Diode Bridges
Book pg. 450 - 454
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Rectification
 The process of converting an ac supply into dc is called
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rectification
The device that carries this out is called a rectifier
Half wave rectifier
 only half of the current is used each cycle
Full wave rectifier
 both halves of the AC cycle are used
 requires a more complex circuit
Diodes are semiconductors that allow current to go in one
direction only
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Semiconductors
 Semiconductors are made from Group 4 silicon and
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germanium doped (covered with) impurity atoms from
group 3 or 5 to modulate its electrical properties
p – type or n – type
Group 4 has 4 valence electrons
Doping with Group 5 elements produces electron rich layers
of n – type semiconductors
Doping with Group 3 elements
produces electron deficit layers
of p – type semiconductors
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Semiconductors
N - type
P - type
 5 valence electrons
 3 valence electrons
 Form covalent bonds with 4
 Net result is a hole as not enough
valence e’s of silicon
 1 free electron left for
conduction
electrons are present to form
covalent bonds
 An electron from a n- type semiconductor can move into the hole
 Electrons can move from hole to hole and create a PD
 These “wafers” can be placed on top of each other to form layers
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Current flow
 Semiconductors allow the current to flow in both directions
 If a p – type is placed next to a n- type current flows easily in one
direction, but almost none in the opposite direction
 This is called a P – N junction diode
Negative charge builds up
holes
Depletion layer
•
•
•
•
Positive charge builds up
This will stop e-movement
A depletion layer forms
Internal electric field
opposes further diffusion
electrons
 Formation and Properties of Junction Diode - Physics - YouTube
[720p].mp4
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 Wider when connected to an emf
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+
 PD creates an electric field that adds up to that in the depletion layer
 Increase in R in the p-n junction
 Diode is reverse biased
 Only a very small current can flow through diode due to the electric field
• This is called leakage current
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Reverse terminals 0fExternal
source
emf
field
Electric field of depletion
+
-
-
 Depletion layers become narrower
Electron flow
 PD creates a field opposite to that in the layer and reduces the net
electric field
 R in the p-n junction decreases
 Diode is forward biased
 Depletion layer is removed and current can flow
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Connecting an ac power supply
 Junction diode
 Load resistor
 Cathode ray oscilloscope
Records current wave
Over time
 Diode
 forward biased
Reverse biased
conducts from x – y
Leakage current y - z
 Only half of available input is used, the other half is wasted
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Diode Bridge
 In order to have most available power, 2 or 4 diodes are used
in a circuit
 This is called a diode bridge
 The magnitude of current through
the resistor changes, but the
direction of current flow is
the same through the resistor
for each half cycle of AC current
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+_
-+
Half – wave rectification
 Basic circuit:
Forward biased diode
x
y
 Load resistor will only have PD across it for half a cycle
 A small forward PD is needed to start conduction
Input: ac
Output: dc
 X – positive: current through diode
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• X – negative: diode is reversed biased, no current
• Although 1 direction only, current is not constant
Increasing current and PD
 To increase a constant value of current or PD
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 use a resistor and capacitor connected in parallel between a
diode and the load
1st half cycle capacitor charges up
PD approaches peak of transformer
output emf
When the current is zero in 2nd half cycle, the
capacitor discharges through the resistor
The rate is determined by the time constant
of the circuit RC = 𝜏
R = circuit resistance, C = circuit capacitance
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Rate of discharge
 RC = 𝜏
 If 𝜏 >> time for a half cycle, Q released and PD will be small
 When the diode conducts again in the next half cycle, the charge
stored in the capacitor will be topped up
 A discharge – charge cycle is created with PD across capacitor
varying much less
 Reservoir capacitor and small variation in output creates a “ripple”
voltage
 A large time constant 𝜏 means
a good smooth curve, but needs
a more expensive capacitor
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Full wave rectification
 Use of two diodes and center tap
on transformer
 Need of a resistor – capacitor pair
 The tap is ½ length of transformer
coil “Y”
 X>0
 𝐷1 will conduct, which is
connected to half of the secondary
coil
 ½ cycle later x < 0, z > 0
 Y always at zero point
 ½ time x is positive and z will be
negative
 During the other half polarities
reverse
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 𝐷2 will conduct
 During both half cycles current is
supplied to the capacitor
 This is a full wave rectification
Disadvantage
 To achieve peak PD, twice as many turns are required on
secondary coil compared to half wave arrangement
 Use a diode bridge to overcome this feature
 Function Of A Bridge Rectifier.flv
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Diode Bridge
 A full coil is used and supplies current
 4 diodes are needed
X>0
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Junction 𝐷1 &𝐷4 > 0, Z < 0
𝐷1 conducts  A is positive
𝐷3 conducts  B is negative
Capacitor charges
Current is supplied to the rest of
circuit
Polarity switches, X < 0, Z > 0
 Conducting now 𝐷2 &𝐷4
 Charge delivered to capacitor is
unchanged