Electronics 1
Lecture 7
Diode types and application
Ahsan Khawaja
ahsan_khawaja@comsats.edu.pk
Lecturer
Room 102
Department of Electrical Engineering
Outline
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Working of Full wave rectifier.
Ripple effect
Clippers
Clampers
Voltage multiplier (Doubler Circuit)
Refrences
Full Wave Rectifier(FWR)
FWR Working
Ripple Effect (RE)...
cont...
• So far this rectifier is not very useful.
• Even though the output does not change polarity
it has a lot of ripple.
• “The variation in the capacitor voltage due to
charging and discharging is called the ripple
voltage”.
• To generate an output voltage that more closely
resembles a true d.c. voltage we can use a
reservoir or smoothing capacitor in parallel with
the output (load) resistance (prev slide fig b).
Ripple Effect...
Ripple Effect...
• For a given input frequency, the output
frequency of a full wave rectifier is twice that
of a half wave rectifier.
• As a result, a full wave rectifier is easier to
filter because of the shorter time between
peaks.
Compraison of HWR & FWR
Ripple Voltage Calculations
Vr(pp) = (1/fRLC)Vp(rect)
VDC = (1 – 1/2fRLC)Vp(rect)
Smoothed Half Wave Rectifier
Circuit with reservoir
Output voltage
capacitor
The capacitor charges over the period t1 to t2 when the diode is on
and discharges from t2 to t3 when the diode is off.
Smoothed Half Wave Rectifier
• When the supply voltage exceeds the output
voltage the (ideal) diode conducts. During the
charging period (t1 < t< t2)
vo = VM sin (t)
(The resistance in the charging circuit is strictly
Rf which we have assumed to be zero. Even for
a practical diode RfC will be very small)
Smoothed Half Wave Rectifier
• When the supply voltage falls below the output
voltage the diode switches off and the capacitor
discharges through the load.
• During the discharge period (t2 < t< t3 ) and
vo = VM exp {- t ’ /RC}
where t’= t- t2
• At time t3 the supply voltage once again exceeds the
load voltage and the cycle repeats
Smoothed Half Wave Rectifier
• The resistance in the discharge phase is the
load resistance R.
• RC can be made large compared to the wave
period.
• The change in output voltage (or ripple) can
then be estimated using a linear
approximation to the exponential discharge.
Smoothed Half Wave Rectifier
• vo = VM exp {- t ’ /RC}  VM [ 1- (t ’ /RC)]
• The change in voltage V is therefore
approximately given by VM t ’ /RC
• For a the half wave rectifier this discharge
occurs for a time (t3 - t2 ) close to the period T
= 1/f, with f= frequency.
• Giving the required result:
VM T
ΔV 
RC
Smoothed Half Wave Rectifier
• We can define a ripple factor as
ΔV
Ripple factor 
Vd.c
where Vd.c. = (VM - V/2)
The lower the ripple factor the better
Half Wave Rectifier
• If we don’t consider the diode to be ideal
then from the equivalent circuit we obtain,
for vi >Vc:
vi – Vc – i Rf - iR =0
i.e.
vi  Vc
i
• Giving
( Rf  R )
R
vo  iR 
(vi  Vc )  vi  Vc
( Rf  R)
Non-Ideal Half Wave Rectifier
VM
Non-Ideal Half Wave Rectifier
• A plot of v0 against vi is known as the
transfer characteristic.
R/(R + Rf)
VC
vi
Non-Ideal Half Wave Rectifier
• We usually have R>> Rf so that Rf can be
neglected in comparison to R.
• Often VM >> Vc so Vc can also be neglected.
The transfer characteristic then reduces to
v0  vi
SURGE CURRENT IN THE CAPACITOR
INPUT FILTER
• When the power is first applied to a power supply,
the filter capacitor is uncharged..
• At the instant the switch is closed, voltage is
connected to the rectifier and the uncharged
capacitor appears as a short.
• An initial “surge” of current is produced through the
forward-biased diodes.
SURGE CURRENT IN THE CAPACITOR
INPUT FILTER
• It is possible that the surge current could
destroy the diodes, for this reason a surge
limiting resistor Rsurge, is sometimes
connected.
• The value of this resistor must be small to
avoid a significant voltage drop across it.
• The diode must have a forward current rating
that can handle the momentary surge of
current.
Diode Clipper Circuits
• Clipper circuits have the ability to ‘clip’ off a
portion of the input signal without distorting
the remaining part of the alternating
waveform.
• Such a circuit may be used to protect the
input of a CMOS logic gate against static errorprone states.
• There are two types of clipper circuit, namely
series and parallel.
Series Clipper Circuits
Parallel Clipper Circuits
Diode Clipper Circuits
• When the diode is off the output of these
circuits resembles a voltage divider
 RL 
vo  
vi

 RL  RS 
• If RS << RL ;v0  vi
• The level at which the signal is clipped can
be adjusted by adding a d.c. bias voltage in
series with the diode.
Diode Clampers (DC Restorers)
A diode clamper adds a DC level to an AC voltage. The capacitor charges to the
peak of the supply minus the diode drop. Once charged, the capacitor acts like a
battery in series with the input voltage. The AC voltage will “ride” along with
the DC voltage. The polarity arrangement of the diode determines whether the
DC voltage is negative or positive.
0V.
Diode Clampers (DC Restorers)
Applications:
Amplifiers of all types use capacitive coupling between stages. Why?
To simplify the DC biasing; allows stage by stage independent biasing.
This capacitive coupling “loses” the DC component, stage to stage. To “restore”
DC, the Diode Clamper can be used.
Here is a –DC Restorer circuit
Voltage Multipliers
Clamping action can be used to increase peak rectified voltage. Once C1 and C2
charges to the peak voltage they act like two batteries in series, effectively doubling
the voltage output. The current capacity for voltage multipliers is low.
Voltage Doubler
Voltage Multipliers
The full-wave voltage doubler arrangement of diodes and capacitors takes
advantage of both positive and negative peaks to charge the capacitors giving it
more current capacity. Voltage triplers and quadruplers utilize three and four
diode-capacitor arrangements respectively.
Voltage Multipliers - Triplers
The voltage tripler arrangement adds another diode/capacitor set.
+ half-cycle: C1 charges to Vp through D1,
- half-cycle: C2 charges to 2Vp through C2,
Next + half-cycle: C3 charges to 2Vp through C3.
Output is across C1 & C3.
Voltage Multipliers - Quadruplers
The voltage tripler arrangement adds another diode/capacitor set.
+ half-cycle: C1 charges to Vp through D1,
- half-cycle: C2 charges to 2Vp through C2,
Next + half-cycle: C3 charges to 2Vp through C3.
Next - half-cycle: C4 charges to 2Vp through C4
Quadruple Output is across C2 & C4.