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
Voltage Doubler On negative half-cycle, D1 charges C1 to Vp. On positive half-cycle D2 adds AC peak to Vp on C1 and transfers it all to C2.
Voltage Doubler • Voltage doublers allow you to develop higher voltages without a transformer. • Stages can be cascaded to produce triplers, quadruplers, etc. • Voltage multipliers usually supply low currents to a high-resistance load. • Output voltage usually drops quickly as load current increases.
Clippers Clippers are used to remove portions of an AC signal
Clampers Clampers are used to add a DC level to 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. More current 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 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 • When a 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 have ESR: 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 of a large filter capacitor charged to only 5 Volts, the ring could get hot enough to burn your finger badly if it hits both the (+) and (-) terminals at the same time.