Mains-Powered White LED Lamp Circuit
A simple LED lamp can provide surprisingly bright illumination while consuming very little power. One interesting way of building such a lamp is to operate a series string of white LEDs directly from the AC mains using a capacitive dropper power supply. The circuit shown here is designed for a 230V AC supply and uses a 220nF capacitor to limit the current flowing through a series string of white LEDs.
Unlike a conventional transformer-based supply, this circuit does not use a bulky transformer. That makes it compact and inexpensive, but it also has an important disadvantage: the LED circuit is not isolated from the mains. Therefore, it must never be treated as a low-voltage circuit.
Know your circuit
How the Mains-Powered LED Lamp Works
The circuit can be divided into four main sections:
Capacitive current limiter
Bridge rectifier
Filtering capacitor
Series-connected white LEDs
The 230V AC input first passes through C1 (220nF, 400V). Resistor R1 (1MΩ) is connected in parallel with C1. The output then goes to bridge rectifier B1, made using four 1N4007 diodes. C2 filters the rectified output, followed by R2 (560Ω) and the series string of white LEDs.
C1 – The Capacitive Current Limiter
The most interesting component in this circuit is C1. Instead of using a conventional resistor to reduce the mains current, the circuit uses a 220nF capacitor as a capacitive reactance.
At 50Hz, a capacitor presents an impedance known as capacitive reactance:
Xc = 1 / (2Ï€fC)
For a 220nF capacitor at 50Hz, the reactance is approximately 14.5kΩ. This limits the available AC to roughly the tens-of-milliamps range, depending on the rest of the circuit.
One major advantage of a capacitor used this way is that it dissipates far less real power than a resistor performing the same current-limiting function.
C1 must be a suitable mains-rated safety capacitor, such as an appropriately rated polypropylene capacitor. An ordinary electrolytic capacitor must never be substituted for it.
The purpose of R1
The 1MΩ resistor connected across C1 acts as a bleeder resistor. When the lamp is disconnected from the mains, it provides a path for the capacitor to discharge rather than allowing it to remain charged indefinitely.
B1 – Full-Wave Bridge Rectifier
The AC coming from the capacitive dropper is fed into B1, which consists of four 1N4007 diodes.
The bridge converts the alternating AC waveform into pulsating DC. Because both halves of the AC waveform are used, this is known as full-wave rectification.
The 1N4007 is a popular general-purpose rectifier diode with a high reverse-voltage rating, making it suitable for many mains rectifier applications. However, the voltage rating of the diode alone does not make the overall circuit safe.
C2 – Smoothing the Rectified Voltage
After the bridge rectifier, C2 (4µF) is connected across the DC output. Its job is to reduce the ripple in the rectified voltage.
Without C2, the LED current would follow the pulsating output of the bridge more closely. The capacitor stores electrical energy when the rectified voltage rises and releases some of that energy as the voltage falls.
This helps produce a steadier LED current and can reduce visible flicker.
However, the 63V rating shown for C2 deserves special attention. A bridge connected to a 230V AC mains source can produce a peak voltage approaching:
230 × 1.414 ≈ 325V
Therefore, C2 cannot simply be assumed safe because the LED string has a relatively low forward voltage. In the circuit as drawn, the capacitor's voltage rating is potentially inadequate unless the circuit has additional voltage-limiting behavior not shown in the diagram. This is an important point that should be corrected before considering practical construction.
R2 and the LED String
The rectified and filtered supply then passes through R2 (560Ω) before reaching the LED string.
The diagram indicates a total of 15 white LEDs connected in series. A typical white LED may have a forward voltage of around 2.8–3.4V, depending on its type and operating current. Therefore, fifteen LEDs can require several tens of volts when operating.
Connecting the LEDs in series has an important advantage: the same current flows through every LED.
R2 provides additional current limiting and helps absorb some of the voltage difference. It also provides some protection against current variations caused by changes in LED characteristics and supply conditions.
Why a Capacitive Dropper Is Used
A transformerless capacitive supply is attractive for low-power LED lamps because it can be:
Compact
Lightweight
Inexpensive
Relatively efficient
Suitable for small LED currents
However, these benefits come with a significant trade-off: there is no galvanic isolation from the 230V mains.
The negative side of the LED circuit can therefore be at a dangerous potential relative to earth, even when the LEDs themselves operate at a comparatively low voltage.
Important Safety Precautions
This circuit is not suitable for direct experimentation on a breadboard and should not be handled while connected to the mains.
There are several important safety considerations:
Never touch any part of the circuit when it is connected to 230V AC.
Use mains-rated components appropriately.
C1 must be a suitable mains-rated capacitor.
Provide proper insulation and enclosure.
Do not expose the LED terminals or PCB tracks.
Ensure adequate creepage and clearance distances.
Use a properly rated fuse or other appropriate protection.
Do not connect the circuit to a USB supply or low-voltage source.
Recheck the voltage rating and position of C2 before building the circuit.
Remember that capacitors can remain charged after the circuit has been disconnected.
For a practical lamp intended for everyday use, an isolated, commercially certified LED driver is considerably safer than an improvised transformerless mains circuit.
Conclusion
The Mains-Powered White LED Lamp circuit demonstrates an interesting application of capacitive current limiting, bridge rectification, filtering and series LED operation. C1 limits the mains current, B1 converts AC into DC, C2 reduces ripple, R2 provides additional current limiting, and the 15 white LEDs convert electrical energy into light.
From an electronics-learning perspective, it is a useful example of how a small capacitor can be used as an AC limiter. But because the entire circuit is associated with potentially lethal mains voltage, safety must take priority over simplicity or low cost.
Most importantly, the circuit shown should not be built exactly as drawn without redesigning and verifying the mains-side protection and the voltage rating of C2. A transformer-isolated or certified LED driver is the preferred approach for a real-world lamp.
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