I want to build a circuit to take the high current off the single point that currently connects one leg of the primary winding of the ignition coil to chassis ground. The other side of the primary goes to +12vdc. The secondary is NOT connected to the primary at one side as they sometimes are. There are two spark plugs in this 2 cylinder engine. One side of the secondary goes to one plug and the other side goes to the other plug. Both plugs fire simultaneously with the circuit completed through the engine block but each cylinder burns on every other spark. There is a capacitor across the point.

One constraint for the desired circuit is that one side of the point is unavoidably connected to battery ground. The other side, of course, should trigger the circuit.

One question is "capacitor discharge" circuit or direct battery current through the circuit and primary? Pros, cons, and recommendations please!

I don't have any SCR's at the moment but I suppose that would be an ideal switch.
I do, however, have a few triac's that, I think, could be used. They have a maximum gate current of 50 milli amps and are rated at 600v and 12 amps. OK guys and gals, what do you suggest as a simple circuit. I don't think any timing adjustment would be required. Right now, a cam on the cam shaft operates the point and all I can do is set the gap and it works fine. Parts can be hard to find for this old engine so I would like to preserve the currently pristine point. Thanks Ed

 

A quick search has a lot of hits.
Here's one.

http://www.cs.berkeley.edu/~wkahan/TransIgn.pdf

Shown with a bypass switch.

 

A capacitor discharge type ignition does provide a fast rise-time, but short spark to help fire fouled plugs. But with the change to unleaded gasoline many years ago, fouled plugs are much less common now since fouling was mostly caused by conductive lead deposits on the plug electrodes.

Transistor (Kettering) ignitions have the advantage of generally having a longer spark duration that a capacitive discharge system, which can better ignite a lean fuel air mixture, as is typical of modern engines. So my preference is a transistor ignition such as inwo posted. It's simpler than a capacitive discharge system and may provide better ignition.

 

there is a similar circuit used by old car buffs, a germanium PNP transistor is connecte dwith the collector to ground, and the bast to the points. the wire that used to go to the points goes to the emitter of the transistor. these igntion circuits spark on opening the points, and current saturation of the coil when the points are closed. the transistor just replaces the points for current, allowing the points to last longer.
use a transistor rated at abouit 60 volts and 10 amps. the grounded collector will make heatsinking easier on a negative ground system.

 

Or you could use this, http://home.comcast.net/~loudgpz/GPZweb/Ignition/GPZheiModForPoints.html Did it with my 1957 Harley and haven't had a problem.

 

I have a feeling this is a automotive modification thread in disguise.

ppl do like to reword their automotive mods threads as some old generator set or lawnmower engine to fly under the radar here, hence the feeling i have.

 

Hello,

It does not need to be automotive, as onan makes generators:
http://www.tradebit.com/filedetail.php/161524291-onan-mdkub-mdkwb-service-repair-manual-installation
Bertus

 

it is also not a modification, it is to prevent old non available parts from wearing out quickly. point ignitions burn a little every time the engine fire. having a transistor take up the current makes them last longer, and does nothing for improving performance or power.

 

I am rather familiar with the Onan two cylinder engines so what exactly is the issue with the points and condenser based ignition system?

I have engines with both the old style P&C ignition and the newer style electronic ignitions and by far I have found the P&C based ignition to be the better system. The electronics ones get buggy and the ignition modals are expensive to buy compared to the P&C replacement parts.

As far as having a hard time to finding replacement parts for either I have a hard time believing that being that quick online search yielded dozens of sources for either of the ignition systems.

 

I have a feeling this is a automotive modification thread in disguise.

ppl do like to reword their automotive mods threads as some old generator set or lawnmower engine to fly under the radar here, hence the feeling i have.

This Is an 18hp Onan P218g engine in a 1987 John Deere 316 garden tractor / mower!

There is no "under the radar" hanky panky going on here!
Thanks for your "feeling"

Ed

 

I am rather familiar with the Onan two cylinder engines so what exactly is the issue with the points and condenser based ignition system?

I have engines with both the old style P&C ignition and the newer style electronic ignitions and by far I have found the P&C based ignition to be the better system. The electronics ones get buggy and the ignition modals are expensive to buy compared to the P&C replacement parts.

As far as having a hard time to finding replacement parts for either I have a hard time believing that being that quick online search yielded dozens of sources for either of the ignition systems.

I have been reconditioning ths tractor for some time now. It came with the electronic ignition which happened to be bad. So, I paid my approx $80 and got a new one which attempted to serve me while I was fixing a horde of other problems. when the horde was finally killed I was left with an engine that would start and run for a couple of minutes and then stop. It turned out the NEW ignition module would heat up and fail even though I did have the thermal gasket in place that is supposed to adequately insulate it from the block Fortunately, this engine still had the port to access the cam on the cam shaft that was formerly used by a point module, E-bay produced a pristine used point module that I installed to cure the last problem. Only a few high current minutes are on that point by my turn of the key and I wish to keep it that way until I bite the dust. Ed

 

Or you could use this, http://home.comcast.net/~loudgpz/GPZweb/Ignition/GPZheiModForPoints.html Did it with my 1957 Harley and haven't had a problem.

Hi shortbus,

Thanks, but that HEI module is the kind of $$ thing I hope to avoid even though it is cheaper than another unit called the "pointsaver". Plus, the HEI still requires a transistor and a resistor. Another aspect of doing it "on the cheep" from discrete components is the greater satisfaction of "rolling your own". Thanks again Ed

 

Cost is next to nothing at a wrecking yard.


Here's some more simple, cheap ones that work. Have the TIM4 on my '58 Harley. http://www.5bears.com/tim4.htm

 

I was left with an engine that would start and run for a couple of minutes and then stop. It turned out the NEW ignition module would heat up and fail

I have a Miller Bobcat 225 welder in my shop I picked up on the cheap with low hours due to the same problem.

The previous owner had the stock module burn out entirely then paid big money to have it replaced only to find it would run for a bout 5 minutes before quitting so he sold it to me cheap.

 

Heres another option using a Ford TFI Module & a GM Ignition coil for twin cylinder waste spark setup. Theres 2 different options of wiring & the module needs a decent heatsink & the module & heatsink need to connect to the engine frame & battery negative. These parts were obtained for free from a wreckers/scrap yard.

 

I have a Miller Bobcat 225 welder in my shop I picked up on the cheap with low hours due to the same problem.

The previous owner had the stock module burn out entirely then paid big money to have it replaced only to find it would run for a bout 5 minutes before quitting so he sold it to me cheap.

Ain't it SWEET! Knowledge wins again!

 

Heres another option using a Ford TFI Module & a GM Ignition coil for twin cylinder waste spark setup. Theres 2 different options of wiring & the module needs a decent heatsink & the module & heatsink need to connect to the engine frame & battery negative. These parts were obtained for free from a wreckers/scrap yard.

Thanks for the info. You and "shortbus" are surely more practical than I am, not that I don't enjoy a certain amount of junk picking. As a kid I loved to prowl the alleys of Detroit in the 1940's and discover treasure.
A large part of my motivation to DIY from discrete components is educational. I'm interested in upgrading my electronics skills and do a little hobby work. A while ago I bought a Model 2204 Picoscope and am dying to use it via my always off-line Windows XP system, as opposed to my always on-line Linux Mint system.
Ed

 

Heres another option using a Ford TFI Module & a GM Ignition coil for twin cylinder waste spark setup. Theres 2 different options of wiring & the module needs a decent heatsink & the module & heatsink need to connect to the engine frame & battery negative. These parts were obtained for free from a wreckers/scrap yard.

will this work with the black tpi module

 

Need a bit more information, or picture. What is a tpi module?

 

How about BTS2140 from aliexpress? I believe all you would need would be a IGBT and a resistor, like here:

I have not tried these, but these might also work: FGB3440G2_F085, AUIRGS30B60K