Capacitive Transformerless Power Supply Calculator
Calculate the capacitor value for a transformerless AC power supply
Volts RMS (V)
Milliamperes (mA)
Hertz (Hz) - typically 50 Hz or 60 Hz
How to Use This Calculator
Enter AC Input Voltage
Enter the AC mains voltage in RMS volts (e.g., 120V for USA, 230V for Europe, 240V for UK).
Enter Load Current
Enter the required load current in milliamperes (mA). This is the current your circuit needs to operate.
Set AC Frequency
Enter the AC frequency (50 Hz for most countries, 60 Hz for USA). This affects the capacitor reactance calculation.
Get Capacitor Value
Click calculate to get the required capacitor value. The result is shown in microfarads (µF) or nanofarads (nF).
Formula
XC = VAC / Iload
C = 1 / (2π × f × XC)
Where:
- XC = Capacitive reactance (Ω)
- VAC = AC input voltage (V)
- Iload = Load current (A)
- C = Capacitor value (F)
- f = AC frequency (Hz)
- π ≈ 3.14159
Example 1:
AC Voltage = 230 V, Load Current = 50 mA, Frequency = 50 Hz
XC = 230 / 0.05 = 4,600 Ω
C = 1 / (2 × π × 50 × 4600) = 1 / 1,445,000 = 0.692 µF
Example 2:
AC Voltage = 120 V, Load Current = 20 mA, Frequency = 60 Hz
XC = 120 / 0.02 = 6,000 Ω
C = 1 / (2 × π × 60 × 6000) = 1 / 2,261,000 = 0.442 µF
About Capacitive Transformerless Power Supply Calculator
The Capacitive Transformerless Power Supply Calculator is a free online tool that helps you calculate the capacitor value needed for a transformerless AC power supply circuit. This type of power supply uses a capacitor as a voltage dropper to limit current, eliminating the need for a transformer. While cost-effective and compact, these supplies require careful design and safety considerations as they are not isolated from mains voltage.
When to Use This Calculator
- Low-Power Electronics: Design power supplies for LED circuits, small microcontrollers, or low-power sensors
- Cost-Sensitive Applications: When transformer cost and size are prohibitive
- Compact Designs: Space-constrained applications where transformer size matters
- Educational Purposes: Learn about capacitive reactance and AC power supply design
- Legacy Equipment: Understand and repair existing transformerless power supplies
Why Use Our Calculator?
- ✅ Accurate Calculations: Uses correct capacitive reactance formulas
- ✅ Easy to Use: Simple interface requiring voltage, current, and frequency
- ✅ Multiple Units: Displays results in µF and nF for convenience
- ✅ Safety Warnings: Includes important safety considerations
- ✅ Free Tool: No registration or payment required
- ✅ Educational: Includes formulas and examples for learning
Common Applications
LED Lighting: Many LED bulbs and small LED circuits use transformerless power supplies. A typical LED string might require 20-50 mA at low voltage, which can be efficiently provided using a capacitive dropper circuit.
Small Electronic Devices: Clocks, timers, and small microcontroller-based devices often use transformerless supplies. These applications typically need 5-50 mA, making capacitive droppers ideal.
Sensor Circuits: Low-power sensor circuits that operate from mains power can use transformerless supplies. However, proper isolation must be considered for safety.
Power Factor Correction: Capacitors in transformerless supplies can also contribute to power factor correction in some applications.
Tips for Accurate Results
- Always use AC-rated capacitors (X2 or Y2 class for safety)
- Include a fuse in series with the capacitor for overcurrent protection
- Add a current-limiting resistor if needed for additional safety
- Use a bridge rectifier and smoothing capacitor after the dropper capacitor
- Consider voltage regulation - output voltage will vary with load
- Always include proper insulation - these circuits are NOT isolated from mains
- For high-power applications, consider using a transformer instead
Frequently Asked Questions
Are transformerless power supplies safe?
Transformerless power supplies are NOT isolated from mains voltage, making them potentially dangerous. They should only be used in properly insulated, double-insulated, or grounded enclosures. For most applications, an isolated transformer is safer.
What type of capacitor should I use?
Use AC-rated capacitors designed for mains voltage, such as X2 or Y2 class capacitors. These are specifically designed to handle AC voltage and have safety features. Never use DC capacitors for AC applications.
Why does frequency matter?
Capacitive reactance is inversely proportional to frequency. At higher frequencies (60 Hz vs 50 Hz), the same capacitor has lower reactance, allowing more current to flow. This means you need a smaller capacitor for 60 Hz than 50 Hz for the same current.
What are the limitations of transformerless supplies?
Transformerless supplies are limited to low-power applications (typically < 100 mA). They provide poor voltage regulation, have no isolation, and current is fixed by capacitor value. They're not suitable for variable loads or high-power applications.
Do I need a rectifier after the capacitor?
Yes, you need a bridge rectifier to convert AC to DC, followed by a smoothing capacitor to reduce ripple. The dropper capacitor only limits current; it doesn't convert AC to DC.