitor acts as impedance to limit current
•Rectification: Diode converts AC to pulsating DC
•Regulation: Zener diode provides stable voltage
•Filtering: Smoothing capacitor reduces ripple

Key Formulas

Output Voltage

V_out = V_z - V_d

Regulated by Zener diode minus forward diode drop

Capacitive Reactance

X_C = 1 / (2πfC₁)

Impedance of the dropper capacitor

Input Current

I_in = √2(V_RMS - V_z) / √(X_C² + (2R₁)²)

Current limited by capacitive reactance

Design Considerations & Safety

Capacitor Selection

Use non-polarized capacitor (polyester/ceramic) rated for at least 400V AC. X2 class capacitors recommended for mains connection.

Current Limitation

Output current is inherently limited by capacitor value. Suitable only for low-power applications (typically <100mA).

Safety Isolation

No galvanic isolation from mains. Use proper enclosure, fusing, and ensure no exposed conductors for safety.

Discharge Resistor

Always include discharge resistor (470kΩ-1MΩ) across capacitor for safety when power is disconnected.

Load Regulation

Poor load regulation. Output voltage varies significantly with load current changes. Best for constant loads.

EMI Considerations

May require additional filtering for EMI compliance. Consider surge protection for robust design.