CHAPTER 2: Diode Circuits potx

CHAPTER 2 Diode Circuits OBJECTIVES Describe and Analyze: • Rectifier Circuits • Voltage Multiplier • Clippers & Clampers • Switching Circuits • Diode Data Sheet Specs • Troubleshooting Intro to Rectifiers • The job of a rectifier circuit is to produce a DC output from an AC input. • Rectifiers are in power supplies where they convert 60 Hz AC into “raw” DC. • Rectifiers are in AM radios where they are demodulators: they convert a radio signal into a DC level that varies with the audio signal. Half-Wave Rectifier The most basic rectifier circuit. Note that the DC output is not steady as a battery is. It’s pulsating DC. Half-Wave Rectifier • Half-wave rectifiers are not efficient for converting 60 Hz AC into DC. Half the input never makes it to output. • Half-wave rectifiers cause DC current to flow in the AC source. If the source is a transformer, DC current could damage it. • The demodulator in an AM radio is a half- wave rectifier. Full-Wave Rectifier Uses all the AC input. Requires a transformer Full-Wave Bridge Rectifier Requires 4 diodes, but does not require a transformer Filter Capacitor Capacitor required to convert “raw” DC to usable DC Filtered DC still has a small AC ripple on top of the DC Ripple Full-wave ripple frequency is twice AC frequency Power Supply System Regulator removes most ripple & keeps DC level fixed [...]... an AC signal Diodes Used As Switches A small AC signal can’t forward-bias a diode When a DC forward-bias is applied,the small AC signal can pass through the diode s low internal resistance Diode Data Sheet Some important diode specifications: VRRM: Peak repetitive reverse voltage Higher voltage will cause reverse breakdown in diode IO: Average forward current The maximum DC current that diode can conduct... can burn up diode IFSM: Peak surge current Maximum current the diode can conduct for a few milli-seconds, such as when it charges the filter capacitor in a power supply Diode Data Sheet (cont) VF: Forward voltage drop Maximum voltage across diode when conducting Usually specified at IO Typically about 0.7 Volts for silicon IR: Reverse current Maximum leakage current in a reverse-biased diode Usually... reverse-biased diode Usually specified at some temperature trr: Reverse recovery time How long it takes for a diode to stop conducting after a reverse bias voltage is applied Important for rectifiers in switching power supplies which operate at frequencies from 20 kHz to 200 kHz to 1 MHz or higher Checking a Diode Using a meter set to Ohms, you can separate the live ones from the dead ones Troubleshooting... piece of electronic equipment fails, the first suspects are the components under high stress Stresses are high current, high voltage, and high temperature Power supplies can have all three ingredients • Diodes can “pop”, often from too much surge current into the filter capacitor Troubleshooting • Aluminum electrolytic filter capacitors can dry out over time, and occasionally spring a leak Capacitors... equivalent series resistance It can increase with age, and causes ripple to increase Troubleshooting CAUTIONS: • A power supply that puts out only 5 Volts DC can have 120 Volts AC or more on the rectifier diodes and filter capacitors That’s the case in “off-line” switching power supplies which, today, are the most commonly used supplies in electronic equipment • If you wear a ring, and you grab the top . CHAPTER 2 Diode Circuits OBJECTIVES Describe and Analyze: • Rectifier Circuits • Voltage Multiplier • Clippers & Clampers • Switching Circuits • Diode Data Sheet. an AC signal Diodes Used As Switches A small AC signal can’t forward-bias a diode. When a DC forward-bias is applied,the small AC signal can pass through the diode s low internal. low internal resistance. Diode Data Sheet Some important diode specifications: VRRM: Peak repetitive reverse voltage. Higher voltage will cause reverse breakdown in diode. IO: Average forward