TOPS Physics
Electronic Trainer
Rectifiers
Electricity is normally supplied to us in the form of alternating current (AC), in which the
voltage continuously varies between negative and positive values. This method is very
convenient, as it allows voltage to be stepped up and down easily as the electricity flow
from the power plant, through transmission lines, and finally to the end user.
Many devices, particularly those that sometimes run from batteries, require a supply of
direct current (DC) in which the voltage is constant and unvarying. This investigation
examines one method of converting AC to DC using rectifiers and capacitors.
Equipment:
Electronics Trainer
iBook computer
Vernier interface
Component Box
Voltage probe
Cautions:
This equipment is delicate. Most of the components are already bent to the correct
shapes. Avoid bending the components if you don’t have to. Do not apply power until
you are instructed to do so!!
Never Never Never short-circuit the power supply! Doing so will (if you are lucky) blow
the fuse. If you’re not lucky, it will blow the whole $200 trainer unit! Don’t plug a wire
into the trainer unless you are told to do so!
Never apply more than 10V to the Vernier voltage probes. The supply voltage test is
critical to this!
Procedure to test the characteristics of capacitors in series and parallel:
The breadboard is set up with power sockets marked with blue and red lines. The rest of
the breadboard consists of sockets in a grid marked vertically with letters and
horizontally with numbers. Each vertical set of five sockets is connected together within
the breadboard. For example, socket 5a is connected to 5b, 5c, 5d, and 5e, but not to 5f.
You will plug components directly into the breadboard sockets and use jumper wires to
connect them together.
You will start by setting up a simple series circuit with two resistors in series You will
attach voltage probes to one resistor that represents the load so that you may monitor the
voltage across and current through the capacitor. You will then modify your circuit with
an eye toward getting the ideal constant DC voltage.
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Getting Started:
1. Open the trainer, make sure that the red rocker switch is toward “OFF” and plug
the line cord into a wall outlet.
Along the top edge of the trainer are several knobs that control the AC frequency
generator. You will use this generator to simulate the AC voltage we get from the
power lines. There are two differences between the AC used here and that from the
power lines. The voltage used here is much lower than the voltage from the power
lines, so it is safe to touch the circuit when it is active. Also, the frequency of the AC
voltage is lower, so that you can see what happens to the voltage level in real time.
2. Make sure that the waveform knob is turned fully clockwise to the sine wave
setting.
3. Make sure that the coarse freq knob is turned fully counterclockwise to the “10”
setting.
4. Make sure that the Fine Adj knob is turned fully counterclockwise.
5. Make sure that the Amplitude knob is turned fully counterclockwise.
6. Make sure that the DC OFFSET knob is midway in its travel.
Constructing the circuit and readying the computer:
Note: This circuit will be constructed entirely on the upper breadboard and upper power
bus. Do not put any components on the lower breadboard or the middle or lower power
buses.
1. Using a short black jumper (a “jumper” is a short piece of wire used to make
connections), connect one of the GND sockets in the power supply to the leftmost
socket just above the blue line on the upper edge of the upper breadboard. This
will be your ground bus.
2. Using a medium red jumper, connect one of the sockets labeled “FREQ” to the
leftmost socket just below the red line on the upper breadboard. This is the power
bus and will have varying voltage, going from positive to negative relative to the
ground bus.
3. Locate a 2200Ω resistor and insert it into sockets 50b and 40b. This load resistor
represents the load that is using the current from our circuit.
4. Locate a 1000Ω resistor and insert it into sockets 40c and 30c. . This source
resistor represents the built-in resistance of your source of voltage.
5. Using a short red jumper, connect any of the power bus sockets (the row directly
below the red line) to socket 50a.
6. Using a short black jumper, connect any of the ground bus sockets (the row
directly above the blue line) to socket 30a.
7. If it isn’t already there, plug a voltage probe into CH 1 of the Vernier interface.
8. Attach the red end of the voltage probe to the resistor lead that goes into socket
50b. Push the button on the end of the probe and “hook” the capacitor lead.
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9. Attach the black end of the voltage probe to the resistor lead that goes into socket
40b. This will sense the voltage across the resistor. This resistor represents the
“load” or the device that is using the voltage. Here is your circuit so far:
AC
Input
Load
Resistor
Source
Resistor
Computer
Ground
10. Make sure that the Vernier interface is plugged into a wall outlet and that it is
connected with the iBook with a USB cable.
11. Turn the iBook on and wait for it to boot up. Logon as a student with a password
of “student”.
12. From the “Student” folder, start the “Buxman Cap item. You will be asked to
clear an “experiment notes” screen. Go ahead and do so. If you get a screen
related to the interface, get assistance from the instructor.
Collecting Data:
You should see a blank graph and a table on the screen. At the top of the screen is a
button labeled “Collect”
1. On the electronic trainer, turn the rocker switch on. It illuminates.
2. Click on the “Collect” button on the iBook. A red trace will start running across
the graph. This trace shows the instantaneous voltage across the load resistor. It
should be close to zero (the center of the graph) if you set the knobs as instructed.
A little roughness of this line is normal.
3. Using the DC Offset knob, move the red trace to the zero volt line.
4. Rotate the amplitude knob clockwise. A sine wave will appear. Adjust the sine
wave until the peaks are at 4 volts.
5. Using the DC Offset knob and the Amplitude knob, make fine adjustments until
the sine wave moves exactly between the +4 and –4 volt grid lines.
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This is the basic waveform that you will be working with. Draw what this waveform
looks like here. Label the axes appropriately.
6. Turn the Fine Adj knob clockwise. What property of the wave changed? Write
your answer here:
7. Return the Fine Adj knob to the fully counterclockwise position.
8. Examine the light emitting diode (LED).
An LED has two functions. The first function is to emit light (duh) when current passes
through it in the forward direction. The other function is to prevent current from passing
through it in the reverse direction. You can think of an LED as a one-way valve for
electricity that lights up when current is passing through. If you look closely, you’ll see
that the base of the LED has a raised ridge that has a flat side to it. Current will flow
from the lead farthest from the flat spot toward the lead nearest to the flat spot. Current
will not flow in the opposite direction.
9. Pull the lead of the 1000Ω resistor from socket 40c, turn the resistor around in the
other socket, and reinsert the free lead in socket 20c.
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10. Insert the leads of the LED into sockets 40d and 30d with the flat pointing toward
socket 40d. It should start blinking, indicating current flow.
11. Observe the waveform on the iBook. The bumps should be pointing downward.
We really want them pointing upward!
12. Reverse the LED in socket 40d and 30d, pointing the flat side toward socket 30d.
The bumps should now point upward. Here is what your circuit looks like now:!
AC
Input
Load
Resistor
Diode
Source
Resistor
Computer
Ground
Draw the new waveform here. Label the axes appropriately.
Examine your data. Note that all negative voltages have been eliminated, so we are
halfway to your goal of a constant DC voltage. The voltage and current is all going in the
same direction but is hardly constant!
In order to smooth out the voltage, you need a device that will store up a little charge
during the peaks of voltage and return that charge to fill in the valleys. The device that
will do this is called a capacitor. Let’s give it a try!
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13. Plug a 470µF capacitor into sockets 40c and 50c, with the negative stripe pointing
toward socket 40c. Your circuit now looks like this:
AC
Input
Load
Resistor
Diode
Source
Resistor
Computer
Ground
Filter
Capacitor
ca
Draw the new waveform
here. Label the axes appropriately. Pay particular attention to
drawing the voltage levels accurately.
Notice that the peak voltage is no longer rising to the 4v level. The voltage also does not
drop to zero. The waveform is therefore considerably smoother than it was, but as you
can see it is still pretty rough. Lets improve on this.
14. Replace the 470µF capacitor with a 1000 µF capacitor. Make sure that the
negative stripe points to the right.
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Draw the new waveform here. Label the axes appropriately. Pay particular attention to
drawing the voltage levels accurately.
Did the large capacitor value improve the smoothness of the wave?
Now, the wave is still pretty rough, but you have been dealing with a frequency of only
about 1 Hz. What will happen if the frequency is increased?
15. Turn the Fine Adj knob slowly clockwise until it is fully clockwise. The
frequency has been increased to 10hz.
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Draw the new waveform here. Label the axes appropriately.
“Real” AC current has a frequency of 60Hz. With the information you have, would you
expect the current curve to be rougher or smoother if the frequency was increased to
60Hz?
Additional explanation:
It was pretty easy to see how the diode created direct current from alternating current. It
simply blocks the current from flowing in one direction. It is a bit harder to see how the
addition of a capacitor can smooth the voltage.
The capacitor is in parallel with the load. When the supply voltage is positive, some
current passes through the load and some gets stored in the capacitor. Later on in the
cycle, the supply voltage drops to zero. The charge stored in the capacitor cannot flow
backwards through the LED, so its only alternative is to flow through the load. This
keeps some current flowing through the load even though the supply current has been cut
off. The result is a smoother flow of current. A larger capacitor will store more charge,
resulting in an even smoother flow. Increasing frequency leaves less time for the
capacitor to discharge, which smoothes the flow even more.
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Cleanup
1. Click on “File”
2. Click on “Quit”
3. Click on “Don’t Save”.
4. Click on “Special”
5. Click on “Shut Down”
6. Remove all components from the breadboard and return the trainer to the
condition in which you found it. You may leave the Vernier interface and probes
set up.
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