Circuit Laws and Simulation
Program with Integrated Circuit
Emphasis (SPICE)
Ohm’s Law, Kirchhoff’s Voltage Law, Single loop
circuits, SPICE simulation
Kevin D. Donohue, University of Kentucky
1
Ohm’s Law
Ø
The relationship between voltage and
current through a material, characterized by
resistance, is given by Ohm’s Law:
v = R×i
Ø
Resistive elements will
always absorb power
(i.e. convert electric
energy to heat):
i(t)
Kevin D. Donohue, University of Kentucky
+
v(t)
-
2
Units for Resistance
Ø
Resistance can be characterized in units of
ohms:
1V
R = 1Ω =
1A
Kevin D. Donohue, University of Kentucky
3
Single loop circuits
Ø
Ø
The current in a single loop circuit is the same everywhere
The sum of the voltage drops over resistors in a single loop
circuit equals the voltage rises from the voltage sources:
+ V1 10 Ω
220 V
- V3 +
40 Ω
Io
+
V2
-
Sum of voltages:
220 = V1 + V2 + V3
50 Ω
Substitute Ohm’s Law:
220 = 10 I o + 50 I o + 40 I o
Solve for Io :
220
Io =
= 2. 2 A
10 + 50 + 40
Kevin D. Donohue, University of Kentucky
4
Spice Analysis
Ø
1.
2.
3.
4.
5.
The previous circuit can be described graphically in SPICE and analyzed
using the .OP (operating point simulation). Steps in using B2SPICE (B2
SPICE Workshop) :
Paste circuit elements into workspace including voltmeters and ammeters
using the “Devices” menu option
Connect elements in proper configuration using the wire tool.
Voltmeters are wired in parallel with the elements whose voltage is to be
measured and Ammeters are wired in series with the elements whose current
is to be measured (hooking up the meters wrong could cause the whole
circuit to fail).
Set element properties by selecting the element in the workspace and use the
“Edit” menu option to select the “Edit Simulation Model” feature. A field
will appear to set values for the resistors and set the type and value of the
source. (Meters do not need to be set.)
Then indicate the type of simulation required by using the “Simulation”
menu option and select the “Set up simulations” feature. Check the .OP
(operating point) simulation only and then run the simulation
Kevin D. Donohue, University of Kentucky
5
Spice Example
Ø
Analyze the previous circuit in SPICE. Find the current in the loop
and voltage drops across all resistors:
Ø
Using the edit simulation model feature set the source to DC and indicate
a value of 220. Likewise set resistors to appropriate values. Include a
ground at the base of the source and wire up meters. Run the .OP
simulation and circuit is pasted below after simulation is finished
IVm1
+22.00
R1
VAm1
10
+2.20
R2
50
V1
220
IVm2
+110.00
R3
40
IVm3
-88.00
Kevin D. Donohue, University of Kentucky
6
SPICE Homework Problem
Ø
Must simulate 3 different circuits for R=0, 20, and 2000
ohms! For R=0 remove the resistor and replace with a
wire (otherwise SPICE will generate an error.)
Battery
20 Ω
Io
+
12 V
R
Vo
-
Kevin D. Donohue, University of Kentucky
7