Circuit Protection, Tips, and TroubleshootingSpring 2015ECE 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

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

- Diode

Polarity: tantalum or electrolytic 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