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Electrical Systems
Choice of Battery
• CAPACITY
– Must adequately supply all power requirements
– Extra margin for battery deterioration (x2) by
temperature and age
• NUMBER
– Twin battery only where heavy and continuous power
needed
– Twin batteries should be switch selectable and not
wired in parallel
• TYPE
– Batteries must be sealed, non spill gel type
Battery Plugs and Connections
• Fuse in positive cable as close as possible to
battery (this is VITAL)
• Battery wiring to carry fuse blow current easily
• Insulation of wiring from fuse to battery must be
perfect and short circuiting virtually impossible
• Battery connections must be rugged, protected
against fatigue and terminals thoroughly insulated
• Preservation of polarity – foolproof standard plug
• Impossible to short live battery plug
• ALL SUBJECT TO MECHANICAL STRESS
AND ABUSE
The Humble Fuse - 1
• Provides low-cost, reliable protection against
overloads and fire – a real lifesaver
• Correct rating depends on MAXIMUM normal
load in circuit (use 1.25 x max normal load as a
guide)
• NB High currents during radio transmit or A/H
startup
• Circuit must NEVER be at risk even at 2x fuse
rating
• Choice of fuse and wiring should be made
together.
The Humble Fuse - 2
• Good quality fuse holder with fuse rating clearly
marked
– May be in-line or panel mounted (preferable)
– Ensure voltage drop across fuse is acceptable at max
normal load
– Ensure plentiful supply of correct fuses in cockpit, all
clearly marked
• Fast blow and slow blow fuses have different
applications and are NOT interchangeable
– Slow-blow fuses withstand temporary startup
overloads
Cable Polarity and Security
• Red for Positive
• Black for Negative
• Secure to structure and not to control
systems
• Protect from abrasion, not too tight, provide
proper support
The Hostile Cockpit
• Instruments and wiring are subject to:
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Vibration and maltreatment
Dirt
Moisture and Corrosion
High and low temperatures
• To function Reliably, wiring must be:
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Tidy
Strong
Protected
Properly joined
Capable of carrying the load
Safe from catching fire
Checked regularly
Instrument Panel Electrics
• POWER
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Main supply plug – robust, low resistance, high capacity, polarised
Battery Select – ON-OFF-ON, capacity related to fuse
Main switch – isolates all circuits, on-off, UP for ON
Distribution block – low resistance, room for expansion
• INSTRUMENTS
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Fuses and switches – placarded ON/OFF, fuse rating and function
Wire thickness and fuse related to upstream fuse and to load
Protection and insulation of all terminals – FOD
Plug to allow easy removal of instrument panel
Plug or terminal block allows for instrument removal for service
• Tidy colour-coded and circuit diagram to aid servicing
Wiring to the ‘stick
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Examples: Press to Talk, cruise-climb
Use very good quality, multistrand cable
Run through ‘stick where possible for protection
Across moving segments – sufficient cable so as
not to obstruct but not enough to catch on other
moving parts
• Cable-tie securely and cut off neatly
• Test very carefully in all possible combinations of
control positions
Ohm’s Law
• Voltage = Current X Resistance (V=IR)
• Current = Voltage/Resistance (I=V/R)
• Resistance = Voltage/Current (R=V/I)
• Units Volts, amps and ohms
COMPONENT CHOICE
• Good quality, new components give little trouble
• Other sources apart from automotive suppliers:
– Dick Smith (limited range with variable quality of
advice)
– Altronics
– Jaycar (Newcastle Street)
– RS Components (cheapest, wide range, little advice and
bulk purchases)
• Cost of good, new parts is tiny compared to cost of
finding a pesky fault in cheap or second-hand parts
• IF IT’S WORTH DOING, ITS WORTH DOING
PROPERLY