Half Wave Rectifier Since a capacitor input filter only draws current from the rectification
circuit in short pulses, the frequency of the pulses is half that of a
full -wave circuit, therefore the peak current of those pulses is so high
that this circuit would not be recommended for DC power more than 1/2 watt.
60 Hz Ripple Depth of ripple slope
is dependent on Capacitance and Load.
Full Wave Center Tapped A full-wave rectifier uses only one-half of the transformer winding
at a time. The transformer secondary rated current should be 1.2 times
the DC current of the power supply. The transformer secondary voltage should
be approximately 0.8 times the DC voltage of the unregulated power supply
per side of the center tap or the transformer should be 1.6 times VDC center
120 Hz Ripple Depth of ripple slope
is dependent on Capacitance and Load.
Full Wave Bridge The full-wave bridge rectification circuit is the most cost effective
because it requires a lower VA rated transformer than a full-wave
tapped rectifier. In a full-wave bridge, the entire transformer secondary
is used on each half cycle, unlike the full-wave center tapped which
only uses one- half the secondary on each half cycle. The transformer
secondary rated con-rent should be 1.8 times the DC current of the
power supply. The transformer secondary voltage should be approximately
.8 times the DC voltage of the unregulated power supply.
Dual Voltage Supply A dual complementary rectifier is used to supply a positive and negative
DC output of the same voltage. In most cases, the negative current is significantly
less than the positive current requirements so the AC voltage and current
relationship to the DC voltage and current should be the same as the full-wave
center tapped described earlier.
Unregulated Linear Power Supply
Unregulated power supplies contain four basic components: transformer,
rectifier, filter capacitor, and a bleeder resistor. This type of power
supply, because of Us simpticity, is the least costly and most reliable
for low power requirements. The disadvantage is that the output voltage
is not constant It will vary with the input voltage and the load current,
and the ripple is not suitable for electronic applications. The ripple
can be reduced by changing the filter capacitor to an LC (inductor-capacitor)
filter but the cost to make this change would make use of the regulated
linear power supply a more economical choice.
Regulated Linear Supply
A regulated linear power supply is Identical to the unregulated linear
power supply except that a 3-terminal regulator is used in place of the
bleeder resistor. The regulated linear power supply solves alt of the problems
of the unregulated supply, but is not as efficient because the 3-terminal
regulator will dissipate the excess power in the form of heat which must
be accom-modated In the design of the supply. The output voltage has negligible
ripple, very small load regulation, and high reliability, thus making it
an ideal choice for use in low power electronic applications.
Switch Mode Power Supplies
The switch mode power supply has a rectifier, filter capacitor, series
transistor, regulator, transformer, but is more complicated than the other
power supplies that we have discussed. The schematic above is a simple
block diagram and does not represent all of the components in the power
supply. The AC voltage is rectified to an unregulated DC voltage, with
the series transistor and the regulator. This DC is chopped to a constant
high frequency voltage which enables the size of the transformer to be
dramatically reduced, and allows for a much smaller power supply. The disadvantages
of this type of supply are that ail of the transformers have to be custom-made
and the complexity of the power supply does not lend itself to low production
or economical low power applications.
Television Sets Are
Dangerous!! This fact cannot be overstated.
Within the average color TV set, Lethal
voltages range from 150_Volts;
1500_Volts; with the Highest Voltage at 25,000_Volts
Line Rectifier Circuits: Line EMI Filter, Automatic Degaussing
Line Rectifier Circuits: Power Transformer, Line EMI Filter,
Automatic Degaussing Coil
The power supply is often the part of the equipment which converts
alternating to direct current. The filter circuit, which includes the ininductor,
smooths out the fluctuating or pulsating direct current until it is nearly
pure direct current. There are two types of filler chokes: the smoothing
choke and the swinging choke. The swinging choke is one in which the I
and E laminates are butted together so that there is a minimum air gap
between them. This makes the amount of inductance vary with the amount
of current. A typical swinging choke may be rated 20 H at 50 mA and 5 H
at 200 ma. The smoothing choke frequently has a small (0.1 mm air gap between
the I and E laminates. This makes the inductance less dependent on the
amount of current because air does not saturate as easily as iron.
more thing to consider about chokes: the "Q" or quality of the inductor
has an effect on its efficiency. As previously stated, the inductor should
appear as a short circuit to the DC power it is carrying, and a high impedance
to any AC, i.e., no series "R." In the practical world this isn't feasible.
However, if heavy current carrying chokes are required, then the choke
must have higher "Q," i.e., less wire which means lower "R." This can be
achieved by using chokes with ferrite cores, which need considerably less
wire for the same value of inductance: it is truly a multiplier of "Q."
Also ferrite beads, i.e., very small donut or tubular shaped ferrite, are
regularly used for circuit isolation, effectively preventing parasitic
oscillations, etc. The down-side of ferrite, is that it will change inductance
as the current or flux changes. In the case of large currents, it can saturate.
However, by correct component choice -- frequency, AC and DC current, etc.
-- ferrite is great tool for the designer.
is used where the supply voltage cannot be lowered, i.e., if one needed
a noise-free +12 volts on a PC bus, say. One could get a "clean" +12 volts
with a voltage regulator... if only there was +15 volts or higher to start
with. But such is not the case. So you use a high "Q" inductor (RFC choke)
along with the proper bypass capacitor to effectively lowpass filter the
+12 volt supply rail. For a real noisy supply you can use more than one
inductor: a "pi" network for example.
2...One of the most efficient
inductors is the ferrite toroid. It has high "Q" -- low "R" -- and because
of its toroidal shape its fields are confined, and therefore has little
stray fields. The super star of high "Q" inductors or transformers is the
pot core. And of course, don't forget the ferrite bead. Thread the wire
through the bead once or several passes and it may be just what the doctor
3...Decoupling is only
as good as the components that you use. The capacitor part of the network
should be high "Q" and minimum inductance: the noise is dropped across
the inductor, and the capacitor must exclude the remaining noise. Another
way of saying it: in a perfect world the inductor is an open circuit to
noise (AC) and the capacitor is a dead short -- Zero, Nada, Caput, Zilch;
"This here parrot is dead." The slightest inductance in series with that
capacitor, and some very high frequency noise will come through like Gang
Busters!.... Anyway nuff said.
4...SMT or chip capacitors
made of ceramic are best. Also, sometimes in critical circuits, several
size caps in parallel are appropriate, e.g., 1ufd || .1ufd || .001ufd,
etc. The reason for this is as the capacitors become smaller in value,
they also get physically smaller, hence less inductance. However this is
less the case with SMT caps: consult your capacitor data sheets for the
impedance verses frequency plots. Didn't he just say that?
Voltage Regulators as Decoupling
Simple Shunt Regulator
Simple Pass Regulator
Pass Regulator with
Simplifier Linear Voltage Regulator
Example of 7805
The 1N4002 is for protection (optional)
NOTE: The following waveforms,
figs 5 through 10, are shown where the top waveform is the initiating signal,
or stimulus, and represents the current drawn by that circuit.
The bottom waveforms are of the voltage variations on the regulated
D.C. output. These voltages are measured at ~500 millivolts per division.
Overshoot & Ringing LVR Recovering from load changes C = 0.033
Damped Oscillations LVR attempting to oscillate C
= 0.005 µF
* LVR oscillating or Squegging due to OPEN
bypass cap C = 0.00 µF
Ripple Caused by squegging oscillation
1..Read the data sheet.
The needes and capabilities of the regulator are in there somewhere; they
might not jump out and bite you right away, but they are there.
2..The use of three terminal
linear voltage regulators, like the 78xx and 79xx devices, is fairly straightforward.
However, there are a few things to remember: Always bypass -- there's that
word again! -- the input pin and the common pin with a ceramic capacitor
no smaller than 0.33 ufd, and use absolutely the shortest leads possible
(there are some transistors with pretty high ft in that regulator,
and if you furnish enough reactance of the wrong kind, Mr. Oscillation
will visit you).
3..If your regulator is
furnishing power to a capacitive load, and the primary power is removed
-- like unplugging a PC card, or disconnecting an experimental setup --
the charge in that capacitive load will cause the secondary or output of
the regulator to be more positive than the primary or input. If this reverse
voltage exceeds the regulator's ratings it will blow up. To prevent this
sort of failure, a diode is placed between the input and output, such that,
when reverse voltages are present, the diode conducts preventing damage.
4..There will come a day
(or night) when you may need an eight volt regulator, and all you have
is a 7805, five volt regulator. By inserting a voltage equal to the difference
in the common lead, "Voila," you have 8 volts. You can do this by inserting
a zener diode or a low resistance voltage divider (or a pot for variability).
If all else fails, insert a series of silicon diodes (cathodes toward Grd.)
@ .6 volts per, until you have the desired output.
5..These regulators don't
need an output capacitor per se, but a minimum of 1 ufd is recommended
to prevent fast load pulses from causing needless error correction by the
regulator. As for the primary or input capacitance, it depends on the ripple
content from the primary voltage: If the voltage is straight from the rectifier,
then obviously large capacitors are required -- assuming a large load on
the regulator's output. The greater the difference between the input voltage
and the output voltage, the less stringent the capacitor requirements.
6...In the data sheet
-- you know, that funny looking piece of paper that causes you to squint,
and makes your head feel funny -- In the data sheet, there is information
on forward drop, Vfwd, of the regulator at some current. This means that
if the primary voltage is near the desired secondary voltage at some current,
you may be in "Deep Dudu." The greater the difference between the input
voltage and the output voltage, the easier life is: if the rating of the
regulator is a 1.1 volt drop at 500 ma, and you have a 5 volt margin --
say -- you are in fairly good shape; if you have, on the other hand, a
10 volt margin, you're in great shape!
Click image to Enlarge
Click image to Enlarge
* Uncontrolled, unwanted or parasitic oscillation, varying in amplitude
from some peak value to very low, or completely off. The frequency of this
oscillation is high compared to the rate at which its varying.
BASIC POWER SUPPLY APPLICATION GUIDE There are four basic types of power supplies
1) Unregulated Linear
2) Regulated Linear
4) Switch Mode
The differences between the four types include
constant voltage output* cost, efficiency, size, weight, and ripple. We
will explain each type of supply, describe the principle of operation,
and the advantages and
3) Ferroresonant Power Supplies
A ferroresonant power supply is very similar
to an unregulated power supply except for the characteristics of the ferroresonant
The ferroresonant transformer will supply
a constant output voltage over a wide variation of the transformer input
voltage. The problems with using a ferroresonant power supply include that
it is very sensitive to slight changes in line frequency and would not
be swrtchable from 50 Hz to 60 Hz, and that the transformers dissipate
more heat than conventional transformers. These power supplies are heavier
and will have more audible noise from the transformer resonance than regulated
linear power supplies.
RECTIFICATION CIRCUITS FOR REGULATED LINEAR POWER
From our previous description, a regulated linear
power supply is the most economical design for lower power, low ripple,
and low regulation which is suitable for electronic applications. In this
section we will explain the four basic rectification circuits that are
1) Half Wave
3; Full Wave Bridge 4) Oual Complementary