This is a selection of electrical information, hopefully useful when wiring a Sonex homebuild



Electric Flap Actuation Switch (schematic)

Electric Flap - Drive Flap Drive Experiments

Preliminary Wiring (schematic)

TVS (transient power pulse suppression)

Wire Gauge and Circuit Breaker Sizing

Calculation of Voltage Drop

Some Wire Mil Standards

Main Buss Voltage

Current Draw of a Conventional VW Ignition


Related Links



The Electric Flap (June. 18th, 2001, updated June 21st)

My ergonomic concept requires the flap actuation to be operated with the right hand (Sonex original plans have the flap lever at the left fuselage side). So the decision is to build an electric operated flap mechanism.

here is a preliminary wiring diagram:

The 'automatic shutoff when next flap position is met' has the advantage that there's no need to watch the flap position indicator.

Here is a Corel Draw 8 version of the diagram

and here is a PDF version

A suitable electric cylinder is the Ball Drive Actuator 85615 form Motion Systems (thanks David)



Flap Drive Experiments (Mar. 6th, 2002)

Sonex #211 will have an electric flap drive, this is for sure. A drive mechanism is available ready to install from Vans Aircraft, for example. But why buy for a few $ when, with lots of sweat and head-scratching it can be hommade? So I went to the hardware store and bought a 'special offer' (20 Euro) cordless drill.

It is a 14.4V type. Max speed is 500 RPM - not bad for my intended application. Next task was to completely disassemble the nice new machine (I like looking inside of such gadgets).









This is the inside view. The motor, planetary reduction gear and torque limiter/clutch is an integrated subassembly. Inside of the switch/trigger box is an electronic speed controller - this I don't need.


This is the flap drive to be. The spring is part of the torque limiter. This torque limiter/clutch is a very simple construction and I will keep it as it is. The drive shaft is of solid steel and roller bearing supported. The end is left-threaded.

Next steps is attaching a trapezoid threaded spindle and nut. Then the whole carboodle will be test fitted on a particle board. Via a steel cable I'll test how much pull the drive can develop and what is the speed under load.



Preliminary Wiring Schematic

same diagram in Corel Draw 8 format

or as a PDF File



TVS (Transient Voltage Suppressors) - the modern way for on-board-power protection (Mar. 05, 2001)

here are some links to General Semiconductor(R) datasheets. This TVS devices may be used for suppression of the high voltage pulses which alwaqys appear when relays or other inductive coils are powered off.

this explains the principle of TVS's operation

this is a diode which can suppress spike-power up to 6000 watts!!!

application notes and sample applications



Wire Gauge and Circuit Breaker Sizing*1 (Mar. 15, 2001, updated June 25, 2001)

*1: always keep in mind that your fuse/circuit-breaker is designed to protect your ~wires~ and not to protect your equipment!

 Wire gauge


 Circuit Breaker Amps



Resistance in Ohms per ft @20 degC Crimp Terminal Color


(sq mm)

Outside Diameter


# of Wires Continuous duty current @ 105 degC *1
 22 *2  5  5  0.0162  red 0.38  1.19  19  3
 20  7.5  5  0.00988  red  0.62  1.4  19  4
 18 *2  10  10  0.00623  red  0.96  1.65  19  6
 16  15  10  0.00481  blue  1.23  1.83  19  7
 14  20  15  0.00306  blue  1.94  2.26  19  10
 12  30  20  0.00202  yellow  2.98  2.74  37  13
 10  40  30  0.00126  yellow  4.74  3.25  37  17
 8 *3  50  50  0.0007    8.61  4.95  133  38
 6  80  70  0.00044    13.6  6.12  133  50
 4*4  100  70  0.00028    21.6  7.87  133  68
 2  125  100  0.00018    33.7  10.36  665  95

*1: the Spec44 wires are good for 150 deg. C

*2: my favoured wiring gauges for low-power devices

*3: my favoured gauge for buss feed wiring

*4: my favoured gauge for starter wiring

Wire sizes form Raychem Spec44/55TM catalog. Wires according to MIL-W-81044

Fuse/circuit braker ratings for selected wire gauges. Taken from AC 43.13-1B Part 11.



Calculation of Voltage Drop

AC 43.13-1B says:

max. allowed voltage drop for nominal 12V systems and continuous operation is 0.5V

max. allowed voltage drop for nominal 12V systems and intermittent operation is 1.0V (e.g. starter)



1. take resistance for selected wire gauge from table above

2. multiply by actual wire length in feet*1 (now you have the actual resistance of your wire piece)

3. multiply by the current actually drawn (this is your actual voltage drop) OR

3a. multiply by the Continuous duty current (last column). This is your max theoretical voltage drop

*1: for non-conductive airframes where airframe cannot be used as an electrical ground, a forward and reverse wire is required. In this case multiply wire length by two.


What is the voltage drop of your starter which draws 250A, wire length is 6 ft of 4AWG wire

Resistance = 0.00028 x 6 = 0.00168 ohms (or 1.68 milliohms)

Voltage drop = 0.00168 x 250 = 0.42 Volts ( 0.42V is less than 1.0V for intermittant operation, thus legal)



Some Wire Mil Standards

Single Conductor:

MIL-W-22759/16-XX or

MIL-W-81044/../XX Manufacturer: Raychem SPEC 44

where XX = AWG Wire Gauge

Coax Antenna Cable:

RG-58 A/U



Main Buss Voltage

should between 13.8V and 14.2V, otherwise check the voltage regulator



Current Draw of a Conventional (points/coil) VW Ignition

Key on, not running. about 3.8
Started and idling about 2.18
about 2500 rpm. about 1.67

measured by jim /




The last days I followed an extensive discussion about the usage of protective diodes in aircraft wiring. I'd like to contribute my point of view of this subject here.


What's all behind this coils (relais, contactors)?

Switching On

Inductive loads like relais or contactor coils build up a magnetic field when a voltage is applied to the coil. A current starts to flow with increasing strength until the current is limited by the coil's ohm's resistance (the resistance of the coil's wire windings). Powering up an inductive load is 'friendly' to the power distribution buss because the current increases slowly, so there is no sudden power surge (contrary to a capacitor - but this is a whole different story).

Switching Off

Very different is the situation if you switch off an inductive load. The coil is now a fully loaded with magnetic energy. When you open the electric circuit (by flipping the switch to the 'Off' position) then the magnetic field is still present and wants to collapse. Collapsing of an electric field always proceeds in the following sequence:

first the magnetic energy is converted to electric energy

which in turn is converted to heat.

Now here lies the problem:

How should this electric energy flow if you have just opened the switch?


The some mils distance between your switche's contacts present a high electrical resistance. So the voltage rises (probably up to several hundreds of volts ) until a small spark gaps this distance, a current flows thru the windings of the coil until all of the magnetic energy is finally converted to heat. This all happens within fractions of milliseconds.

What we don't like:

We don't like this spark, because it creates an electromagnetic impulse in all frequency bands which can cause all sorts of bad habits (like knack sounds in your radio[could live with this], crash of your flight computer [worse], irritating the electronic ignition computer [very bad]).

Where is Help?

The >>>DIODE<<< is the solution! (or not?). If you connect a diode in parallel to the coil such that the excessive current can flow thru the diode (and thru the coil's windings) when the electric circuit is opened, then the energy will circulate thru the windings as long as the last drop is converted into heat. -Perfect-

No, not perfect, because there's a drawback. It takes time to convert the magnetic energy to heat. This makes your relais contacts open (or close) ............s..l..o..w..l..y.....

This is not what we want, because an intermediate state of contacts between open and close will lead to high transition resistance which leads to heat build up which leads to high contact wear.

So what the @$%& shall I do then?

Let's face the subject more pragmatic. You have an avionics master switch (have you? - you should have one - even if 'Electric Bob' has another opinion about this theme). Your procedure for startup is like:

switch on the battery and alternator contactor

start the engine

switch on the avionics master switch

And the shutdown is in the reverse order. So why bother about the battery relais or the starter coil? Your precious avionics gadgets are not online while this gremlins spew the high voltage transitions on your power distribution buss.

My suggestion is:

Protect the power input of your expensive electronic devices (if this makes you sleep better at night). This can be done by varistors (traditional/old-fashioned method) or by TVSes (the modern way).

Leave the coils and contactors alone.

-loef, 29th of June 2001



Related Links

EAA's document ressources (some nice electrical stuff from Tony Bingelis)

RS Components in case you need a few small parts ~fast~ and (like to pay a little more)

Farnell another large distributor of electrica/electronics stuff