CHAPTER 2

CHAPTER 2

DiodeCircuits

OBJECTIVESDescribe 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 DCFiltered 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 SheetSome 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.