IGNITION
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IGNITION MODULES
> HOW TO BUILD
HOW TO BUILD TRANSISTOR IGNITION MODULES
"Most carburetor problems are
electrical" That was told to me by a savvy old auto mechanic
long ago and it has proven true more times than I can remember.
The standard Kettering points/condenser ignition timing setup works
just fine if the point faces are parallel and clean, are closed with
proper pressure, and the condenser (capacitor is the current term)
is good and of the correct value. A lot of ifs, don't you think?
Also, unfortunately for us model engine builders, either a grossly
oversize points/condenser set from older style lawn mower engines
must be used or a miniature points set will have to be fabricated
from questionable materials and with questionable accuracy. Most
model engines don't have shaft oil seals and just a little oil
leakage onto the points will cause major problems. Ever wonder why
so many model gas engines on display at shows are never ran? Do you
suppose it's because they are easy starters and good runners? Some
may be. But how many really otherwise great engines won't run or are
so hard to start because of ignition problems, that the owner won't
even bother? What a shame! If you don't like having your engines
ending up as just shelf models, then keep reading!
I came across the answer some years ago in a magazine article
written by Floyd Carter and all my spark plug ignition model gas
engines use it with great results. The original Transistor Ignition
Module (TIM - 4) is a simple two transistor circuit that can easily
be home built. TIM-4 was designed to operate on 3.6 volts (three
Ni-Cad cells in series) for use with model airplane engines. It
eliminates all the problems of standard points systems. The coil
will give a good hot spark every time. The circuit requires very
little current to trigger (25 ma). This allows use of a tiny micro
switch for the points which can be easily hidden. There is no
arcing, so the contacts in the micro switch will never burn. If you
want your model antique engine to be authentic, or on already built
engines which you don't want to change, the old point set can be
used if desired. A "condenser" is not needed but can be
included for looks.
And now for the really BIG advantage........ Since we now have a
circuit that is so easy to trigger, we can use a tiny magnetic
sensor instead of mechanical point contacts (high amperage switch)!
The magnetic sensor is called a "Hall Effect Device". They
are really tiny, measuring just .125" x .170" x .060"
thick (3mm x 4.3mm x 1.52mm). Instead of a cam to operate contacts,
a tiny magnet (only 1/8" diameter by 1/16" thick - or
smaller) mounted on a drum or disk (cam gear) triggers the Hall
device which is mounted in close proximity. The Hall sensor is
located remote from the circuit board which can be hidden under the
engine, or wherever you wish. Now you have the ultimate in small and
reliable ignition, no mechanical parts, rub blocks or contact points
at all! The circuits are extremely reliable.
Floyd is a retired aerospace electronics expert now enjoying life
and intends to continue doing so. He sells his TIM-4 units ready
made. He does not make any of the units available as kits. There is
actually nothing at all difficult in building these circuits except
a little care and the exercise of some common sense. With some help
and advice from Floyd (and against some!) I am making these kits
available under the following conditions: If you are not proficient
at soldering, don't have a 25 to 35 watt (max.) soldering pencil (no
150 - 300 watt solder guns), don't have some previous experience
with electronic parts and circuit boards, then you probably
shouldn't order these kits because I positively will not replace any
damaged part at my expense for any reason. I will sell replacements
for damaged parts at very reasonable prices in the unlikely event
that you should need them.
I changed some of the components of the original TIM unit for 6 volt
operation on stationary engines. I designate this as the TIM-6.
To run an engine with electronic ignition you will need: TIM-6
module, a suitable 6 volt ignition coil (see below), spark plug and
a good 6 volt battery that can supply at least 5 amps.
These ignition modules may be used on multi cylinder engines if
model ignition coils such as the Exciter, Modelectric or Gettig and
having a primary winding resistance of not less than 1 ohm. This
combination runs my V-Twin, V-Four and other engines with no
problems at all.
If you want to use automobile or motorcycle ignition coils with a
primary winding resistance less than 1 ohm, use a proper ballast
resistor in series with the coil so the current draw is not over 4.5
amps.
Ignition Dwell Angle
A rule of thumb to
calculate dwell angle is Cam Shaft RPM x .0075 for 4 cycle engines,
or Crankshaft RPM x .0075 for 2 cycle engines. This will determine
the shaft rotation in degrees that the coil should be energized
(points closed or Hall Sensor turned "ON"). Too little
dwell angle will limit top engine RPM as the spark will be weak or
non existent - too much dwell angle will overheat the coil and
electronics at low RPM. A simple calculation or two will determine
the radius from the center of the shaft to mount the magnet and Hall
Sensor. High speed engines need small radius (or several magnets in
an arc) to get enough dwell angle, slow running engines require
greater radius (or a smaller magnet) to prevent excessive dwell
angle. Calculate the dwell angle for the highest expected engine
RPM. Therefore, if a 2-cycle engine has a top RPM of say 6,000 RPM,
then .0075 times 6,000 = 45 degree dwell angle. In the above case,
draw a circle representing the smallest radius you can mount magnets
in a disk and also have a Hall sensor mounted at the same radius.
Engine features will determine this. Draw the 45 degree angle lines
from the center of that circle. The arc on the circle between the 45
degree lines is the length of arc you need to have magnets. For a
4-cycle engine at 6,000 RPM, use the cam shaft speed of 3,000 RPM
which gives a 22.5 degree dwell angle. If the Hall sensor can be
mounted so that it can be rotated around the center of the shaft,
ignition timing can be adjusted for "advance" or
"retard". Try to get the dwell right for a nice running
engine. See the diagram below.
Very few model engines will need more than one magnet to obtain
correct dwell angle - none of my engines needed more than one
magnet/cylinder.
The following measurements are for the Hall sensors and magnets that
I currently have available.
TIM-6 magnets are 1/8" dia. by 1/16" thick. With the
1/8" dia. rare earth magnets at .030" away from the Hall
Sensor face, the sensor will be turned "ON" during the
time it takes for the magnet to move .125" across the sensor
face. In other words at a certain point, as one edge of the magnet
starts to move cross the Hall sensor face, the sensor will turn
"ON" and stay "ON" until the 1/8" diameter
magnet has moved across the Hall Sensor face for a distance of
.125". As the magnet moves beyond that point, the Hall sensor
will turn "OFF" again. The distance the magnets move
during turn "ON" does not change significantly with the
magnets from .025" to .035" from the Hall Sensor, so
distance away is not that critical.
For the 2mm diameter magnet at a distance of .030" away from
the Hall Sensor face, the Hall Sensor will be turned "ON"
during the time it takes for the 2mm dia. rare earth magnet to move
a distance of .050" across the face of the sensor. All these
measurements were made using the DRO on my milling machine.
More info is in Strictly I.C. magazine #27 and #36. Back issues
available robert@strictlyic.com
Please Note
I get asked a lot of
questions about using these ignition modules on chain saw, weed
eater and other non-stationary and/or non-model engines. Many of
these types of engines have been converted and are operating in
various applications. However, I make no claim of suitability of any
of the above ignition units for non-model engines. Some of these
engine types may be suitable and some may not be. If you want to
convert these engines, you are on your own, so you should consider
the use of these ignition modules and/or coils on non-model engines
to be experimentation on your part. Please also note - electrical
items are not returnable for refund for obvious reasons.
With the above
understanding, you want to go ahead anyway, here are some
guidelines. With the right battery voltage and - very important - an
ignition coil with a primary resistance of not less than 1 ohm, I
see no reason why an enterprising person shouldn't be able to
convert most, if not all, of these engines. In a nutshell, determine
from your engine the minimum circle diameter you can use to get the
proper dwell angle (see above) from the rotating magnet(s) which are
mounted on a drum or disk somewhere on the crankshaft (2 cycle) or
cam shaft (4-cycle) and suitable mounting of the stationary Hall
Sensor in close proximity to the rotating magnet(s) and make that
installation. That is all the modification you need to do to a
single cylinder engine. There are many ways to set up multi-cylinder
engine ignition systems. Usually multiple magnets and a distributor
are required. Again, I don't do consulting so you are on your own.
Also, if you intend to use the engine with radio control, remember
that the entire ignition system - TIM module, coil, plug wire, plug,
etc. should be shielded and grounded to the engine to prevent radio
interference and possible loss of control of your model. On the
other hand, I have talked to some fellows who say they have not
found this necessary with their particular radio by installing the
radio as far aft in the plane fuselage as possible.
You decide what you are comfortable with. Good
luck!
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