Transcript Document

Admin for next week:
Mid-term date Friday March 20th.
•Review lecture Monday (16th). Any requests?
•Labs as usual
•NO DISCUSSION SECTIONS NEXT WEEK
•Extra Office hours
• 2-4pm Monday: Dr Holder Sharp Lab 222
• 2-4pm Tuesday: Dr Holder Sharp Lab 222
• 2-4pm Wednesday: Seth Sharp Lab 308
• 2-4pm Thursday: Zach Sharp Lab 320
•Email is also OK – or stop by my office any time
Monday or Tuesday
•I will be away Wednesday – Friday.
•Dr Ji will give Wednesday’s class and the exam.
Copyright © 2009 Pearson Education, Inc.
Admin:
• Assignments:
• Fourth assignment due Monday (16th)
• Any questions?
• Fifth Assignment is posted.
• Not due until Monday (23rd), but good exam practice
• Three questions on Thevenin circuits
Copyright © 2009 Pearson Education, Inc.
Thevenin and Norton Equivalent Circuits
Any network of sources
and resistors will appear
to the circuit connected
to it as a voltage source
and a series resistance
vTH= open circuit voltage at
terminal (a.k.a. port)
RTH= Resistance of the
network as seen from port
(Vm’s, In’s set to zero)
Thevenin and Norton Equivalent Circuits
Léon Charles Thévenin
1857-1926
vTH= open circuit voltage at
terminal (a.k.a. port)
RTH= Resistance of the
network as seen from port
(Vm’s, In’s set to zero)
Norton Equivalent Circuit
Any network of sources
and resistors will appear
to the circuit connected
to it as a current source
and a parallel resistance
Ed Norton – Bell Labs, 1898-1983
The Thevenin and Norton equivalent cicuits are just
different representations of the same thing.
They are used to simplify circuit analysis
RTH=RN
VTH=INRTH : so you only usually need IN or VTH
Calculation of Thevenin and Norton
Resistance (RTH = RN)
• RTH=RN ; same calculation (voltage and current sources set to zero)
• Remove the load.
• Set all sources to zero (‘kill’ the sources)
– Short voltage sources (replace with a wire)
– Open current sources (replace with a break)
• Calculate the equivalent resistance
Remove the load.
Kill the sources
Calculate the equivalent resistance
RTH=R3 + R1 || R2
Calculation of the Thevenin Voltage (VTH)
• Put the sources back. Remove the load and calculate
the open circuit voltage
Note:
How much current flows through R3?
What is the Voltage across R3?
The Thevenin equivalent is then just VTH in series with RTH
Calculation of the Thevenin Voltage (VTH)
• Put the sources back. Remove the load and calculate
the open circuit voltage
Note:
How much current flows through R3?
What is the Voltage across R3?
Voltage divider!
VTH = VR2 =
R2
VS
R1 + R2
The Thevenin equivalent is then just VTH in series with RTH
Exercise: Draw the Thevenin Equivalent as
seen by the 40Ω load resistor
• To find RTH, remove the load, kill the sources (short
voltage sources, break current sources) and find the
equivalent resistance.
• To find VTH Remove the load and calculate the open
circuit voltage
Exercise: Draw the Thevenin Equivalent
• To find RTH remove the load, kill the sources (short
voltage sources, break current sources) and find the
equivalent resistance.
Exercise: Draw the Thevenin Equivalent
• Replace the sources
• To find VTH remove the load resistor and calculate the
open circuit voltage
VAB = 20 - (20Ω x 0.33amps) = 13.33V
Exercise: Draw the Thevenin Equivalent
• Draw the Thevenin equivalent
What about the Norton equivalent?
What about the Norton equivalent?
But what if we don’t know VTH?
Remember – if you
suppress the “kilo” in
the resistance, the
current calculated is in
milliAmps
STOP HERE
for mid-term 1
• Anything you’d like me to cover in detail on
Monday?
• Requests so far:
mesh analysis 2
voltage divider & current divider
light bulbs
Thevenin