Capacitive power supply

A capacitor power supply is a special construction of an electric power supply that uses the reactance of a capacitor in order to reduce the supply voltage (low voltage) at low voltage. Because of the demands on the capacitor and the ohmic resistor it is only used for low power consumers. Due to lack of galvanic isolation so that the supplied electrical equipment must be sealed against accidental contact.

Construction

A capacitor power supply consists of a capacitor C1 whose reactance limits the current through the rectifier D1. To protect against spikes during switching operations a resistor R1 connected in series. An electrolytic capacitor C2 smoothes the DC voltage, and buffers the current peaks in the ampere - range switching operations. It is a low-drop voltage regulator downstream, comprising a current-limiting resistor R3 and a cross regulator IC1. When requirements with voltage stability you can use a reversely biased Zener diode.

Of voltage and reactance C1 practically creates a power source. This means that the current flow through R3 is nearly constant. Therefore, capacitor power supplies are ideal for constant current loads, such as LEDs. The real power consumption of this circuit is proportional to at C2 -adjusting output voltage.

The capacitor C1 moves in the order of 100 nF - 1000 nF. He should be a resistor R2 of 470 kOhm to 1 M are connected in parallel. He discharges the capacitor after the device is disconnected from the network. Otherwise, you can get a usually harmless but unpleasant electric shock to the extracted plug of the unit. This is known eg from older appliances when the suppression capacitor is still charged after pulling the power plug.

Despite the series ohmic resistance ( usually in the range 330-5600 Ω ) must be used for a capacitor power supply so-called X - capacitors (eg MKP 630V = / 250V ~ ). Because in addition to the turn-on pulse in the 230 V mains come also extremely short pulse-like voltage peaks, so-called transients, that can have up to 6 kV and up to 2 kV quite normally occur several times a day within the range. The series resistor begins on this part, but the capacitor has to withstand them. The X capacitors are constructed so that they can withstand voltage spikes without damage and heal flashovers no fire risk.

If a simple, inexpensive capacitor used, which leads to an extremely short lifetime of the circuit by penetration of the capacitor after a few hours to days of operation, with consequent risk of fire!

The resistor used also should be a so-called "fusible type", that is suitable in addition to its original function as a backup, so it does not inflamed, if the capacitor but breaks down once in the ideal case. Reason, and also because of the higher dielectric strength to recommend metal film resistors. In addition, the resistance should be pulse resistant to not fend for themselves at high transients. Attention, the resistance may ( depending on the connected load) are very hot, and should therefore have a sufficiently large distance to plastics.

Capacitor power supplies are due to the limited size of the capacitors for low power. They hardly produce power loss. They are smaller, cheaper and lighter than conventional transformer (switching) power supplies. Due to the lack of electrical isolation, the use is normally limited to built- in electronic control devices with a network connection, such as for self-supply of twilight switches, motion detectors, remote control receiver, electronic switches and similar devices. In the range of a few watts rather non-isolated buck converter can be used, but they require by their pulsed operation corresponding interference suppression.

Centralized pulses and similar high-frequency superimposed on the power supply noise can damage the capacitor power supplies and the loads connected to it. However, the need to suppress harmful frequencies reactors are rapidly larger and more expensive than a conventional transformer -isolated power supply.

Example

If you choose an X2 capacitor of 330 nF, resulting in a current of approximately 20 mA. This enables up to 30 white LEDs (eg 3.1 V/20mA/20000mcd ) - to be supplied - connected in series. The picture shows the open network part of a 48 -fold LED lamp. The capacitor 125 = 1.2 uF has a reactance of 2.6 kOhm, and limits the current to about 90 mA. At the rectifier in parallel with the 10 uF electrolytic smoothing capacitor, there are four current branches of about 20 mA with 12 LED. The light emitting diodes limit the voltage to about 40 V per branch. Since the circuit is connected directly to the network, a contact protection for the entire circuit including the LED is required.