Download Circuit Protection, Tips, and Troubleshooting

Circuit Protection, Tips,
and Troubleshooting
Revised Fall 2013
ECE 445
Becoming a Good Design
Engineer
• Understand the Problem
• Understand the constraints you have
• Understand previous approaches to the
problem at hand
• End Goal of Senior Design:
– Solve problems you haven’t been faced with and
be innovative
• But First: Background knowledge is required
Circuit Connections
• Pay attention to
Polarity
Longer Lead: Positive
Electrolytic
capacitor
Shorter Lead: Negative
GOOD
BAD
Ratings
• Determine part ratings
• All components are rated
• Capacitors:
– If the voltage across the capacitor is going to be 50 [V], should you use a
capacitor that is rated to 50 [V]?
•
Resistor: Maximum Power Dissipation
P = i2 * r or P = v2/R
– Example:
– p = (5e-3)2 * 1e4
– I.E The power dissipated without burning it out 250 mW
From Newark Electronics
Data Sheets
• By Reading the Data Sheet:
– Provide pin-layouts
– Device Ratings
– Potential Applications
Voltage Current Limiting
• Fuses: Typically allow for
passage of “normal” current
• A fuse will “blow” above its
current rating
• Diodes:
• Conduct when V>0.7 V
• Best Solution: Use both diodes
and fuses
Fuses
Voltage/Current Protection
Examples
Current across resistor typically = 20 mA
Fuses and Diode Combination
Current Limiting Resistor
Device Polarity
•
•
The longer length is the (+) terminal
Capacitor
Polarity: tantalum or electrolytic
(-)
•
(+)
Diode
No polarity: ceramic or polyester
(-)
(+)
The bar indicates cathode
(+)
(+)
(-)
(-)
http://academic.evergreen.edu/projects/biophysics/technotes/electron/leds.htm/
http://electrapk.com/zener-diode/
Reverse Polarity Protection
Below are 2 different configurations to ensure correct voltage polarity to the circuit.
Circuit will not operate with incorrect polarity
“Diode Bridge”
Circuit will operate under either polarity,
but will have higher losses
Wikipedia
Power Supply
Bypass/Decoupling Capacitors
• Bypass Capacitors
• Takes noise to ground
• Rephrased: Basically
shunts AC signals to
ground
• Typical values range
from: 0.1 uF
• Larger size capacitors
for higher supply
voltages
*Picture http://www.physics.udel.edu/~nowak/phys645/The_bipolar_transistor_files/image013.jpg
Earth Ground vs “Ground”
• Green terminal is
earth ground
• Black terminals are
signal grounds
• Know the
difference!
Earth Ground
Floating Ground
Potentiometers
• Variable Resistors
– Also known as trimpots
– Example if a trimpot is
R=10 K Ohms
– Then from (a) to (c) 
R1 = 6 K Ohms
– Then R2 = 4 K Ohms
(b) R2 = 4k
(a) R1 = 6k
To Rest of
Circuit (c)
Resistor Codes
METRIC-TO-AWG
CONVERSION TABLE
• Reading Surface
Mount Resistors
• I.E. a resistor
marked 332 is
3.3 kilo-ohms
• I.E or 3K3 is 3.3
kilo-ohms
• Know wire gauges
Resistor Value Chart
Metric Size
mm2
AWG
Size
0.5
20
0.8
18
1.0
16
2.0
14
3.0
12
5.0
10
8.0
8
13.0
6
19.0
4
32.0
2
52.0
0
Wire Gauges
•
Wire gauge is a standard for the size of the wire
(proportional to current rating)
•
Typical wire in lab is 22 AWG
(American Wire Gauges)
– 52.9 mΩ/meter
– 7 A for short wiring
– 0.92 A for power transmission
http://www.powerstream.com/Wire_Size.htm
•
Breadboard in the lab can only use 22 AWG or
smaller, otherwise it will damage the clips
http://www.how-to-wire-it.com/romex-cable.html
Driving High Current Load
• Most microprocessor/TTL can drive <20mA, that is
approximately an LED.
– Interface microprocessor I/O with a gate.
Let the gate break instead of the microprocessor!
• Methods
– Relays
• may wear out and have delays
– Transistor
• fast switching but have current limit
– H-bridge
• allows forward and reverse current
• good for motors
http://www.acroname.com/robotics/info/articles/drivers/drivers.html
Troubleshooting Steps (1/2)
1. Check supply voltage using the multimeter
– Is power plugged in? Is any switch off? Is the fuse blown? Are all the
breadboard bus strips connected to VDD/GND?
2. Probe signal at intermediate stages or individual function blocks I/O
– Equipment available:
• For digital signals: Logic Analyzer, LEDs
• For analog signals: Oscilloscope, Voltmeter, Spectrum Analyzer
3. Check interconnections
– Is anything mis-wired? Are any wires loose? Are any contacts bad? Is
any signal floating?
Troubleshooting Steps (2/2)
4. Double check the design
– Check the pin diagram
– Check that you have the correct datasheet for the part number
– Re-analyze the logic, go through some calculation
– Ensure correct polarity (refer to the next slide)
5. Faulty devices/breadboard (Last resort if all else fails!)
– Replace/rewire one part at a time, test after every change
– Isolate the parts under test from the rest of the circuit
References
• http://www.intersil.com/data/an/an1325.p
df
• http://en.wikipedia.org/wiki/Diode
• http://en.wikipedia.org/wiki/Fuse_(electric
al)
• http://www.learnaboutelectronics.org/resistors_07.php
References Cont
• http://www.rbeelectronics.com/wtable.ht
m
• Previous ECE 445 Lecture Slides
• Staff of the ECE Electronics Shop
Dan Mast, Mark Smart, Skot Wiedmann