Download Lab Writeup Diodes and AC

AC and Diodes
LBS 267L
Objectives
(1) To learn about the characteristics of diodes and how they are used for rectification.
(2) To learn about filters which employ capacitors and resistors
(3) To be able to: (a) construct correct rectifying circuits with and without filters from schematic
drawings; (b) to distinguish between full and half-wave rectification; and (c) recognize circuits
appropriate for each case.
Theory
Electric power is generated and transmitted from generating plants to businesses and homes in the
form of an AC (originally "alternating current", but now any time varying signal in either current or
voltage) signal such as that shown in Fig. l. The frequency of this AC signal is carefully controlled at 60
Hz (Cycles/sec), so that power from sources can be reliably combined. AC signals have the advantage that
they can be conveniently "transformed" to larger or smaller values via transformers. AC currents and
voltages can also be used for many purposes, such as electronic control and heating.
Figure 1
For some purposes, however, it is essential to have a DC (time independent) signal. Uses include
power supplies for heating and cooling control, bias voltages in electronic tubes and integrated circuits,
etc.
In this experiment we examine how an AC voltage can be turned into a DC voltage using a device
called a DIODE. Unlike an electrical resistor, a diode is a NON-LINEAR device, that is, its output is not
simply directly proportional to its input. A typical current-voltage characteristic for a Diode is shown in
Fig. 2. For the present experiment, the most important feature of Fig. 2 is that the current that passes
through the diode is large for positive applied voltage and very small for negative applied voltage. We can
thus approximate the diode as a current gate, which passes current in one direction only. In this
experiment we shall see how this property permits a diode to be used to convert AC current into DC
current.
In the circuit of Fig. 1 voltage is supplied by an AC (alternating current) generator. Its function is to
produce the voltage shown on the right side of in Fig. l. Consider point a to be at ground (zero) potential
(voltage), then the potential at point b varies with time as indicated. The voltage at point b changes
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periodically from positive to negative with respect to a. The current that flows through the circuit is, by
Ohm's Law:
I = V/R = (V0/R) sin t
This current thus also changes periodically with time. When the polarity of V changes, the direction
of the current also changes. The electrons carrying the current oscillate backward and forward about a
mean position.
Suppose now that we add to the circuit a diode as indicated. A diode usually consists of a junction of
n- and p-type semiconducting materials. The current ID through the diode varies with the voltage VD
applied across it as shown in Fig. 2. Current flows easily in one direction but not in the other. The
direction of current flow is given by the direction of the arrow in the circuit symbol of the diode. It takes a
certain finite voltage to turn on the current in the direction of the arrow in a diode, about 0.2 V for a
germanium diode and about 0.5 V for a silicon diode. (If you want to learn more about diodes, you can
read, e.g., chapter 4.9 of the book "Introductory Electronics for Scientists and Engineers" by R.E.
Simpson. Allyn and Bacon, Inc., Boston 1977.)
Alternatively, one may regard the resistance to current flow as being much greater in one direction
than in the other or that the diode acts as a valve which lets current flow in one direction but not in the
opposite one. In our circuit, appreciable current flows only in alternate half cycles of the applied potential.
We can observe this by looking at the voltage across R with an oscilloscope. Since VR = IR, VR should
only be non-zero on alternate cycles. The current and the voltage VR are called half wave rectified (Fig.
3). Full wave rectified current has the form also shown in Fig. 3.
Figure 3
In both half and full wave rectified currents the current flow is always in the same direction.
(Clockwise in the above circuit; by convention the direction of the current is taken parallel to the velocity
of positive charge carriers and antiparallel to the velocity of negative charge carriers.)
Filters
For the operation of most electronic equipment, one needs a steady DC voltage rather than a full wave
rectified voltage that varies with time. A DC voltage is obtained by applying the full wave rectified
voltage to the input of a filter circuit. The output voltage obtained is shown in Fig. 4.
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Figure 4
It consists of an average DC voltage with a small "ripple" voltage superimposed upon it. In the
experiment that follows, you will attempt to obtain an output for which the ratio of ripple voltage to DC
voltage is a minimum, i.e. the purest DC voltage.
You will examine both DC and ripple voltages. Both voltages will be measured quantitatively using
an oscilloscope and a voltmeter. The amount of ripple is usually given as a percentage
percent ripple, %R = Vrms(ripple)/VDC
and the rms voltage is given in terms of the amplitude of a sine wave as
Vrms = V0/1.414
Procedure
Make sure that you understand how to connect the circuits correctly, and that you have them correctly
connected before you turn them on!!!
1. Half-wave Rectifier Circuit:
(a) Assemble the circuit of Fig. 5. The voltage V and ground G are applied to the red and black leads,
respectively, of the oscilloscope (which in this case is analog input A of the PASCO interface) and the
voltmeter.
How does this circuit behave during the half cycle when current flows down (from 1 to 2) through the
transformer? How does this circuit behave during the half cycle when current tends to flow up through the
transformer? What would you thus predict for the voltage variation across the 5 k resistor? Verify by
observing the wave form on the oscilloscope. The 5k resistor limits the current in the circuit to a value
that is probably too small to give an appreciable reading on the ammeter. Note that a voltmeter is a DC
meter. It therefore reads zero for AC signals and reads only the effective DC component of a rectified
signal; the larger this DC reading, the more effective the rectification. Record the AC and DC voltages
from the voltmeter and save the oscilloscope display for your report.
(b) Insert the capacitive filter shown into the circuit of Fig. 5, first using the 2F capacitor and then
the 20F capacitor (Fig. 6). POWER SHOULD BE OFF ON THE DIODE CIRCUITRY WHEN
CHANGING THE CIRCUIT. THE SCOPE MAY BE LEFT ON.
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Observe and the resulting wave form and save it for your report. Estimate the DC component and the
amount of ripple in percent from the trace on the oscilloscope. Then verify this by recording the AC and
DC voltages on the voltmeter which is attached to the resistor also.
(c) Insert into the original circuit of Part (a) a C-R-C filter shown in Fig. 7.
Observe and save the resulting wave form for your report. Estimate the amount of ripple from the
oscilloscope trace and with the voltmeter.
2) Full-wave Rectifier
A circuit which uses the center tap of the transformer to make a full-wave rectifier is shown in Fig. 8.
For simplicity in this and the next case we do not include a filter circuit, but obviously one could be added
to smooth out the output voltage.
Consider the half cycle for which point A has a negative potential with respect to ground and point B
as a positive potential with respect to ground. Then the diode between 4 and 5 is conducting; the diode
between points 9 and 10 is blocking (why?). Current will flow along the path A, 2, 32, 24, 23, 31, 30, 29,
5, 4, 1, A. Point 23 will be at a negative potential with respect to ground. (note: points 2, 32, 24 are at
ground potential). For the other half cycle, point A has a positive potential with respect to ground and
point B has a negative potential with respect to ground. Now the diode between points 4 and 5 is blocking
(Why?). Current will flow along the path B, 2, 32, 24, 23, 31, 30, 29, 5, 10, 9, 3, B. Point 23 will, once
again, be at negative potential with respect to ground. Record the wave form on the scope and record the
DC voltage.
3) Bridge Rectifier
A way to achieve full-wave rectification without the need for a transformer center tap is shown in Fig.
9. Explain in detail how this circuit gives full-wave rectification. Check with the scope that it does, once
more save the scope display for your report, and estimate the percent ripple both from the display and the
voltmeter.
Your report should include scope displays from each of the circuits and a description of each. Be
sure to include the DC voltage and percent ripple for the cases you have measured. Do not repeat the
material in this write-up, but do try to describe what it is you have observed and what it means.
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