Rectifier/filter circuit

bahmotherElectronics - Devices

Oct 7, 2013 (3 years and 11 months ago)

132 views

,{
|
Rectifier/filter
circuit
PARTS AND MATERIALS
.
Low-voltage AC power sLrpply
.
Bridge rectifier pack (Radio
Shack caLalog# 276-1 185 or equivalent)
.
Electroly'tic capacitor, 1000 pF, at least 25 WVDC (Radio
Shack catalog # 272-
1047 or equivalent)
.
Fout "banana"
jack
style binding posts,
or other terminal hardware, for connection
to potentiometer circuit (Radio
Shack catalog # 274-662 or equivalent)
.
Metal box
.
12-volt light bulb, 25 watt
.
Lamp
socket
A bridge rectifier "pack" is highly recommended
over constructing a bridge
rectifier
circuit from individual diodes,
because such "packs" are made to bolt onto a metal heat
sink. A metal box is recommended
over a
plastic
box for its ability to function as a heat
sink for
the rectifier.
A larger capacitor value
is fine to use in this experiment, so long
as its working voltage is
high enough. To be safe, choose a capacitor with
a working voltage rating at least twice
the RMS AC voltage
output of the low-voltage AC power
supply.
High-wattage 12-volt lamps n-ray
be
purchased
from recreational vehicle
(RV)
and
boating supply
stores. Common sizes are 25 watt and 50 watt.
This lamp will be used as a
"heavy" load for the power supply.
CROSS-REFERENCES
Lessons In Electric
Circuits,
Volume
2, chapler 8: "Filters"
LEARNING
OBJECTIVES
.
Capacitive fllter function
in an AC/DC power supply
.
Importance
of heat sinks for power serniconductors
SCHEMATIC DIAGRAM
Rectifi*r
!
DC
trut
ILLUSTRATION
INSTRUCTIONS
This experiment involves constructing
a rectifier and filter circuit for attachment to the
low-voltage AC power supply constructed earlier. With this device,
you
will
have a
source of low-voltage, DC
power
suitable
as a replacement for a battery in battery-
powered experiments. If you would like to make this device its own, self-contained
120VAC/DC power supply,
you
may add all
the componentry of the low-voltage AC
supply to the "AC in" side of this circuit:
a transformer, power cord, and plug. Even if
you don't choose to do this, I recommend using a metal box larger than
necessary to
.*
ffi
ff
(-)
provide room for additional voltage
regulation circuitry
you
might choose to add to this
project later.
The bridge rectifier unit
should
be rated
for a current at least as high as the transforrner's
secondary winding is rated for, and for a voltage at least twice as high as the RMS
voltage of the transformer's output
(this
allows for
peak
voltage, plus an additional safety
margin). The Radio Shack rectifier specified in the parts list is rated for 25 amps and 50
volts, more than enough for the output of the
low-voltage AC power supply specifled in
the AC experiments chapter.
Rectifier units of this size are often equipped with "quick-disconnect" terminals.
Complementary "quick-disconnect" lugs are sold that crimp onto
the
bare
ends
of
wire.
This is the preferred method of terminal connection. You may solder wires directly to the
lugs of the rectifier, but I recommend against direct soldering to any serniconductor
component for two reasons:
possible
heat damage during soldering, and difficulty of
replacing
the
component in the event of failure.
Semiconductor devices are more
prorle
to
failure than most of the components covered in
these experiments thus far, and so if you have any intent of making a circuit permanent,
you should build it to be maintained.
"Maintainable
construction" involves, among other
things, making all delicate components replaceable. It also nleans making
"test points"
accessible to meter probes throughout the circuit, so that troubleshooting may be
executed with a minimum of inconvenience. Terminal strips inherently
provide
test
points for
taking
voltage measurements,
and
they also allow for
easy disconnection of
wires without sacrificins connection durabilitv.
Bolt the rectifier unit to the inside of the metal box. The box's surf-ace area will act as a
radiator, keeping the rectifier unit cool as it passes high currents. Any metal radiator
surf-ace desigr-red to lower the operating temperature of an electronic component is called
a heat sinlr. Serniconductor devices in
general
are
prone
to damage from overheating, so
providing a path for heat transfer from the device(s) to the anbient air is very important
when the circuit in question may handle large amounts of power.
A capacitor is included in the circuit to act as afilter to reduce ripple voltage. Make sure
that you connect the
capacitor
properly
across
the
DC output terminals of the
rectifier, so
that the polarities match. Being an electrolytic capacitor, it is sensitive to damage by
polarity
reversal. In this circuit especially, where the internal resistance of the transformer
and
rectifier
are low and the short-circuit current consequently is high, the
potential for
damage is great. Warning: a failed capacitor in this circuit will likely explode with
alarming force!
After the rectifier/filter circuit is built, connect it to the low-voltage AC power supply
like
this:
lt
TJ
{9.
@
Measure the AC voltage output by the low-voltage power
supply. Your meter should
indicate approximately 6 volts if the circuit is connected
as shown. This voltage
measurement is the RMS voltage of the AC power supply.
Now, switch your
multimeter to the DC voltage function and measure the DC voltage
output by the rectifier/filter
circuit. It should read substantially higher than the RMS
voltage
of the AC input measured
before. The filtering action of the capacitor provides a
DC output
voltage equal to the peak AC voltage, hence
the
greater
voltage indication:
Full-wav*,
rectified Dfr valtage
tt
-',
il,
,:
'i
,l
'l
r.i -i
Time
---*'F
Ful
yayl
*iflld
fc
vutt.*E*,,*iilr
1il1,yilu
Time
-**--*
Measure the AC ripple voltage magnitude with
a digital voltmeter set to AC volts (or
AC
millivolts). You should
notice a much smaller ripple voltage in
this circuit than what was
measured in any of the unfiltered rectifier circuits previously
built. Feel free to use
your
audio detector to "listen" to the AC ripple
voltage output by the rectifier/filter unit. As
usual, connect a small "coupling" capacitor
in series with the detector so that it does not
respond
to the DC voltage, but only the AC ripple. Very little
sound should be heard.
After
taking unloaded AC ripple voltage measurements, connect the 25 watt light
bulb to
the output of the rectifier/filter circuit like
this:
Re-measure the ripple voltage present between the rectifierlflter unit's "DC out"
terminals. With
a healy
load,
the filter capacitor becomes discharged between rectified
voltage peaks, resulting in greater ripple than before:
Full-wav*. filtered DC vcrlta6e
under heavy
load
Time
-ts
If less ripple is desired under heavy-load conditions, a larger capacitor may be used, or
a
more complex filter circuit may be built using two capacitors and an inductor:
DC
out
If
you
choose to build such a filter circr-rit, be sure to use an iron-core inductor
for
maximum inductance, and one with thick enough wire to safely handle the full rated
current of power supply. Inductors used for the purpose of filtering are sometimes
referred to as chokes, because they "choke" AC ripple voltage from
getting
to the load. If
a suitable choke cannot be obtained, the secondary winding of a step-down power
transformer like the type used to step 120 volts AC down to 12 or 6 volts AC
in
the
low-
voltage power
supply
may
be
used. Leave the prirrary (120 volt) winding open:
i-eave flmse r{rires
dlsconnerredj
DC
0ut
COMPUTER SIMULATION
Schematic with SPICE node numbers:
)
D--
Lt
Netlist
(make
a text file containing the followingtext. verbatim):
r..-
..,:r'o
h,
,
cae
y
ecr if ier
v1
-
0 srn(C 8.485 60 0
C)
rload 2 3 10k
cI23100Curc:O
dl 3 I modl
d2 1 2 modl
d3 3 O modl
d4 O 2 modl
.model
modl d
.tran .5m 25m
.plot tran v(i,O) v(2,3)
. end
You may decrease the
value of Rroao in the simulation from 10 kQ to some
lower value to
explore the effects
of loading on ripple voltage. As it is with a 10 kO load
resistor, the
ripple is undetectable
on the waveform plotted by SPICE.