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
Source: commons.wikimedia.org
Peltier Devices – Exploit the Thermoelectric Effect to convert
a temperature differential into an electric voltage
(Reversible!)
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 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)
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 nonconductive
 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: 1926.441 (www.osha.gov)
 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