Download Power Supply Circuits and Diodes

Power Supply Circuits and Diodes
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The Junction Diode
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Diode Operations
Forward Biased
Reverse Biased
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A diode application
Encoder Circuit – A circuit that produces different
output voltage codes, depending on the position of a
rotary
switch.
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Junction diode
Junction Diode – A semiconductor diode whose
ON/OFF characteristics occur at a junction between
the n-type and the p-type semiconductor materials.
Bias Voltage – The voltage that causes the diode to
operate in a certain manner.
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Junction diode characteristic
curve
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Breakdown voltage
Breakdown Voltage or Peak Inverse Voltage
(PIV) – The point on the reverse voltage
scale at which the diode breaks down and
there is a sudden increase in the reverse
current.
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Testing junction diodes
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Zener diode
Zener Diode – A diode constructed to operate at
voltages that are equal to or greater than the reverse
breakdown voltage rating.
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Characteristic curve of the
zener diode
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Zener diode current and voltage
Reverse Leakage Current (IR) – The undesirable
flow of current through a device in the reverse
direction.
Zener Voltage (VZ) – The voltage drop across the
zener when it is being operated in the reverse zener
breakdown region.
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Testing zener diodes
1. Check the source voltage to see if it is greater than VZ.
2. Check for an open or short in RS.
3. Check for a short across the load.
If all 3 checks are good, the zener is at fault.
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Light-Emitting Diode (LED)
A semiconductor device that produces light
when an electrical current or voltage is applied
to its terminals.
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LED characteristics
Typical
VF = 1.8V
IF = 15mA
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Transient suppressor diode
Transient Suppressor Diode – A device used to
protect voltage-sensitive electronic devices in danger
of destruction by high energy voltage transients.
Transorb – Absorb transients. Another name for a
transient suppressor diode.
Metal Oxide Varistors (MOV) – Devices that are
replacing zener diode suppressors because they are
able to short a much higher current surge and are
cheaper.
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Transient suppressor diodes
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Varistor
Varistor – Voltage-dependent resistor.
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DC power supply
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Rectifiers
Half-Wave Rectifier Circuit – A circuit that converts
ac to dc by allowing current to flow one-half of the
ac input cycle.
VAVG = 0.318 × VPIVpeak
Vout = Vin−VDiode
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Half-wave rectifier
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Output polarity
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Ripple voltage of half-wave
rectifier
Output pulsating dc ripple frequency =
input ac frequency
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Full-wave rectifiers
Full-wave Center-Tapped Rectifier – A
rectifier circuit that makes use of a centertapped transformer to cause an output current
to flow in the same direction during both halfcycles of the ac input.
Vavg = 0.636 x ½ VS peak
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Full-wave center-tapped rectifier
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Ripple voltage and output
voltage of a full wave-rectifier
Output pulsating dc ripple frequency = 2 × the
input frequency
Vout = ½ × VS peak – 0.7V
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Bridge rectifier
Bridge Rectifier Circuit – A full-wave
rectifier circuit using four diodes that will
convert an alternating voltage input into a
direct voltage output.
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Bridge rectifier (continued)
Vavg = 0.636 × VS peak
With a center-tapped transformer
Vavg = 0.636 × ½VS peak
Output pulsating dc ripple is twice the input ac frequency.
Vout = VS peak – 1.4V
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Filters
Capacitive Filter – A capacitor used in a power
supply filter system to suppress ripple currents while
not affecting direct currents.
RC Filter – A selective circuit which makes use of a
resistance-capacitance network.
LC Filter – A selective circuit which makes use of
an inductance-capacitance network.
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Capacitive filtering of a full-wave
rectifier output.
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Percent ripple
Percent Ripple =
VRMS of
Ripple
V AVG of Ripple
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× 100
Regulators
Voltage Regulators – A device or circuit that
will maintain the output voltage of a voltage
source constant despite variations in the input
voltage and load resistance.
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Percent of regulation
percent of regulation =
Vnl − V fl
Vnl
Vnl = no − load Voltage
V fl = full − load Voltage
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× 100
Zener regulator
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IC regulator
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Typical power supply circuit
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Troubleshooting a dc power
supply
Step 1
Diagnose
Step 2
Isolate
Step 3
Repair
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Diagnose
Diagnose the problem as either an operator
error or as a circuit malfunction.
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Isolate
1. Check for obvious errors.
a. Power fuse blown
b. Wiring error
c. Devices installed incorrectly
2. Check for broken wires, loose connections, signs of
overheating or smoking components.
3. Use cause and effect troubleshooting.
4. Split the system in half to eliminate working components.
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Repair
Repair the circuit.
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This ends
Chapter 16
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