Circuit Protection, Tips, and Troubleshooting
Spring 2015
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
innovative
• But first: background knowledge is required
be
Circuit Connections
• Pay attention to polarity
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]?
• Resistors: 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
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
Device Polarity
• The longer length is the (+) terminal
- Capacitor
Polarity: tantalum or electrolytic
- Diode
No polarity: ceramic or polyester
The bar indicates cathode
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
Power Supply Bypass/Decoupling Capacitors
• Bypass capacitors
- Shunts noise (AC signals) to ground
- Larger size capacitors for higher
supply voltages
Earth Ground vs “Ground”
• Green terminal is
earth ground
• Black terminals are
signal grounds
• Know the difference!
Potentiometers
• Variable resistors
- Example is a trimpot with
R=10 K Ohms
- From (a) to (c),
R1 = 6 K Ohms
- From (b) to (c),
R2 = 4 K Ohms
Resistor Codes
• Reading Surface
Mount Resistors
• A resistor marked 332 is
3.3 kilo-ohms
• 3K3 is 3.3 kilo-ohms
• Be aware of reference
tables for resistors and
wire gauges
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
• Breadboard in the lab can only use
22 AWG or smaller, otherwise it will
damage the clips
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
- Simple but may wear out and have delays
- Transistor
- Fast switching but have current limit
- H-bridge
- More involved but allows for forward
and reverse current
- Good for motors
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
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.pdf
• http://en.wikipedia.org/wiki/Diode
• http://en.wikipedia.org/wiki/Fuse_(electrical)
• http://www.learnabout-electronics.org/resistors_07.php
• http://www.rbeelectronics.com/wtable.htm
• Previous ECE 445 Lecture Slides
• Staff of the ECE Electronics Shop,
Dan Mast, Mark Smart, Skot Wiedmann