Schematics - Schematic devices
and diagrams
• Typically there are several types of
schematics.
• The primary type tries to depict the circuit in a
simple format that shows the electrical
relationship between all the parts and circuits.
• Typically wires are solid lines, and parts may
be drawn using common symbols or a
generic “box” for more complex “units”
Schematics - Schematic devices
and diagrams
• A set of diagrams may put certain types of
information in a common area, like all the
grounding point locations may be drawn on
one page.
• Another may be all the connector/pin
diagrams.
• A third may include drawings of the various
devices.
Schematics - Schematic devices
and diagrams
• Another type of schematic is one that doesn’t
depict circuits it depicts diagnosis decision
making, called a flow chart.
• These can be useful, or not, but they will
contain data about testing results.
• This can include both known good and known
bad values for a specific test.
Schematics - Schematic devices
and diagrams
• In general component symbols are usually
similar, with some attempt at showing
function or purpose.
• In spite of that one must memorize some of
them to be able to easily read a schematic.
Schematics - Schematic devices
and diagrams
• wire and wire intersections
Schematics - Schematic devices
and diagrams
• Resistors
Schematics - Schematic devices
and diagrams
• Capacitors
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Schematics - Schematic devices
and diagrams
• Switches
Schematics - Schematic devices
and diagrams
• Batteries & antennas
Schematics - Schematic devices
and diagrams
• Coils
Schematics - Schematic devices
and diagrams
• Solid State Devices
• In general if the arrow points from positive
towards negative current will flow from
negative to positive.
• The straight line defines the cathode region,
the arrow defines the anode region.
• If the anode is positive and the cathode is
negative current will flow.
Schematics - Schematic devices
and diagrams
• Diodes
Schematics - Schematic devices
and diagrams
• Junction Transistors
• PNP
NPN
Schematics - Schematic devices
and diagrams
• Emitter is the arrowed leg and emits holes or
inputs electrons
• The Collector is the collector of holes or
output of electrons
• The base controls
• If its an N region and goes negative, current
will flow.
• If its a P region and goes positive, current will
flow.
Schematics - Schematic devices
and diagrams
• In Junction Field Effect transistors there is a
source, drain, and gate.
• Current flows from source to drain.
• Gate can either be P or N type channel
• Is less sensitive to voltage and noise output is
less but is slower and can’t handle as much
current.
Schematics - Logic theory
• One last type of schematic is a block
type that is designed to show logic flow.
• It does not necessarily show current
paths.
• It often doesn’t show power and ground
supplies.
Schematics - Logic theory
• The primary purpose is to show how the
circuit processes decisions and
information.
• These can most often be used to
determine when a component or
module is bad, but are not that great for
diagnosing a specific failure within that
module.
Schematics - Logic theory
• The basic building blocks for this are the
and, or, nand, and nor gates as well as
the amplifier and the inverter.
• These will almost always be packaged
into a circuit “chip” called and integrated
circuit.
Schematics - Logic theory
• The common form for these are either a
multi-pin chip with pins on either side or
the bottom.
• Or they may be a SMT device (surface
mount technology) that has pads which
are soldered to a “board”
• This is often done by machine in mass.
Schematics - Logic theory
• Other than being fried, the most
common failure of theses devices is
broken solder joints due to poor
soldering technique or excessive
vibration.
Wiring
– Connectors
– Identification, routing and mounting
Wiring – Connectors
• There are as many different types of
connection strategies as there are
manufacturers.
• They can be single pin, multi-pin,
plastic, metal, clipped, positive locked,
threaded, and sealed in a myriad of
ways.
Wiring – Connectors
• A good connection is one that
– Provides continuity with little resistance
– Is mechanically sound and unlikely to
break the wire in the event of relative
motion.
– Prevents corrosion of the wire or connector
components.
– Is easily disconnectable, but won’t do so
on its own. (excludes permanent cnxtns)
Wiring – Connectors
• AMP and Molex are the common
players of many years, but they have a
lot of competition.
• In general the two types are
male/female arrangements or contact
pads with spring pins.
Wiring – Connectors
• Typically the strategy will be to put the
most protected side on the power side
of the connection.
• This will most often be the female pins,
although the connector may appear to
fit inside the male pin housing.
Wiring – Connectors
• The connector pins are either some
derivative of copper, or they may be a
gold plated type.
• They can be crimp or solder type, and
they can be pre-mounted solder leads
or loose insert types.
Wiring – Connectors
• Most loose types will have some tools
that allow one to “eject” the pin back out
of the connector housing.
• These pins get bent easily, and the
retention tangs break off when
removed.
Wiring – Connectors
• 90% of electrical failures occur at a
connection.
• It may be bent pins, wire pull out from
the pin, corrosion, misalignment of the
pins, pin push back, mis-installed pins
(wrong hole), pin crimped onto
insulation, or pin crimped onto shield.
Wiring – Connectors
• Quality connectors provide a means to
secure a good electrical connection that
is different than the means to ensure a
good mechanical connection.
• The connector housing may have screw
plates that trap the wire bundle aft of the
connection pins.
Wiring – Connectors
• Connectors may have various mounting
features such as a bulkhead flange,
pressurized or not.
• Connectors may provide a means to
maintain shielding through the
connector.
Wiring – Connectors
• In general use the right tool for the job.
• All crimpers are specialized to a specific
task, some more than others.
• Trimming insulation is critical, and very
difficult to get right.
• Cut or nicked strands compromise the
current and mechanical capacity of the
wire.
Wiring – Connectors
• Always test new connections with a pull
and an Ohm meter.
• Always re-verify connector pin-outs prior
to the application of power.
Wiring - Identification, routing and
mounting
• The Advisory Circulars have great data about
how to route, where to route and how to
identify wiring harnesses.
• In general limit the number of wires in any
bundle. This reduces heat, and provides
redundancy in the event a mechanical event
destroys that one harness.
Wiring - Identification, routing and
mounting
• Identification is either alpha/numeric coding
or by color.
• This value can change at a connector.
• There can be hidden connections within a
harness such as a solder joint.
• Identification can also be by size, IE, two red
wires, one #10 and one #16.
Wiring - Identification, routing and
mounting
• Wire sizing relates to current being carried,
length of run, and thermal capacity of run.
• AC 43.13 1XXX has charts for bundled and
unbundled sizes.
Wiring - Identification, routing and
mounting
• The typical mounting device for a harness is
the adel clamp.
• These come in sizes from 1/8” up to 4” or 5”,
in 1/8” increments.
• Two can be doubled to attach a harness to a
tube structure.
Wiring - Identification, routing and
mounting
• In general routing should always include
some sag between hard points.
• Wires can be laced together, or zip tied into a
harnesses.
• Zip ties are not structural nor are they
suitable for high heat environments.
• Lacing does a much cleaner job, but is harder
to service or install a new wire into.
Wiring - Identification, routing and
mounting
• Another method of securing wires into a
bundle or harness is spiral wrap, or split tube
“Spaghetti”.
• These are easily installed and removed for
reuse after a wire is added or removed.
• Not good for any heated environment.
• They do provide some mechanical protection.
Wiring - Identification, routing and
mounting
• Harness shields and stand offs are a
common strategy to prevent mechanical or
thermal damage.
• Never run a harness under fluid devices or
lines.
• Keep wires bundled cleanly, with concentric
turns, and even feeds in and out of the
harness.
Wiring - Identification, routing and
mounting
• 90° turns between relative motion points such
as an engine and mount provide room for
motion.
• Three or four loops in a hard wire bundle
provide flexibility, EG a thermal-couple lead
going to an engine.
Wiring - Identification, routing and
mounting
• Route wires away from likely hand holds or
stepping zones.
• Twist wires if a paired circuit, eg wires to a
landing light and back. This will help cancel
inductive effects from the wires.
• Install an appropriate connector if often
disconnected, eg device mounted on a cowl
such as a light.
Busses
• Typically provide common distribution
points for power and ground.
• Must provide a means to mechanically
secure connectors.
• No more than four per lug.
• Hardware specific to metals being used.
Busses
• Not uncommon to have several power
busses, eg one master, one for radio
stack, panel lights, etc
• Buss design and insulation will be
appropriate to the voltage and current
being carried on that system.
Busses
• Are often made from copper or plated
copper.
• Insulation block will be a hard phenolic
composite.
Busses
• The general use of a buss is to have a
common point to shut down a group of
devices at one time.
• The down side is they reduce
redundancy.
• If one thing in the group shorts, they all
go down, unless independently
protected.
Busses
• Typical circuit will see power supply to
shut off device for circuit, then to the
buss and then a short run to the circuit
protection devices.
• On the ground side it may be grounded
right where its mounted, or the main
ground may carry back to some more
significant point.
Busses
• Is not the best practice to use metal
structures for ground paths in aircraft.
• This makes a lot of noise on those
sensitive devices.
• As well it can effect performance of the
device load.
Busses
• Radios, antenna leads and
intercom/microphone circuits are very
particular to noise.
• Should always be fully separate and
shielded as much as possible.
Controllers
– Mechanical
– Solid State
Controllers – Mechanical
• These are typically switches such as a
toggle or limit switch.
• They will often be incorporated into a
system of levers, cranks and pulleys.
• They may be a little micro switch or a
big rotary selector device.
Controllers – Mechanical
• Generally their maintenance is the
same as any other mechanical device.
• Properly secure and seal them, keep
them lubricated and adjusted correctly.
Controllers - Solid State
• Are most often a black magic box.
• Will have inputs and outputs.
• Inputs are usually analog devices and
outputs are a mix of both.
• Output control is often on the negative
side of the circuit.
• NPN Power Transistors generally drive
outputs.
Controllers - Solid State
• Repair is usually replacement.
• Failure is often due to output device
using too much current. (overloads the
NPN driver)
• Be sure to verify all outputs are good
when replacing a bad solid state
controller.
Controllers - Solid State
• Heat, vibration, and broken solder joints
are the common enemy of these
critters.
• Mis-installation of a power or ground
source can buy one of these quickly.
• Transient voltage/current spikes can do
the same.
Controllers - Solid State
• Static protection when working with
these is sometimes called for.
• Will be a grounding device for you, the
unit, the aircraft, or all three.
• Packaging/mounting may also be
designed for this.
Controllers - Solid State
• Controller location may be
environmentally controlled.
• Could be for cooling, or moisture
content, or pressurization.
• These units are generally not field
repairable.
Circuit rating and protection
• Circuit protection is just that, a device
that protects the circuit.
• Typically includes fuses and circuit
breakers.
• They only protect the circuit, not the
load device in the circuit.
Circuit rating and protection
• Any circuit must be so designed such
that all its power sources and loads will
not exceed the current carrying capacity
of the conductors, and the voltage
insulation capacity of the insulation.
Circuit rating and protection
• The voltage issue is not big for most
insulation materials used unless you
exceed several hundred volts.
• Other factors that may effect the choice
of insulation will be mechanical
protection, and thermal characteristics
of the material and the environment.
Circuit rating and protection
• Current carrying capacity is a function of
material choice, cross sectional area,
and length.
• But, since any conductor provides some
resistance there will be some loss in
heat.
Circuit rating and protection
• Since heat can effect a material’s
resistance, as well as effecting the
insulation, current capacity will change if
the wire is bundled or not and shrouded
or not.
Circuit rating and protection
• In general the designer will add up all
the loads for that one circuit, the
distances of the runs, and then
determine a wire size that won’t cause
an unacceptable line loss.
• Once these values are known, then the
proper type and rating of circuit
protection device can be selected.
Circuit rating and protection
• These devices are rated for voltage,
current, and speed of activation.
• Fuses are commonly designed to be
either fast or slow blow.
• This allows for balancing protection
needs with a dirty circuit.
Circuit rating and protection
• In most circuits a high flow of current
won’t do damage if it occurs for a short
period (milliseconds)
• This might toast a load device, but the
circuit will be OK as it takes time for it to
heat up and let the smoke out.
Circuit rating and protection
• If a load device needs specific
protection that should be integrated into
its internal power supply.
• This can be both voltage and current
protection.
• It can be on the input, power/grnd
sources, and on the output side.
Circuit rating and protection
• The power supply must be rated
appropriately as well.
• A 2 Amp/Hour Battery won’t cut the load
of starting an engine.
• The same is true with a generator.
• Generator gauge (amps) can be wired
between the buss and battery, or the
buss and generator.
Circuit rating and protection
• If on battery leg it will be called an amp
meter.
• It will read plus and minus for battery
charge and discharge.
• All system users (except starter) must
not exceed 80% max Generator output.
• This allows for battery load during
recharge.
Circuit rating and protection
• If on generator leg it will be called a load
meter.
• Will read plus only.
• All system users (except starter) must
not exceed 100% max Generator
output.
Load devices
• Electro-mechanical - Motors
• Lights/heaters
• Others
Load devices - Electromechanical – Motors
• When engaged motors are typically the
highest power consumer on most aircraft.
• But, most motors have a limited duty cycle.
• The one exception is fans and drive servos.
• Fans have limited use on turbine aircraft
because the engine provides a lot of
customer air.
• Drive servos can be in constant use
particularly with autopilots and trim systems.
Load devices - Electromechanical – Motors
• They are all inductors.
• As such in a DC circuit one must design for
inductive spikes.
• In an AC circuit one must account for
inductive reactance in the system design.
Load devices - Electromechanical – Motors
• Most of these devices are not field repairable,
but they may need servicing and
adjustments.
• Typical failures include overloading/fried, and
worn brush/commutator assemblies.
Load devices - Lights/heaters
• In the long run of time lights are
probably the highest user of power on
any vehicle.
• Typical cruise ship will use 1/4 to 1/3 of
total power production for lighting.
Load devices - Lights/heaters
• Although heaters are sometimes used, in
most aircraft the engine provides plenty of
heat sources.
• Some medium size aircraft do use a
specialized combustion heater.
• The electrical power these use is mostly
for driving their blower fans.
• Heat comes from a combustion process.
Load devices - Lights/heaters
• Lights are most often high intensity
heaters that act like resistors and
inductors in a circuit.
• They can also function by exciting
specialized gasses such that photons
are emitted in a more diffused manner
over a larger area.
Load devices - Lights/heaters
• In the case of lights, heaters and motors
they all have what’s called In-Rush
current.
• Because of EMF, Counter EMF, and
varying degrees of resistance due to
temperature when current is first applied
to these devices it flows freely in high
quantities.
Load devices - Lights/heaters
• It will do this until the device loads/heats
up to operating parameters.
• Since the circuit is turned on with this
low relative resistance the switch will be
conducting a lot of current while it is
making contact.
Load devices - Lights/heaters
• For this reason all switches for these
devices must be de-rated.
• “In-rush” current generally calls for the
switch to be rated at a higher voltage
and a higher current then the load
device’s listed ratings.
Load devices - Lights/heaters
• As previously stated many load devices
will also unload current when they are
shut off.
• For this reason there may be circuit
devices to reduce impact.
• EG: diodes used on master and starter
solenoid relays, or the capacitor used in
every ignition circuit.
Load devices – Others
• Heaters, motors and lights tend to be
the greatest current users in most
aviation circuits.
• But devices such as transmitters use a
lot of current when they are transmitting.
• As such the circuit (and generator) must
be rated for maximum output conditions.
Load devices – Others
• Typical radios need about 5-8 amps so
are often protected at 10 amps, with
wiring to support 10 amps.
• Avionics are often place on one
common master buss so that they may
all turned off during start, but easily
turned back on.
Load devices – Others
• Most new age avionics use variable
input power supplies of 10-33 volts.
• They will give different current/power
specs for the two common system
voltages, 12/24
Load devices – Others
• These units if stacked may produce a
lot of heat.
• They may need a cooling blower.
• Many will have a 5/8 hose connector in
their chassis for cooling air.
• Typical stack chassis should have
spacing between units for circulation.
Load devices – Others
• Stack assembly of trays may be tied
together to increase structure.
• Typical installation should include 12/18
inches of service loop in all wiring.
• Wire routing should separate signal
lines from power/ground lines and from
antenna lines.
Load devices – Others
• Avoid any routing near magnetic
sensing devices such as flux gates and
compasses.
• Many installations will require additional
certification such as navigation or
transponder devices.
Load devices
• On a final note for load devices, many, if
not most electrical installations are
considered to be a major alteration.
• This is one area where it behooves the
installer to check with your local FSDO
Avionics Inspector prior to the
installation.
Circuit diagnosis
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Plan of attack
Testing techniques and analysis
Verification
Failures – causes and patterns