Mechatronics 1 Filters & Regulators

Filters & Regulators Power Supply Review Rectifier Review Filters transformer rectifier filter regulator Rectifier Review diode review half wave operation center tap full wave operation bridge operation Filters capacitor review RC time constant review capacitor filter operation ripple voltage Regulators IC regulators line regulation load regulation zener diodes

Power Supply Overview

Power Supply Overview Transformer – sets the appropriate voltage level Rectifier – rectifies AC input voltage to pulsating DC voltage (can be half wave or full wave rectified Filter – eliminates fluctuations in the rectified voltage and produces a relatively smooth AC voltage (this function was performed by the capacitor in the last part of Lab 5) Regulator – maintains a constant voltage despite variations in the input line voltage or the load

Power Supply Overview

Rectifier Review Diode review Half wave operation Center tap full wave operation Bridge operation

Diode Review Key things to remember: General Forward bias Reverse bias P material is the anode N material is the cathode Forward bias the applied voltage is more negative at the cathode with respect to the anode if barrier voltage is overcome, current will flow acts like a closed switch Reverse bias the applied voltage is more negative at the anode with respect to the cathode no current will flow (unless breakdown voltage is achieved – zener diode) acts like an open switch

Forward bias When a pn junction is forward-biased, current is permitted. The bias voltage pushes conduction-band electrons in the n-region and holes in the p-region toward the junction where they combine. p-region n-region The barrier potential in the depletion region must be overcome in order for the external source to cause current. For a silicon diode, this is about 0.7 V. p n R + - VBIAS The forward-bias causes the depletion region to be narrow.

Reverse bias When a pn junction is reverse-biased, the bias voltage moves conduction-band electrons and holes away from the junction, so current is prevented. p-region n-region The diode effectively acts as an insulator. A relatively few electrons manage to diffuse across the junction, creating only a tiny reverse current. p n R - + VBIAS The reverse-bias causes the depletion region to widen.

The P-N Junction “The polarity of applied voltage which causes charge to flow through the diode is called Forward Bias.” “The polarity of applied voltage which can't produce any current is called Reverse Bias.” Source : http://www.st-and.ac.uk/~www_pa/Scots_Guide/info/comp/passive/diode/diode.htm

Bias Recognition Forward Reverse Reverse Forward Forward Forward

Bias Recognition Reverse Forward Reverse

Half Wave Rectifier

Average Voltage Value the average voltage is a measure of the efficiency of the rectifier circuit the “straight line” dc equivalent of the pulsating dc created by half wave rectification the value you would measure on a dc voltmeter V p(out) V AVG = ---------- π

Center Tapped Full Wave Rectifier

Bridge Full Wave Rectifier

Since 2/ π = 0.637, you can calculate Average Voltage Value twice that of half wave rectified output 2V p(out) V AVG = ---------- π Since 2/ π = 0.637, you can calculate V AVG = 0.637 V p(out) The full wave rectifier is twice as efficient as the half wave rectifier

Filters Capacitor review RC time constant review Capacitor filter operation Ripple voltage

The Basic Capacitor Capacitors are one of the fundamental passive components. In its most basic form, it is composed of two conductive plates separated by an insulating dielectric. The ability to store charge is the definition of capacitance. Conductors Dielectric

The Basic Capacitor Initially uncharged Source removed Fully charged Charging The charging process… A capacitor with stored charge can act as a temporary battery.

Charging

Discharging

How fast does a capacitor charge or discharge?

The RC time constant When a capacitor is charged through a series resistor and dc source, the charging curve is exponential.

Capacitor Charging Voltage Curve

Tau (T) The voltage across a capacitor cannot change instantaneously because a finite time is required to move charge from one plate to another The rate at which the capacitor charges or discharges is determined by the RC time constant of the circuit The time constant of a series RC circuit is a time interval that equals the product of the resistance and capacitance T = RC

The RC time constant When a capacitor is discharged through a resistor, the discharge curve is also an exponential. (Note that the current is negative.)

Capacitor Discharging Voltage Curve

Universal exponential curves Specific values for current and voltage can be read from a universal curve. For an RC circuit, the time constant is Rising exponential Falling exponential

Half Wave Rectifier with Capacitor Filter

First Quarter Cycle

Remainder of Cycle

Second Cycle

Ripple Voltage the variation in the output voltage much improved when you add filtering the smaller the ripple, the better the filtering and the better quality dc output

Half Wave & Full Wave Ripple Comparison

Ripple Voltage Make RC > 10T The ripple factor (r) is an indication of the effectiveness of the filter and is defined as the ratio of the ripple voltage (Vr) to the dc (average) value of the filter output voltage (VDC) r = (Vr / VDC) x 100%

Regulators IC regulators Line regulation Load regulation

IC Regulators Filters reduce ripple from a power supply to a relatively low level (<10%) Integrated circuit regulators connect to the output of a filtered regulator and reduce the ripple to a negligible level Regulators maintain a constant output voltage despite changes in the input voltage, load current or temperature Available in a variety of voltages

A Basic Regulated Power Supply

Percent Regulation Regulation as a percentage is a figure of merit used to measure performance of a voltage regulator Line Regulation How much change occurs in the output voltage for a given change in the input voltage Line Regulation = (ΔVOUT / ΔVIN)100% Load Regulation How much change occurs in the output voltage from no load to full load Load Regulation = (VNL – VFL / VFL)100%

Measures Efficiency – Average Voltage Value (VAVG) Filter Quality – Ripple Voltage (Vr) & Ripple Factor (r) Regulator Quality – Line & Load Regulation