Download Kirchhoffs_Laws

Week 3: Experiment 4
Kirchhoff’s Laws
Circuit to be Constructed
Changes from circuit in lab manual.
1. The DC voltage supply to use is +5V, not +9V as is shown in
the lab manual.
2. A second circuit will be constructed in which a trim
potentiometer (trim pot) replaces the 8.2 kW resistor.
Trim Potentiometers
• Trim pots, for short
– A resistor whose values depends on the position
of the wiper (middle terminal – Pin 2).
• Used as a voltage divider
– All three terminals are connected in the circuit.
• Used as a variable resistor
– Either pins 1 and 2 or 2 and 3 are connected in the circuit.
Pinout
•
Pins 1 and 3 are labeled on the top surface of
the trim pot.
– The resistance between pins 1 and 3 is the
maximum resistance of the trim pot (Rpot).
•
The middle pin (2) is connected to the wiper.
– The resistance between pins 1 and 2 is x Rpot,
where x is the fraction of the total number of turns
of the knob.
– The resistance between pins 2 and 3 is (1 – x) Rpot,
where x is the fraction of the total number of turns
of the knob.
– There may be a notation on the top surface about
the direction that the knob should be turned
[Clockwise (CW) or Counterclockwise (CCW)] to
increase the value of the resistance between pins
1 and 2 and decrease the value of the resistance
between pins 2 and 3.
http://www.solarbotics.com/assets/images/rt1k_t/rt10k-tdscn3762_pl.JPG
Pspice Symbol
POT:
Trim Pot
R_Var:
Variable Resistor
Reading the Value of Your Trim Pot
• On one surface of the pot are markings
– The maximum resistance of the trim pot
– The part number
• The value of the resistance is calculated as follows:
– The first two digits of the three digit number is the
number that is then multiplied by 10 raised to the third
digit.
» For example: 102 = 10 x 102 = 1 kW
Resistance Between Pins
• When measuring the resistance of your trim pot
– Resistance between pins 1 and 3 is the maximum
resistance of the trim pot.
– Resistance between pins 1 and 2 added to the resistance
between pins 2 and 3 is equal to the resistance between
pins 1 and 3.
Rpot  R1,2  R2,3
• Turning the knob on the trim pot clockwise changes the fraction of
the maximum resistance that is between pins 1 and 2.
R1,2  x Rpot
where0  x  1
– All resistance measurements should be made when the trimpot is not
connected in the circuit.
Pre-Lab
• Perform the steps in the Analysis and Modeling
sections of Procedure for Experiment 4.
– Analysis section are calculations that you perform
by hand.
– Modeling section are simulations that are
performed using PSpice.
PSpice
• The download site and installation instructions are posted
on the ECE 2074 Scholar site.
• Please install both Schematics and Capture if you decide to
install PSpice Version 9.1 (which I strongly encourage you to do).
• Example circuits for Version 9.1 or 10 are posted on
Resources/Technical Support: Circuit Simulation
• Find the appropriate folder and download either Example1.sch
(Schematics) or Example1.zip (Capture)
• For Version 16
– http://filebox.vt.edu/users/kameehan/pspice
• Download Examples.zip
• Extract the folders and files from the zipped files
• Follow instructions posted on Scholar to verify that the installation is
correct if you are using PSpice versions 9.1 or 10
– See me if you plan to use Version 16.2 or Cadence Virtuoso.
Bias Point Calculations:
PSpice Schematics Version 9.1
After Launching Schematics
Select Draw/Get New Part, which will cause
a pop-up window to open with a list of all of
the parts available in the student version of
Schematics.
- Note that there is a key stroke shortcut .
Pick a Part
Type the name of the part in the
box located by Part Name or
Scroll through the list of parts and
click on the name of the one that
you want put into the circuit.
Then, click Place if you have more
parts to select or Place and Close
if this is the last part.
If You Know the Name of Your Part
You can type it in directly into the box below the Toolbar
Help and then hit Enter.
– Either way, a symbol for a dc battery will appear on
the schematic when you move your cursor on to it.
• Left Click to place the part in a specific location on the
schematic.
• Right Click to end the process of placing the part Vdc.
Names of Components
Component
PSpice Name
Component
PSpice Name
DC voltage supply
Vdc
Resistor
R
DC current supply
Idc
Capacitor
C
Ground
gnd_earth
Inductor
L
Connect Parts
Click on the pencil.
Then click the pencil tip to one
end of a part and then to the
end of another part to place a
wire between them.
All circuits in PSpice must
include a ground before a
simulation will run.
Setting the Value of the
Component
• For the voltage source and resistor, double
click on the default value, which is 0V for the
voltage source and 1k (which is short hand for
1 kW) for the resistor.
– In the pop-up window that opens, replace the
default value and click OK.
• The component value will change on the schematic.
Setting the Value for R_VAR
• Double click on the variable
resistor symbol, which will be
highlighted in red.
– Click on the word VALUE in the list
or by typing VALUE into the box
labeled Name. Then enter 8.2k
into the box labeld Value.
– Click Save Attr.
– Select SET and change its value to
1.
– Click Save Attr. and then Click OK.
Save Your Schematic
• PSpice will not run a simulation unless your
circuit is saved.
– You can select any directory in which to save the
.sch file.
• Note: You can send the .sch file to me if you need
assistance finding errors that prevent the simulation
from running.
Select Type of Simulation
• Click on Analysis/Setup or the
button.
– Bias Point Detail should be clicked. If not, do so.
Run the Simulation
• Click Analysis/Simulate or click on the
A pop-up window should
open, text should scroll by in
the bottom left box. After
“Simulation complete” is
printed, the run is finished.
button.
Common Error
There must be a dot, inserted by the program, at the end of the ground to
connect it to the rest of the circuit. The simulation uses this point as 0V and
calculates all currents referenced to this voltage.
Correct placement of dot
Incorrect placement of dot
To Display DC Values
• Click on Analysis/Display Results on Schematic
and enable the voltage and/or current display.
Screenshot for Pre-Lab
• Should include:
– The schematic of the circuit in Experiment 4
– The voltage and currents should be displayed.
• You can move the position of the displayed voltages
and currents by clicking and dragging them to a new
location.
– This should be done if any of the voltage or currents overlap
one another or hide the resistor or voltage source in the
schematic.
Comment on Analysis
• Note that the calculation performed by PSpice
Schematics is a nodal analysis.
• The voltage drop across a component is the
difference between node voltages:
– The first voltage is the node at which current is
entering the component and the second voltage is
the one at which the current is leaving the
component.
Arrow show the direction of the current that PSpice has assumed when
you click on the current.
The voltage across R1, VR1, is equal to 9V – 2.25V = 6.75V
Output File
• If you go to Analysis/Examine Output, a text
file will open and contains:
– The net list, a list of the names of the nodes in the
circuit, the components connected to the nodes,
and their values.
– The type of simulation run.
– Table of the nodes and the voltages at each node.
– A list of the voltage sources in the schematic , the
currents flowing out of the sources, and the total
power dissipated by the resistors in the circuit.