Capacitive Dropper

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2016-06-28 | By Bob Paradiso

License: None

A cap dropper circuit is a type of transformerless power supply, converting AC mains input to a much lower voltage output for low-power components without the cost and bulk of a transformer. I built two circuits to test some ideas out (with and without a rectifier).

CAUTION:You should not attempt to build, test, or use this type of circuit unless you are experienced and competent in handling AC mains and high voltage. Do not touch this circuit. A low voltage between two points in a circuit, DOES NOT mean that there is not a high voltage between one of those points and earth ground.

The first circuit, very minimal, powers standard red LEDs from mains. The 100 nanofarad capacitor provides about 26 kilo-ohms of impedance, which can be calculated with:

Z = 1/Cs = 1/(100(nanofarads) * 10^-9 * 60(Hz) * 2 * PI) = 26,525

This drops enough voltage and limits current through the LEDs. First, choose the current you want to get, then divide your expected voltage (less the LED voltage drop) by the current you want. Rough estimation example: (170V{peak} – 2V{LEDs})/0.006{6 milliamps} = 28K.

Only non-polarized capacitors can be used. Cap must be rated at least for the peak voltage (not just the stated RMS voltage), which, excluding spikes, will be 120 Vrms * sqrt(2) = 170 V in the US.

Diagram

AC-powered ArduinoThe second circuit outputs almost 5V DC for Arduino. Like CAP droppers in general it is not designed to source much current. Even the Arduino drops voltage a bit below 5V. Since capacitor does not convert AC to DC there is a diode rectifier. The rectified DC is capped by a 5.1V Zener diode and buffered in the output capacitor to smooth it.

Capacitive dropper

Similar to the first circuit, a resistor is used to limit inrush current when AC power is first applied. Also the resistor limits the effect of a transient voltage spike. Without these, in the first circuit for example, the LEDs will burn out after turning the power ON and OFF a few times. The resistors do somewhat limit the current, but in these circuits the capacitor’s reactance is doing most of the work.