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 <<

SafetyNEVER WORK HOT!All Design Work must account for Human

Exposure to Electric Hazards (Idiot-Proof)

Safe by DesignSafe 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”)WeatherproofingHazardous Materials/Containment

Battery AcidLeadOil

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 SwitchesMust completely De-Energize Project If your project is mobile, it will require a Local Kill

Switch (Big RED Button, Easily Accessible)

Source: Cartek

Remote KillA remote Kill Switch could act via one of

many “wireless” technologies: Infra-Red, AM Radio, FM Radio, IEEE 802.11g, Bluetooth, etc.

Range and Noise will be issues in the Contest Environment

Example: http://www.kitsrus.com/pdf/k180.pdf UHF Radio 2-Channel (Control 2 things) Relay Backend (Heavy Duty)

GroundingGrounding too often an ‘Afterthought”Soild, Reliable Ground Plane eliminates many

Noise and Reliability ProblemsGround 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 GroundsIn 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 ProtectionElectrical Circuits Require Protection against Fault,

Failure or Improper UseAlways Know the Failure Modes of Equipment

used in your Design (e.g. A Diode can fail short)Types of Circuit Protection:

Over CurrentOver/Under VoltageOver HeatingOver/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 ProtectionCircuit Breaker – Rated for a maximum

application voltage, interrupting level and maximum interrupting current (or volt-amps)

Over Current Protection (Cont.)

Fuses Inexpensive Over Current ProtectionOne – ShotFast or Slow Be Aware of resistance

Over Current Protection (Cont.)

Self-Resetting FusesThermistor that is conductive at room temperature If current exceeds rating, heats up and becomes non-

conductiveConductive 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 circuitZener 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 IsolationIMPORTANT to electrically isolate delicate

electronics from power circuits (Pulse Width Modulation motor drives, etc)

TransformersProvide Electric Isolation (energy transfer is

through magnetic circuit in core)AC Signals ONLY – V2 proportional to change in

flux

RelaysProvide Electric Isolation (magnetic circuit)

Provide “electro-mechanical Amplification”Low Power Signal Controls Large Power Circuit

AC or DC

Not for Repetitive Operations

Opto-CouplersProvide 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

BatteriesIt is IMPORTANT to understand that Batteries have

significant internal resistance: Fast Charge or Discharge leads to Internal Heating and Loss of Life.OSHA Std: 1926.441 (www.osha.gov)

Energy Storage in batteries is rated in Amp-HoursA 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 SuppliesConventional 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 RegulatorsSolid State DC/DC Buck ChopperCan provide a fixed output voltage: +5, -5, +12,

etcSome 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