ELECTRIC FLIGHT SPEED CONTROLLER
by Ken Hewitt

Originally published in the September 1994 issue of
Radio Control Models and Electronics


Introduction

When the urge came to try electric powered flight a few years
back time for building was limited so a short cut was needed.
The answer appeared in the form of a Galaxy minnow which at the
time had a Cox 049 powering it. It was reasoned that a 550 buggy
motor and six cell pack that was laying in the corner of the
workshop might just give enough power to give electric powered
flight a try. The Cox was removed and the buggy motor fitted in
a couple of hours. The motor was switched on and off with a
micro-switch operated by a Futaba S133 micro servo, which had
been repaired by fitting a new home made amplifier (and hence
not trusted for more important duties), using a ZN409 IC. This
set up gave reasonable results but it was felt that a fully
proportional throttle would be desirable. Thinking back to my
car racing days and designs for speed controllers that had been
built at time, it was decided to adapt one of the old designs
for electric flight. Having built a replacement amplifier for
the micro servo it was obvious that a very small speed
controller could be constructed using a ZN409 IC, and some of
the now relatively cheap power Mos-Fet transistors that were not
available in the car racing days. The design that evolved is the
one presented here, it has given good service at a low cost, and
is even being used in my electric powered helicopters. As can be
seen from the circuit diagram, the design is quite simple with a
low component count, the simplicity of the design has been
achieved by leaving out functions like BEC (Battery Eliminator
Circuit) which allows the RC receiver to be powered off the
flight pack. The additional components needed to included a BEC
function would lead to a larger and more expensive speed
controller. For most applications I think that a separate RX
battery is advisable on safety grounds anyway. The design as it
stands at the moment can be constructed for a cost of around 16
- 20, depending on where the components are purchased from.


Circuit Description

The circuit is based around a ZN409/419 servo IC driving five
power Mos-Fet transistors. The ZN409/419 is used as a linear
pulse width amplifier,  the motor is driven with a train of
pulses whose mark/space ratio can vary between zero and one, to
control the motor speed from zero to maximum, via a power
amplifier formed by the five Mos-Fet transistors. The input
signal from the receiver is coupled via C1. RV1 along with C2
determine the amount of pulse expansion, and RV2,R1,R2,C4 set
the reference timing. The output is taken from pin 9 and used to
drive the power amplifier made up of TR1 - 5. Each Mos-Fet
device is capable of passing 40 amps continuous or 160 amps
pulsed, and when switched on has an ON resistance of only 0.038
ohms. As there are five of these devices connected in parallel,
in theory they could carry 200 amps continuous or 800 amps
pulsed, and have an effective resistance of 0.0076 of an ohm. In
practice though they are limited by the cable used to connect
the battery and motor,  also the size of the track on the PCB,
but in this design they can carry a current in excess of 20
amps, (7 cells and ASTRO 05) with an On resistance of
approximately 0.013 of an ohm, giving an efficiency better than
95%. 


Board Assembly

As the printed circuit board for this project is quite small it
is advisable to hold it in a small bench vice while soldering if
possible. Some of the components may have their legs preformed
when purchased, if so these may need to be straightened to allow
them to fit in the PCB, the tantalum capacitors will sit 2mm
lower on the PCB if the leads are straightened and as the case
is only 10mm deep every little bit helps. The presets may have a
different base lay out than the ones used in the prototypes, so
the PCB has been designed to use both types. Also it is
important that after the legs of the components have been
soldered they are cut off as close to the board as possible, to
enable the assembly to fit into the case specified in the parts
list. Start assembly by inserting all of the components that lay
flat on the board, ZN409 and the diode, next solder in the
components that sit up from the board, resistors, capacitors and
presets. Last of all insert the Mos-Fets making sure that they
are all the same height from the board and then solder the
middle lead of each one, this allows you to then align the
devices , and then solder the remaining leads. When assembling
the board make sure that the diode and the tantalum capacitors
are inserted the correct way round.


Component Identification

The only parts that may prove a little difficult to identify are
the capacitors. The 220nF Capacitor may be marked 224, and the
2.2uF tantalums may be marked either 2.2 or 225. The 10uF
tantalum marked 10 or 106. Which ever way the tantalums are
marked the lead nearest to the solid line or + will be the
positive lead. And on the 1N4001 the end with a ring around it
will be positive. The presets will be marked 102 for the 1k and
104 for the 100k.


Wiring up The Parts

When connecting the wiring for the flight pack and motor it is
possible to take the positive from the battery, directly to the
motor (and not loop it round on the PCB),and then take the
negative of the motor to the speed controller. This makes for an
installation that has one less joint in the wiring and cuts down
on the length of cable and hence resistance in the motor
circuit. It is also a bit easier to get the wires into the case
if you are using fairly thick cable, as you will only have to
take two and not four cables into the case to attach to the PCB.


Case Assembly

The case used to house the speed controller is a standard off
the shelf item and needs to be modified to allow the board to
fit correctly. The mods needed are shown in figure X. Once the
case has had all the mods done the board can be inserted and the
base screwed on. Take care not to over tighten the screws as
this can crack the base plate.


Setting up

When fully assembled the unit requires setting up to your
particular RC system. The best way of doing this is with the aid
of an oscilloscope if you have access to one, and a dummy load
replacing the motor, a 100 ohm resistor is ideal for this. With
every thing connected and switched on, connect the scope probe
to one of the Mos-Fets metal tabs and the earth lead to the
flight pack negative. Set the throttle stick on the transmitter
to minimum, and adjust RV1 for no negative going pulses on the
scope, as you advance the stick to full throttle you should see
negative going pulses appear that get wider as you advance the
stick. With the stick now set at full throttle adjust RV2 until
no pulses can be seen and the scope trace is showing about 100mv
DC above ground, return the stick to minimum and check for no
negative going pulses, if needed repeat the process.

If you do not have access to an oscilloscope, then the speed
controller can be set up when installed in a model. Set RV1 so
that for minimum throttle on the transmitter stick the motor is
stopped,  with the stick set for full throttle adjust RV2 to
give full power from the motor. Repeat the process until the
unit gives these results for full stick travel. 


Parts List

C1,C2           2.2uF Tantalum
C3              10uF  Tantalum
C4              220nF

R1              82k  1/4W
R2              4.7k 1/4W
RV1             100k Preset
RV2             1k   Preset

D1              1N4001
IC1             ZN409/419
TR1-5           BUK555-60A or B

Servo Lead	Available from local model shop	

PCB             Available from author   2.50 UKP

                                 Post and packing is included in the price for
                                 the UK and Europe. Please add 0.50 UKP per
                                 order for the rest of the world
                                 Make Cheques/ PO payable to K.Hewitt.
Author

Ken Hewitt,
21 The Brambles
Welwyn
Herts, AL6 0PG
England

or
C/O RCM&E,Nexus House, Broundary Way, Hemel
Hempstead, Herts. HP2 7ST.  








