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Capacitive Transformerless PSU - Reactance-Based Current Limiting

Transformerless power supplies use capacitive reactance Xc = 1/(2πfC) to limit AC current instead of a transformer. Compact and cheap but NOT isolated from mains—X2-rated capacitors and bleeder resistors are essential for safety.

Design Transformerless PSUEnter AC voltage, desired current, and output voltage for component sizing.

Why This Physics Calculation Matters

Why: Capacitive dropper supplies are used in LED drivers, sensors, and low-power applications where isolation is not required. They are compact and efficient but require careful safety design—always use X2 capacitors and bleeder resistors.

How: Current I ≈ V_ac/Xc where Xc = 1/(2πfC). Zener diode clamps output voltage. Bleeder resistor R ≥ 100kΩ discharges capacitor when unplugged. Safety margin required for capacitor voltage rating.

  • ●X2-rated capacitors are designed for AC mains applications
  • ●Bleeder resistor ≥100kΩ discharges capacitor when power off
  • ●NOT isolated—circuit is at mains potential
  • ●Typical applications: LED drivers, low-power sensors
Sources:NIST - Electrical StandardsHyperPhysics - Capacitive Reactance

Input Parameters

Sample Examples

⚠️For educational and informational purposes only. Verify with a qualified professional.

🔬 Physics Facts

⚡

Capacitive reactance limits current without resistive power loss

— HyperPhysics

✓

X2 capacitors are certified for across-the-line AC applications

— IEC

🔌

Bleeder resistor prevents dangerous charge retention when unplugged

— NIST

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Circuit is NOT isolated—extreme caution required for mains voltage

— IEC

📋 Key Takeaways

  • • Capacitive transformerless PSUs use capacitive reactance (Xc = 1/(2πfC)) to limit current instead of transformers
  • • These circuits are NOT isolated from mains voltage - extreme safety precautions required
  • • Use X2-rated capacitors for AC applications to ensure safety and reliability
  • • Always include a bleeder resistor (≥100kΩ) to discharge the capacitor when power is removed

💡 Did You Know?

⚠️Capacitive dropper supplies are NOT isolated - the output can be at mains potential. Never use for touchable circuits.Source: IEEE Standards
💡A 0.47μF capacitor at 120V/60Hz provides approximately 20mA of current - perfect for LED stringsSource: Electronics Tutorials
🔌X2 capacitors are specifically rated for AC line applications and can handle voltage spikes up to 2.5kVSource: NIST
⚡The capacitor must discharge through a bleeder resistor - without it, lethal voltage can remain for minutes after power-offSource: All About Circuits
📊Capacitive reactance is frequency-dependent: Xc = 1/(2πfC). At 60Hz, reactance is lower than at 50HzSource: TI App Notes
🛡️Zener diodes provide voltage regulation but dissipate power as heat - always calculate power dissipationSource: IEEE Standards
💰Transformerless supplies cost 70-90% less than transformer-based designs but require careful safety designSource: Electronics Tutorials

📖 How Capacitive Transformerless Power Supplies Work

A capacitive transformerless power supply uses a capacitor's reactance to limit AC current flow. The capacitor acts as a current-limiting impedance, allowing controlled current to flow while blocking DC.

The Basic Principle

Capacitive reactance (Xc = 1/(2πfC)) opposes AC current flow. By selecting the right capacitor value, you can control the current delivered to the load. The capacitor drops voltage across itself, leaving the remaining voltage for the load.

Circuit Components

  • Capacitor: Limits current via reactance (must be X2-rated for AC)
  • Rectifier: Converts AC to DC (half-wave or bridge)
  • Zener Diode: Regulates output voltage
  • Bleeder Resistor: Discharges capacitor safely (≥100kΩ)
  • Fuse: Protects against overcurrent (recommended)

🎯 Expert Tips

💡 Always Use X2 Capacitors

X2-rated capacitors are designed for AC line applications and can handle voltage spikes. Never use standard DC capacitors.

💡 Include Bleeder Resistor

A bleeder resistor (≥100kΩ) ensures the capacitor discharges safely when power is removed. Without it, lethal voltage can remain.

💡 Calculate Zener Power

Zener diodes dissipate power as heat. Always calculate power dissipation and use appropriate heat sinking if needed.

💡 Safety First

These circuits are NOT isolated. Never use for touchable circuits. Always include proper fusing and follow electrical safety codes.

⚖️ Capacitive vs Transformer-Based Power Supplies

FeatureCapacitive PSUTransformer PSU
CostLow (70-90% cheaper)High
SizeCompactBulky
Isolation❌ Not isolated✅ Isolated
Efficiency60-80%70-90%
Power RangeLow power (<100mA)Any power level
Safety⚠️ Requires care✅ Safer
ComplexitySimpleModerate
ApplicationsLEDs, sensors, indicatorsGeneral purpose

❓ Frequently Asked Questions

Are capacitive transformerless power supplies safe?

They can be safe for non-touchable applications when properly designed with X2 capacitors, bleeder resistors, and fuses. However, they are NOT isolated from mains voltage, so extreme caution is required. Never use for touchable circuits.

What capacitor rating should I use?

Always use X2-rated capacitors for AC line applications. These are designed to handle voltage spikes up to 2.5kV and are self-healing. Standard DC capacitors will fail in AC applications.

How do I calculate the capacitor value?

Use the formula: C = I/(2πf×V), where I is load current, f is frequency (50Hz or 60Hz), and V is input voltage. Our calculator handles this automatically.

Why do I need a bleeder resistor?

A bleeder resistor (≥100kΩ) discharges the capacitor when power is removed. Without it, lethal voltage can remain for minutes, creating a serious shock hazard.

What is the maximum current I can get?

Practical limits are typically 50-100mA. Higher currents require very large capacitors and create safety concerns. For higher power, use a transformer-based supply.

Can I use this for battery charging?

Not recommended. These supplies are not isolated and lack proper current limiting for battery charging. Use isolated supplies for battery applications.

What happens if the capacitor fails?

X2 capacitors are self-healing and designed to fail open-circuit. However, always include a fuse to protect against catastrophic failures. Regular inspection is recommended.

Is a bridge rectifier necessary?

A bridge rectifier provides full-wave rectification, doubling efficiency compared to half-wave. For low-power applications, half-wave may be sufficient, but bridge is recommended for better performance.

📊 Capacitive PSU by the Numbers

0.47μF
Typical Capacitor Value
20mA
Typical Current Output
60-80%
Typical Efficiency
X2
Required Capacitor Rating

📚 Official Data Sources

IEEE Standards ↗
Electrical engineering standards and specifications
Updated: 2026-01-15
NIST Electrical Engineering ↗
Physical measurement laboratory standards
Updated: 2026-01-20
Electronics Tutorials ↗
AC circuit analysis and capacitive reactance
Updated: 2026-01-10
All About Circuits ↗
Comprehensive electronics education
Updated: 2026-01-18
Texas Instruments App Notes ↗
Power supply design application notes
Updated: 2025-12-01

⚠️ Disclaimer: Capacitive transformerless power supplies are NOT isolated from mains voltage and can present serious shock hazards. These circuits should only be used by qualified personnel for non-touchable applications. Always use X2-rated capacitors, include bleeder resistors, follow local electrical codes, and ensure proper fusing. This calculator provides theoretical calculations - actual designs require professional review and safety testing. Not intended for medical devices or safety-critical applications.

What is a Capacitive Transformerless Power Supply?

A capacitive transformerless power supply is a cost-effective AC-to-DC converter that uses a capacitor's reactance to limit current instead of a transformer. This design eliminates the bulky and expensive transformer, making it ideal for low-power applications like LED drivers, night lights, sensor modules, and indicator circuits.

Current Limiting

The capacitor acts as a current-limiting element, allowing controlled current flow while blocking DC.

Key Concept:

  • Capacitive reactance limits current
  • No transformer needed
  • Cost-effective solution

Key Components

Essential components include capacitor, rectifier, Zener diode, and bleeder resistor.

Components:

  • X2-rated capacitor
  • Bridge rectifier
  • Zener voltage regulator
  • Bleeder resistor

Applications

Ideal for low-power applications where cost and size are critical factors.

Use Cases:

  • LED driver circuits
  • Night lights
  • Sensor power supplies
  • Indicator circuits

How Does a Capacitive Power Supply Work?

The capacitive transformerless power supply operates by using a capacitor's reactance to limit AC current flow. The capacitor blocks DC while allowing AC to pass, creating a voltage drop that limits current. A rectifier converts AC to DC, and a Zener diode provides voltage regulation.

🔬 Design Process

Step 1: Determine Load Requirements

  1. 1Identify required output voltage (V)
  2. 2Determine load current (mA)
  3. 3Calculate load power (mW)

Step 2: Calculate Capacitor Value

  • Use formula: C = I/(2πf×V)
  • Add safety margin (20-30%)
  • Select X2-rated capacitor
  • Verify voltage rating ≥ √2 × V_in

When to Use Capacitive Power Supplies

Capacitive transformerless power supplies are ideal for low-power applications where cost, size, and simplicity are more important than efficiency and isolation. They're commonly used in consumer electronics, lighting applications, and sensor modules.

LED Driver Circuits

Perfect for driving LED strings with constant current requirements. Simple and cost-effective.

Typical Specs:

  • 10-50 mA current
  • 12-24V output
  • Low cost design

Sensor Modules

Ideal for powering low-current sensor circuits and microcontroller modules.

Typical Specs:

  • 5-20 mA current
  • 3.3-5V output
  • Compact design

Night Lights

Perfect for low-power night light applications with minimal component count.

Typical Specs:

  • 3-10 mA current
  • 5-12V output
  • Simple circuit

Capacitive Power Supply Formulas

Understanding these formulas is essential for designing safe and efficient capacitive transformerless power supplies. Each formula relates key parameters for proper component selection and safety analysis.

📊 Core Formulas

Capacitor Current (Ic)

IC=2pifCVI_C = 2\\pi f C V

The current through a capacitor in an AC circuit, where f is frequency (Hz), C is capacitance (F), and V is voltage (V).

Capacitor Value (C)

C=fracI2pifVC = \\frac{I}{2\\pi f V}

The capacitance required to provide a specific current at a given frequency and voltage.

Capacitive Reactance (Xc)

XC=frac12pifCX_C = \\frac{1}{2\\pi f C}

The opposition to alternating current by a capacitor, measured in ohms.

Bleeder Resistance (R)

R=fracVIdischargeR = \\frac{V}{I_{discharge}}

Resistance value for safe capacitor discharge, typically 100kΩ to 1MΩ for safety.

Zener Power Dissipation (Pz)

PZ=IZtimesVZP_Z = I_Z \\times V_Z

Power dissipated in the Zener diode, where Iz is Zener current and Vz is Zener voltage.

Efficiency (η)

eta=fracPoutPintimes100\\eta = \\frac{P_{out}}{P_{in}} \\times 100\\%

The efficiency of the power supply, typically 50-70% for capacitive designs.

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