#472 Capacitive Dropper Power Supply
Understanding how to design a capacitive dropper, demonstrated with a low-voltage LED driver circuit.
Capacitive power supplies aka capacitive droppers, are cheap and simple designs for generating a current-limited, rectified voltage, usually from mains power.
See LEAP#067 for an example of this used for mains powered LED lamp.
The essence of a capacitive dropper is to use a mains-rated capacitor in series with the power line. The capacitive reactance at the supply frequency will limit the current that can be drawn.
Give a capacitive reactance
Xc = 1/ωC = 1/2πfC, the available current is
I = Vac/Xc.
There are particular safety concerns with capacitive droppers:
- the rectified circuit is not isolated, and will float relative to mains power.
- capacitors may retain a mains charge after power is disconnected if bleeder resistors are not included in the design
Typical capacitive droppers include the following circuit stages:
- AC power input with live and neutral connections. Most dropper circuits are not earthed or isolated.
- Capacitive current limiter C1 with parallel residual voltage protection R1
- Series resistor R2 to limit the impact of power-on voltage spikes on C1
- Full-wave rectifier
- DC bypass/smoothing capacitor C2
- C2 residual voltage protection R3.
- Finally, the load - often just LEDs in series with a small resistor R4 to take up the slack in the voltage drop.
Voltage regulation may be added to the load circuit if it is important.
Low-power Demonstation Circuit
A good way of experimenting with dropper circuits is to avoid the high voltage risks of mains supply and use a low voltage AC adapter. In this example, I’m using a 12V AC power adapter rated for 10VA.
I am using a 0.1µF X2 capacitor for C1. In my location, our mains power is 50Hz, so the capacitive reactance Xc = 1/ωC = 31.8kΩ, and at 12VAC, the circuit should be limited to around 0.38mA. That’s quite low, but ample for the LEDs I am driving.
C2 is quite a chunky DC bypass capacitor of 220µF. This does provide very smooth power, but of course also means a slow ramp of the voltage at startup and shutdown. With R3 at 100kΩ, the discharge time constant on power off is also very slow at 22s. A better choice for C2 may be say 4.7µF, for a discharge time constant of 0.47s.
Initial testing on a breadboard:
Here’s a scope trace of:
- CH1 (Yellow) - AC input
- CH2 (Blue) - AC voltage across the bridge rectifier and R2 i.e. after C1
Just for fun, I whipped up a freeform realisation of the circuit.
Credits and References
- capacitive power supply - wikipedia
- Some good capacitive dropper circuit/project write-ups:
- one of a few questions on EE.SE on the subject.