Transcript CAD Tools for Circuit Design Electronics Unit – Lecture 5

CAD Tools for Circuit Design
Electronics Unit – Lecture 5
Schematic Diagram Drawing
Etched Circuit Board Layout
Circuit Simulation
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Schematic Capture
Many CAD packages available for design, simulation,
and layout.
Will use ExpressPCB suite for example
Extensive library of standard components
Custom components can be created
Does not include a simulation function
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ExpressSCH Main screen
User’s manual and Quick Start Guide
Component selection tools
After startup, maximize
the window, choose
View menu, then Zoom
To Fit
Pick out a library
component, for example
a 555 timer chip.
View Help screens as
needed
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Choosing Components from Menu
Click on the desired
component. A cursor
will appear.
Position the cursor and
click, the symbol will
be added to the
drawing. Click again to
duplicate the symbol.
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Placing Components
Click and drag to move
the component around
the drawing
Zoom tool
Use the Zoom tool to
get a closer look.
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Using the Component Manager
Choose Component
menu, then select
Component and
symbol manager
Component menu
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Using the Component Manager
Choose Component
menu, then select
Component and
symbol manager.
Select a component,
then click Insert into
schematic.
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Component Properties
Right click on a
component, then open
the Set component
properties window.
Part ID and value can
be set, as well as other
properties.
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A closer look at components
Note U1 has numbered
pins, Q1 does not.
Pin numbers may be
hidden. Click on a pin
to view the pin
number.
Pin number display
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Adding a Wire Connection
Left click at the start
and at each desired
corner.
Wire tool
Right click when done.
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Power and Ground
Library components
include symbols for
power sources and
ground (common)
connections.
NOT a connection
IS a connection
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Creating Custom Components
Components are
created as groups of:
Rectangles
Lines
Arcs
Pins
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Custom Components
Draw the component.
line
Assign pin numbers and
names.
arc
Select the component.
pin
Access the Component
menu and choose Group to
make component.
rectangle
Assign component
properties, such a ID and
type.
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Component Properties
Afterwards, choose
Save custom
component from
Component menu
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Ready for Use
Connect the custom
component into the
circuit.
If desired, use
Component manager
to add the component
to your Favorites list
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Circuit Board Layout
After drawing the circuit
with ExpressSCH, use
ExpressPCB to design a
pattern for an etched circuit
board.
Schematic can be linked to
the board layout to aid in
placing components and
routing interconnections.
Correct pin numbering will
be essential.
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Boards Have Three Layers
Top Layer – an etched copper pattern
Bottom Layer – an etched copper pattern
Silk Screen – an inked pictorial pattern
Top and bottom layers may include
Traces – a copper path interconnecting two or more points
Vias – a connection between the top and bottom layers, with a
continuous plated copper path existing through the hole
Pads – a point where a component lead will be connected
most pads exist on both top and bottom layers and incorporate a
through-hole, thus also serve as a via.
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Component patterns
are saved arrays of
pads which must
match the shape of a
physical component.
place a pad or via
place a component
layer selection
place a trace
It usually includes a
silkscreen outline of
the component.
Custom components
can be created, just
as in ExpressSCH.
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Pad Characteristics
Pad inner diameter sets the size of the through-hole.
Must be large enough to accept the wire lead of the
component.
Outer diameter controls how much solder-accepting
surface is available.
OD = 0.062” and ID = 0.029” is a common pad for small components
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Trace Characteristics
Trace width determines the amount of current the
trace can carry without excessive voltage drop or
heating.
See Design Tips under the ExpressPCB Help menu
for suggested trace widths.
Examples: a 0.025” trace can carry up to about 1 amp
a 0.050 trace is good for up to about 2 amps
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A Very Simple Design Example
Design a one transistor amplifier, with capacitively
coupled input and output.
A protection diode should be used to protect the
transistor from reverse polarity.
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First Draw the Schematic Diagram
After all components
are named and
labeled, select Check
schematic for netlist
errors from the File
menu.
Leave this window
open, then open an
ExpressPCB window.
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Now Place All Components
Don’t worry about
connecting them with
traces yet.
Name each component
to match the name on
the schematic.
Select Link schematic
to PCB from File
menu.
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Connect Components Together
Use the Highlight net
connections tool to
identify which pads
should be joined with
traces.
Blue dots indicate
points that should be
connected, according to
the linked schematic
Highlight net connections tool
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Connect Components Together
Re-arrange the
components for the
most efficient
interconnects.
Highlight net connections tool
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Connect Components Together
Re-arrange the
components for the
most efficient
interconnects.
Then add the traces.
Use the bottom side as
much as possible.
This example uses
much too much space.
It’s just an illustration.
Highlight net connections tool
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Activity E5
Design a simple power supply using a three
terminal regulator integrated circuit.
Draw the schematic.
Lay out a circuit board.
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