How Power Supplies Filter Rectified Current in Electronic Circuits
Although the output from an electronic rectifier circuit is technically direct current because all of the current flows in the same direction, it isn't stable enough for most purposes. Even full-wave and bridge rectifiers produce direct current that pulses in rhythm with the 60 Hz AC sine wave that originates with the 120 VAC current that’s applied to the transformer. And that pulsing current isn’t suitable for most electronic circuits.
That’s where filtering comes in. The filtering stage of a power supply circuit smoothes out the ripples in the rectified DC to produce a smooth direct current that’s suitable for even the most sensitive of circuits.
Filtering is usually accomplished by introducing a capacitor into the power supply circuit. Here, the capacitor is simply placed across the DC output.
A capacitor has the useful characteristic of resisting changes in voltage. It accomplishes this magic feat by building up a charge across its plates when the input voltage is increasing. When the input voltage decreases, the voltage across the capacitor’s plates decreases as well, but more slowly than the input voltage decreases. This has the effect of leveling out the voltage ripple.
The difference between the minimum DC voltage and the maximum DC voltage in the filtering stage is called the voltage ripple, or just ripple, which is usually measured as a percentage of the average voltage. For example, a 10% ripple in a 5 V power supply means that the actual output voltage varies by 0.5 V.
The filter capacitor must usually be large to provide an acceptable level of filtering. For a typical 5 V power supply, a 2,200 μF electrolytic capacitor will do the job. The bigger the capacitor, the lower the resulting ripple voltage.
Don’t forget to watch the polarity on electrolytic capacitors. The positive side of the capacitor must be connected to the positive voltage output from the rectifier, and the negative side must be connected to ground.
One way to improve the filter circuit is to use two capacitors in combination with a resistor. In this circuit, the first capacitor eliminates a large portion of the ripple voltage. The resistor and second capacitor work as an RC network that eliminates the ripple voltage even further.
The advantages of this circuit are that the resulting DC has a smaller ripple voltage and the capacitors can be smaller. The disadvantage is that the resistor drops the DC output voltage. How much depends on the amount of current drawn by the load.
For example, if you use a 100 Ω resistor and the load draws 100 mA, the resistor will drop 10 V (100 0.1). Thus, to provide a final output of 5 V, the rectifier circuit must supply 15 V because of the 10 V drop introduced by the resistor.
You can also use an inductor in a filter circuit. Unlike a resistor-capacitor filter, an inductor-capacitor filter doesn't significantly reduce the DC output voltage. Although inductor-capacitor filter circuits create the smallest ripple voltage, inductors in the range needed (typically 10 henrys) are large and relatively expensive. Thus, most filter circuits use a single capacitor or a pair of capacitors coupled with a resistor.