If you’ve ever needed a compact way to power a small electronic circuit directly from mains, without using a bulky transformer, you’ve probably come across something called a transformerless power supply.
These supplies are simple, cheap, and very compact. Instead of stepping down mains voltage with a transformer, they use a series capacitor to limit current, followed by rectification and voltage regulation. The circuit we’re looking at today outputs 12V DC from a 220V AC mains input.
It’s not a design for high-current applications, but it works perfectly for powering low-power circuits like sensors, LEDs, microcontrollers, and other small loads.
The Basic Idea
Traditional power supplies use a transformer mainly to step down voltage and provide isolation. But in cases where isolation isn’t required and you want to save space (like in night lamps, small chargers, or appliances), you can get rid of the transformer.
Instead, you use a capacitive dropper at the input. This capacitor behaves as a reactance (like an AC resistance), limiting current entering the circuit without dissipating power like a resistor would.
That’s exactly what’s happening in this 12V transformerless design.
Circuit Overview
Looking at the schematic:
- Input: 220V AC mains is applied at the left side.
- Series Resistor (100Ω, 1W): Acts as a safety resistor. Limits inrush current and provides some short-circuit protection.
- Capacitor (474nF, 400V): This is the capacitive dropper. It reduces AC to a manageable level before it enters the rectifier stage.
- Rectifier Diodes (1N4004): A pair of diodes is configured to convert AC into DC.
- Zener Diode (12V, 1W): Provides voltage clamping. Keeps the DC regulated around 12V.
- Filter Capacitor (470µF/16V): Smooths out ripples in the rectified DC, producing a relatively clean 12V output.
Step-by-Step Working
Step 1: AC Input and Series Resistor
The input is 220V AC. The first component it encounters is the 100Ω, 1W resistor. This resistor doesn’t do the heavy lifting of voltage reduction, but it improves safety:
- Limits surge current when plugging in.
- Provides damping to avoid oscillations in the capacitor.
Step 2: Capacitive Dropping
Then comes the CBB21 474nF/400V capacitor. Its job is to limit current according to its capacitive reactance, defined by:
[ Xc = \frac{1}{2 \pi f C} ]
At 50 Hz and 474nF, the reactance is around 6.7 kΩ. This means only a small current can flow through, effectively dropping the power available while keeping the AC voltage level the same. This is how the circuit avoids dissipating power like a resistor would.
Step 3: Rectification
The next stage is handled by the 1N4004 diodes, which form a rectifier. The positive half cycles are passed to charge the output capacitor, while the negative half cycles are clamped. This produces pulsed DC.
Step 4: Voltage Clamping with Zener
Left alone, the rectified voltage would still be too high for most circuits. That’s where the 12V zener diode comes in. The zener conducts in reverse when the voltage exceeds its rated voltage, clamping the supply to about 12V.
Step 5: Filtering
Finally, a 470µF electrolytic capacitor smooths the DC voltage by charging on peaks and discharging when the supply dips. This reduces ripple and ensures a reasonably steady 12V output across the load.
Components Recap
- Resistor (100Ω, 1W): Current limiter/ inrush protection
- Capacitor (474nF/400V, non-polar): Capacitive dropper, main current limiter
- Diodes (1N4004): Rectifier (1A, 400V rating)
- Zener Diode (12V, 1W): Maintains regulated voltage
- Electrolytic Capacitor (470µF/16V): Output filter
- Input Voltage: 220V AC
- Output Voltage: ~12V DC
Advantages of Transformerless Power Supply
- Small size: No bulky transformer. Great for compact gadgets.
- Low cost: Only requires a few inexpensive passive components.
- High efficiency for low current: Doesn’t waste much energy as heat (compared to resistive dropping).
- Simplicity: Easy to build and understand.
Limitations
- No isolation from mains: The 12V output shares a direct reference with mains. This is the biggest safety risk. Never touch the circuit or connect directly to devices that humans might handle.
- Limited current: Capacitive current limiting doesn’t allow large current delivery. Usually below 50–100 mA, depending on capacitor size.
- Dangerous for beginners: A direct connection to the mains always carries risk if not handled carefully.
- Sensitive to surges: Input spikes from mains can damage the circuit if not well-protected.
Applications
Despite its limitations, this type of supply is widely used in:
- LED lamps and night lights
- Electric mosquito repellers
- Small appliance control circuits
- Simple 12V sensor systems
- Embedded microcontrollers, where the current demand is low
Practical Safety Advice
This is a circuit that works directly with 220V mains. That means the shock hazard is real. If you’re building and testing it, follow these precautions:
- Always enclose the circuit in an insulated case.
- Never work on it while plugged in.
- Use a good quality non-polar capacitor rated for AC mains (X-rated safety capacitors are recommended).
- Do not use it to power devices that humans will directly handle (like mobile chargers).
- Fuse the input for added protection.
Possible Improvements
If you want to make the circuit more reliable:
- Use an X2 safety capacitor instead of a generic film capacitor for long-term safety.
- Place a TVS diode or MOV across the input to absorb mains surges.
- Add a fuse in series with the input for overload protection.
- Include a bleeder resistor across the 474nF capacitor to discharge it when disconnected from the mains.
Conclusion
The 12V transformerless power supply shown in this schematic is a compact way to power small circuits directly from the mains. It uses a capacitive dropper (474nF, 400V) to limit current, rectifier diodes to convert AC to DC, a 12V zener diode for regulation, and a 470µF capacitor to smooth the output.
It’s efficient and cheap, making it great for small embedded devices and appliances, but it comes with a serious tradeoff: no galvanic isolation from mains. That means you have to design and use it with care, always prioritizing safety.
For low-power, enclosed, and non-user-accessible circuits, transformerless supplies like this make sense. For anything people directly interact with, a transformer-isolated or switch-mode power supply is far safer.
