ECE 4591 – Design Workshop Lecture 1: Safety and Protection

The Three Laws of Robotics 1. A robot may not injure a human being or, through inaction, allow a human being to come to harm. 2. A robot must obey the orders given to it by human beings, except where such orders would conflict with the First Law. 3. A robot must protect its own existence as long as such protection does not conflict with the First or Second Laws. >> Isaac Asimov <<

Safety NEVER WORK HOT! All Design Work must account for Human Exposure to Electric Hazards (Idiot-Proof)

Safe by Design Safe Design Practices include: Enclosing Electrical Components to avoid incidental human contact Providing a Discharge Path for all Energy Storage Devices (like Capacitors) Design for discharge to under 5V in 2 seconds

Safe Design Procedures (cont.) Design for Environment Internal Cooling/Heating (entire exterior must be “touchable”) Weatherproofing Hazardous Materials/Containment Battery Acid Lead Oil Peltier Devices – Exploit the Thermoelectric Effect to convert a temperature differential into an electric voltage (Reversible!) Source: commons.wikimedia.org

Safe Design Procedures (cont.) Kill Switches Must completely De-Energize Project If your project is mobile, it will require a Local Kill Switch (Big RED Button, Easily Accessible) Source: Cartek

Remote Kill A remote Kill Switch could act via one of many “wireless” technologies: Infra-Red, AM Radio, FM Radio, IEEE g, Bluetooth, etc. Range and Noise will be issues in the Contest Environment Example: UHF Radio 2-Channel (Control 2 things) Relay Backend (Heavy Duty)

Grounding Grounding too often an ‘Afterthought” Soild, Reliable Ground Plane eliminates many Noise and Reliability Problems Ground Plane on Mobile Platform cannot reliably be joined to Earth Ground. (Floating with Respect to Earth) Induced Voltages on Vehicle must be considered when designing and maintaining Vehicle

Proper Grounding (Bonding) Good Example of Terminating a Ground Wire (or any wire) – Be aware of Vibration!

Proper Grounding (Cont.) Proper Termination of Shielded Cable

Need for Separate Grounds In the case where a low power analog signal (such as from a radio antenna) requires a ground plane, AVOID direct connection to a ground plane used for digital equipment (Noisy). In the absence of Earth Ground, establish an Analog Ground Plane and a Digital Ground Plane and connect the two with a resistive path to attenuate noise.

Circuit Protection Electrical Circuits Require Protection against Fault, Failure or Improper Use Always Know the Failure Modes of Equipment used in your Design (e.g. A Diode can fail short) Types of Circuit Protection: Over Current Over/Under Voltage Over Heating Over/Under Frequency (AC Systems)

Over Current Since most circuit designs assume established voltage levels within the circuit, regulating current will regulate the power in the circuit. Conductors must be sized to handle the maximum load current and any transient short-circuit current level available. Example: NEC indicates 14 Gauge Solid Copper will safely carry 15 Amps when properly protected by a circuit breaker Know When to Use Solid Wire and When to Use Braided Wire

Over Current Protection Circuit Breaker – Rated for a maximum application voltage, interrupting level and maximum interrupting current (or volt-amps)

Over Current Protection (Cont.) Fuses Inexpensive Over Current Protection One – Shot Fast or Slow Be Aware of resistance

Over Current Protection (Cont.) Self-Resetting Fuses Thermistor that is conductive at room temperature If current exceeds rating, heats up and becomes non- conductive Conductive again after cooling down Source: commons.wikimedia.org

Simple Fuse Status Indicator Size R to limit current through LED

Over Voltage Protection It is often desirable, especially in power electronics, to limit transient over-voltages in a circuit Zener Diodes are an inexpensive means of limiting low-power over-voltages MOV (Metal-Oxide Varistor) Surge Suppressors provide a heavier duty solution Bring Critical Voltage Test Points Out to A Measuring Block

Circuit Isolation IMPORTANT to electrically isolate delicate electronics from power circuits (Pulse Width Modulation motor drives, etc)

Transformers Provide Electric Isolation (energy transfer is through magnetic circuit in core) AC Signals ONLY – V2 proportional to change in flux

Relays Provide Electric Isolation (magnetic circuit) Provide “electro-mechanical Amplification” Low Power Signal Controls Large Power Circuit AC or DC Not for Repetitive Operations

Opto-Couplers Provide Electric Isolation (Energy Transfer via Photons) Many Types of Output: BJT, Darlington Pair, SCR, etc

Tri-State Drivers (Buffers) Enable Pin = 0 puts driver in High Impedance State (Open Circuit A to B) High Input Z, Low Output Z (10 GE output) Non-Inverting or Inverting

Batteries It is IMPORTANT to understand that Batteries have significant internal resistance: Fast Charge or Discharge leads to Internal Heating and Loss of Life. OSHA Std: ( Energy Storage in batteries is rated in Amp-Hours A general rule of thumb is that storage batteries should never be discharged below 50% of capacity to avoid undue aging

Lead-Acid Storage Batteries Oldest technology, but still the most cost-effective in terms of energy density (most amp-hours per kilogram per dollar) Checking Charge level with a voltmeter: Open Circuit (Resting) Voltage VS. State of Charge 12V Lead Acid:12.66V 100% 12.45V 75% 12.24V 50% 12.06V 25% 11.89V 0%

Power Supplies Conventional Power Supplies: AC to DC using Transformer and Diode Bridge Switch-Mode Power Supplies: AC to DC using Solid State Electronics DC to DC Conversion is called Chopping Solid State Voltage Regulators

Conventional PS

DC-DC Chopper

Chopper Output Waveforms Power MOSFET “Chops” the DC supply voltage Inductor smoothes the Current, Capacitors smooth the Voltage

Voltage Regulators Solid State DC/DC Buck Chopper Can provide a fixed output voltage: +5, -5, +12, etc Some can provide a variable output: +4 to +12 Volts, etc

Assignment: Using a breadboard, LM7805 Voltage Regulator, a 5.1 V Zener Diode and any other parts: Build a simple 12 Vdc to 5 Vdc step-down circuit and protect the output against voltage transients Submit to me: An electronic copy of your working design schematic