Electronics Design Laboratory
Lecture #5
ECEN 2270
Electronics Design Laboratory
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Notes
• Experiment 2 demo tomorrow in the lab
– Know how to probe and measure for Ton, Tenc, PWM, etc.
– Show LTspice circuit on the screen
– Use 2 scope probes: show encoder pulses and 555 output (PWM signal) on the scope, display measurement of the PWM signal duty cycle
• Experiment 2 report due via D2L. Double check your submission!
• Quiz 3 today. Ask questions before we start.
– Covers last weeks lecture and work… RC circuits, 555 timer operation, etc
ECEN 2270
Electronics Design Laboratory
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Experiment 3 – Motor Drivers/Speed Control
Speed
Control (Lab 3B)
Motor
Driver (Lab 3A)
Speed
Sensor (Lab 2)
ECEN 2270
Electronics Design Laboratory
3
Experiment 3a
0‐10V
0‐10V
Forward
Controller
Reverse Controller
5VDC
NOT Forward
Forward
Speed error
‐
+
0‐5V Speed Output proportional to desired motor speed
Vspeed
0‐5V Speed Output proportional to actual motor speed
Speed Sensor and Filter
GND
Experiment 3b Experiment 1 & 2
ECEN 2270
Electronics Design Laboratory
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DC Motor Driver
+10VDC
GND
From Feedback Controller
+10V to ‐10V Outputs
From Feedback Controller
• Motor requires up to 1 A in each direction
• Bipolar Junction Transistors (BJTs) used to drive the motors
• BJTs have high current gain, which reduces the output current required from the feedback circuit op‐amps
ECEN 2270
Electronics Design Laboratory
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PN Diode
Forward Current
Ideal
Voltage Drop
Piecewise Linear
Reverse Voltage
“ON”
Forward Voltage
“knee”
0.7v Silicon
“OFF”
Reverse Current
www.electronics‐tutorials.ws
• Needs positive voltage to conduct
• Current flow from Anode to Cathode only
• Multiple models used, depends on how much information you need
– Always use simplest model that works.
ECEN 2270
Electronics Design Laboratory
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NPN Bipolar Junction Transistor (BJT)
• Three port device
– Voltage differences between ports control device operation
• Three common modes of operation
– Cutoff ( Off‐State Switch)
– Active ( Current Amplifier )
– Saturation ( On‐State Switch )
Physical device model
VBC
‐
C
+
B
+
VBE
VBC
C
Saturation
‐
E
VBE
BC Diode Threshold Voltage
Cutoff
ib
B
ic
+
vBE
ic =  ib

-
Active
E
BE Diode Threshold Voltage
ECEN 2270
NPN equivalent active mode model
Electronics Design Laboratory
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PNP Bipolar Junction Transistor (BJT)
• Dual of the NPN device.
• All characteristics are the same, voltages are simply reversed
• Chapter 5 of Sedra/Smith for more information on both NPN and PNP
Physical device model
VEB
E
+
‐
B‐
VCB
VCB
E
Saturation
+
C
+
vEB
VEB
EB Diode Threshold Voltage
Cutoff
ic =  ib

-
Active
ic
B
ib
C
CB Diode Threshold Voltage
ECEN 2270
PNP equivalent active mode model
Electronics Design Laboratory
8
DC Motor Driver
+10VDC
GND
From Feedback Controller
+10V to ‐10V Outputs
From Feedback Controller
• BJTs provide current gain, β ≈ 200
• Limiting the base current can be used to limit he motor current
ECEN 2270
Electronics Design Laboratory
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Motor current limit design
B1
10VDC
Rb1
Q1
Q2
Rmotor
Q3
Q4
Rb2
B2
GND
•
Consider a test at maximum motor current
–
•
•
Wheel locked.
Full speed in one direction: B1 = 10V, B2 = 0V
What mode does each transistor operate in?
– Only one device is ‘on’ per side
ECEN 2270
NPN
PNP
Mode
Active
VEB > 0.8V
VCB < 0.8V
VBE > 0.8V
VBC < 0.8V
Sat.
VCB&VEB > 0.8V
VBC&VBE > 0.8V
Cutoff VCB&VEB < 0.8V VBC&VBE < 0.8V
(C/O)
Q1
Q2
Q3
Q4
Active
Cutoff
Cutoff
Active
Electronics Design Laboratory
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Motor current limit design
B
Rb1
ic
+
vBE
B1
10VDC
C
ib
Q1
Q2
ic = ib
-
Rmotor
NPN equivalent active mode model
Q3
Q4
E
Rb2
E
B2
GND
•
•
•
+
vEB
Insert equivalent models into circuit
Solve Rb1 = Rb2 = Rb to limit the motor current
What is the voltage on the motor?
– Wheel is locked
– Wheel is at maximum speed
•
What is the current through the motor?
– Wheel is locked
– Wheel is at maximum speed
ECEN 2270
ic =  ib

-
B
ic
ib
PNP equivalent active mode model
C
Q1
Q2
Q3
Q4
Active
C/O
C/O
Active
Electronics Design Laboratory
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B
Rb1
ic
+
vBE
B1
10VDC
C
ib
ic = ib
-
Rmotor
NPN equivalent active mode model
E
B2
GND
E
Equivalent Motor Circuit
ECEN 2270
+
vEB
ic =  ib

-
B
ib
PNP equivalent active mode model
Electronics Design Laboratory
ic
C
12
DC Motor Driver Board
ECEN 2270
Electronics Design Laboratory
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Soldering
BAD
suspect
Good! Do this!
• Be careful with the soldering irons!
– Do not burn anyone
– Do not burn anything
• Make sure you know what you want to do before you do it. Double check everything before soldering.
ECEN 2270
Electronics Design Laboratory
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