IGNITION SYSTEMS > TRANSISTOR 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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