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LED Technology

How does a Full Wave LED Christmas lights work?

A '''diode bridge''' or '''bridge rectifier''' is an arrangement of four diodes connected in a bridge circuit as shown below, that provides the same polarity of output voltage for any polarity of the input voltage. When used in its most common application, for conversion of alternating current (AC) input into direct current (DC) output, it is known as a bridge rectifier. The bridge rectifiers provide full wave rectification from a two wire AC input.

diode

The essential feature of this arrangement is that for both polarities of the voltage at the bridge input, the polarity of the output is constant.

==Basic operation== When the input connected at the left corner of the diamond is positive with respect to one connected at the right hand corner, Current (electricity) flows to the right along the upper colored path to the output, and returns to the input supply via the lower one.

diode1

When the right hand corner is positive relative to the left hand corner, current flows along the upper colored path and returns to the supply via the lower colored path.

diode2

In each case, the upper right output remains positive with respect to the lower right one. Since this is true whether the input is AC DC, this circuit not only produces DC power when supplied with AC power: it also can provide what is sometimes called "reverse polarity protection". That is, it permits normal functioning when battery (electricity) batteries are installed backwards or supply wiring "has its wires crossed" (and protects the circuitry it powers against damage that might occur without this circuit in place).

Prior to availability of integrated electronics, such a bridge rectifier was always constructed from discrete components. Since about 1950, a single four-terminal component containing the four diodes connected in the bridge configuration became a standard commercial component and is now available with various voltage and current ratings.

AC half wave and full wave rectified signals

wave

The gap in the half wave causes the flicker. The full wave takes the gap out of the power cycle making it flicker free and 40% brighter.

== Output smoothing == For many applications, especially with single phase AC where the full-wave bridge serves to convert an AC input into a DC output, the addition of a capacitor may be important because the bridge alone supplies an output voltage of fixed polarity but pulsating magnitude (see photograph above).

The function of this capacitor, known as a 'smoothing capacitor' (see also Filter capacitor) is to lessen the variation in (or 'smooth') the raw output voltage waveform from the bridge. One explanation of 'smoothing' is that the capacitor provides a low impedance path to the AC component of the output, reducing the AC voltage across, and AC current through, the resistive load. In less technical terms, any drop in the output voltage and current of the bridge tends to be cancelled by loss of charge in the capacitor. This charge flows out as additional current through the load. Thus the change of load current and voltage is reduced relative to what would occur without the capacitor. Increases of voltage correspondingly store excess charge in the capacitor, thus moderating the change in output voltage / current.

The capacitor and the load resistance have a typical time constant <math>\tau=RC</math> where ''C'' and ''R'' are the capacitance and load resistance respectively. As long as the load resistor is large enough so that this time constant is much longer than the time of one ripple cycle, the above configuration will produce a well smoothed DC voltage across the load resistance.

 

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