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Crossed
Wires |
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By
Stan Yeo
Recently
I was asked to finish fitting out a four servo wing sports soarer
and program the transmitter. No problem I thought, three or four
hours toil and the job would be finished. Wrong. I ended up spending
nearly fifteen hours on the job. A number of things went wrong prompting
me to write this article and make a mental note not to take over
a part completed task in the future!
The
problems I encountered ranged from poor servo installation, non-conventional
wiring and programming an unfamiliar transmitter. Servo installation
is an article by itself and is frequently covered in kit instructions,
reviews and plans etc. Programming the Tx. Well just because the
word computer is used in the product description is no excuse to
adopt the computer industry's habit of writing instruction manuals
to baffle all and sundry. To add insult to injury they are in my
experience getting worse rather than better. To give credit where
credit is due the technical departments of the importers are generally
very good at answering your questions when they are contacted.
This
leaves one problem, that of wiring etc. I am aware that not all
radio control systems sing from the same hymn sheet when it comes
to wiring, the colours used are frequently different as is sometimes
the connection lead wiring order. Being aware of these differences
etc. should have protected me from cross wiring mistakes. Unfortunately
when I carried out polarity checks on a pre-wired wing I was working
on I failed to spot the red and black wires were reversed. Result
four blown servo amplifiers! I am not the first to do this and undoubtably
will not be the last.
This
article is therefore a discussion of various the wiring
tasks associated with today's radio control systems.
Transmitters
An
increasing number of modellers, particularly those with computer
sets are replacing their original transmitter batteries with higher
capacity ones to give longer Tx. usage times. Unfortunately the
set chargers are seldom able to deliver the higher charge currents
required to charge the batteries properly so a higher capacity charger
is needed. A word of warning though. Most transmitters have a diode
(electrical non-return valve) in the charging circuit to prevent
the Tx battery being discharged through the charging socket. This
means that cycling Tx batteries in-situ is not possible
and there is a danger when fast charging of 'blowing' this diode
if the charging current is too high.
All
the main UK imported R/C systems (Futaba, JR, Hitec Sanwa) use a
similar DC power socket for charging but, there are two sizes and
two ways of wiring them. One uses the centre pin as negative whilst
on the others the centre pin is positive. Likewise there is a difference
in size of the centre pin, one is 2.1mm diameter and the other 2.5mm.
The plugs are also available in two lengths so always buy the longer
one.
Tx
Charging Socket Polarity Table
| Transmitter |
Hole
Diameter |
Centre
Pin Polarity |
| JR |
2.1mm |
Negative |
| Hitec |
2.1mm |
Positive |
| Futaba |
2.5mm |
Positive |
| Sanwa |
2.5mm |
Positive |
Some
Tx batteries have pressure or push fit connectors whilst others
have a polarised plug and socket arrangement. When buying replacement
packs from other than the equipment importer it is not always possible
to buy a pack with the correct plug fitted. However most plugs can
be made to fit but this usually means the plug is not polarised
so be very very careful. Check and double check that
the wiring is the same as the original when you plug the battery
in otherwise you could have an expensive repair bill on your hands.
Airborne
Wiring
Fortunately
all the current production R/C systems mentioned polarise their
airborne equipment the same way i.e. Negative, Positive and Signal
(but be careful if you have any old Sanwa black plug equipment as
the positive and negative are reversed!). Unfortunately there are
two plug and socket systems and four different colour codes! The
main difference is Futaba has a polarising tab whilst the others
are polarised with bevelled corners on the top face of the plug.
A tip if mixing plugs and sockets and they are tight, scrape the
sides of the female with a scalpel blade as opposed to trimming
or filing. This reduces the grabbing effect of a slightly
rough surface finish.
Airborne
Equipment Colour Codes.
| Make |
Negative |
Positive |
Signal |
Plug |
| Futaba
|
Black |
Red |
White |
Futaba
(tab) |
| JR |
Brown |
Red |
Orange |
'Uni-Plug' |
| Hitec |
Black |
Red |
Yellow |
'Uni-Plug' |
| Sanwa |
Black |
Red |
Blue |
'Uni-Plug' |
| Multiplex |
Black |
Red |
Yellow |
'Uni-Plug' |
The
Plea
Sometimes
it is not always possible for a number of reasons to use matching
colour coded servo wire when manufacturing wiring looms. If this
is the case please do not make up your own colour code but use the
colours indicated above for the different polarities i.e. Negative
should be either Black or brown. This will help you and subsequent
owners should the wiring require modification or repairs.
Servo
Reversing (Does NOT apply to digital servos)
Even
with the advent of built in transmitter servo reversing and computer
Txs it is still sometimes not possible to get one servo of a pair
to operate in the required direction. In this situation you have
two choices, 1. To install a servo reverser on one of the servos
or 2. To electronically reverse the servo. The most common instance
of this problem is when using two servos to operate the flaps via
a 'Y' lead. Sometimes the problem can be overcome by turning the
servo 'over' but this is not always possible. The problem is not
always solved using a computerised transmitter and using two channels
to operate the flaps because the servo reversing on some computerised
Txs is carried out AFTER mixing rather than before i.e. JR388. Tip.
If no matter what you do one servo is always operating in the wrong
sense when using flapperons, V tail mixing or elevons swap the channels
the servos are plugged into. Also if it is not a permanent connection
tag the plug and socket you take apart.
Diagram
of wing with flap servos
The
principles of servo reversing are very straightforward. Basically
the servo motor terminals must be reversed along with the two outer
terminals of the feedback potentiometer (pot). The problems arise
mainly from two sources 1. the servo amplifier is hard-wired to
the motor and pot i.e. soldered directly onto them i.e. not via
fly-leads, and 2. Getting access to the servo pot. My advice is
remove the servo pot. Do NOT attempt to reverse the leads on the
amplifier as apart from being extremely fiddly there is a danger
of damaging the surface mount components on the amplifier PCB (printed
circuit board). Another piece of advice is make a sketch of the
wiring before you start. If after completing the task the servo
runs full travel it is likely that only one of the operations has
been carried out.
Diagram
of servo amplifier wiring
Aileron
Extension
Leads
When
the aileron servo lead needs to be extended to connect the servo
to the Rx please do not cut the servo lead and insert an extra length
of cabling. Try and use a propriety servo extension lead. This way
the servo is easier to replace should it fail or get damaged and
it does not invalidate the warranty. To pass the extension lead
through the wing remove the plastic holder from the Rx end of the
lead. The connectors are held in the holder either by a plastic
tongue on the holder or a metal tongue on the connector. Either
way disengage the tongue using the point of a scalpel knife and
remove the connector. After passing the extension lead through the
wing re-assemble the plug ensuring all tongues are properly engaged
and the wires are in the correct positions.
Where
the aileron servo lead, including any extensions, exceed 500mm it
is possible for the lead to act as an aerial and pick local transmitter
signals and feed them back into the receiver thereby increasing
the risk of interference. One way to minimise this risk is to fit
Toroidal Chokes to the servo lead as close to the receiver as possible.
These chokes look like lamb's tail docking rings i.e. small fat
rubber bands. The value of the impedance (alternating current resistance)
produced is a function of the square of the number of turns so the
more turns the more efficient they are. To fit them dismantle the
servo plug as above and wrap the lead around the choke as per the
photograph.
Photograph
of choke fitted to servo lead.
Batteries
Finally
a brief note about battery charging as most early life battery problems
can be attributed to inappropriate charging. A standard
charge is a charging current of 10% of the capacity of the battery
i.e. a 600mAhr battery would be charged at 60mA. A fast
charge is one where the charging current is typically the capacity
of the battery i.e. for the battery above 600mA. Some batteries
can be fast charged at 2 times the rated battery capacity but this
is not recommended as it shortens the life of the battery and the
charge is not as complete as it would be if charged at a lower current.
Basically the faster you put it in the less you get out. Likewise
the faster you take it out the less you get out! (batteries are
rated at their 5 hour discharge current). Also it is recommended
that the first few charges are standard charges to
condition the battery. If charging at currents greater
than 10% of the capacity of the battery then it is recommended that
a peak detect charger is used to avoid overcharging and damage to
the battery.
Farewell
I hope
the above has been useful. If you do carry out any of the above
tasks please take care, be methodical, produce wiring diagrams before
you start, follow the colour code conventions, use a small pointed
soldering iron, thin multi-cored fluxed solder and heatshrink sleeving
to insulate solder joints. Do not use insulating tape, it looks
untidy and offers no support to the solder joint. Also be aware
that modifying you R/C equipment will in all probability invalidate
your guarantee.
Finally
my apologies to the professionals. I am aware of your views re modifying
equipment etc. but a lot of modellers are going to do it no matter
what advice to the contrary they are given so in my view the safer
option is provide assistance in order to reduce the number of mistakes
made. For other associated model flying articles please visit my
website phoenixmp.com
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