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Circuit Protection, Tips, and Troubleshooting

Circuit Protection, Tips, and Troubleshooting. Revised Spring 2012 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:

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Circuit Protection, Tips, and Troubleshooting

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  1. Circuit Protection, Tips, and Troubleshooting Revised Spring 2012ECE 445

  2. 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

  3. Circuit Connections • Pay attention to Polarity Longer Lead: Positive Electrolytic capacitor Shorter Lead: Negative GOOD BAD

  4. 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

  5. Data Sheets • By Reading the Data Sheet: • Provide pin-layouts • Device Ratings • Potential Applications

  6. 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

  7. Voltage/Current Protection Examples Current across resistor typically = 20 mA Fuses and Diode Combination Current Limiting Resistor

  8. Device Polarity • The longer length is the (+) terminal • Capacitor • Diode Polarity: tantalum or electrolytic No polarity: ceramic or polyester (-) (-) (+) (+) The bar indicates cathode (+) (-) (-) (+) http://academic.evergreen.edu/projects/biophysics/technotes/electron/leds.htm/ http://electrapk.com/zener-diode/

  9. 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

  10. 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

  11. Earth Ground vs “Ground” • Green terminal is earth ground • Black terminals are signal grounds • Know the difference! Earth Ground Floating Ground

  12. 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)

  13. Resistor Codes • 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

  14. 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

  15. 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

  16. Troubleshooting Steps (1/2) • 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? • 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 • Check interconnections • Is anything mis-wired? Are any wires loose? Are any contacts bad? Is any signal floating?

  17. Troubleshooting Steps (2/2) • 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) • 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

  18. 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

  19. References Cont • http://www.rbeelectronics.com/wtable.htm • Previous ECE 445 Lecture Slides • Staff of the ECE Electronics ShopDan Mast, Mark Smart, Skot Wiedmann

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