Scratch Built Ignition???
 

Browsing through the RC Modeler archive index, I found that they published an article on building a CD ignition system in the 12/80 issue. Anyone have that issue?

RE: Scratch Built Ignition???
 

I have built the TIM-6 ignition and installed it on a 18cc weedeater engine. The ignition works great. Wish I could say the same for this engine. (really weak!) I calculated the dwell angle using the formula from (av8tor1977)'s post in this thread. I had to mount the magnet as close to the shaft as I could to come close to the dwell angle. I calculated at 7500 RPM's I did notice that with a smaller prop that as I approched 8000 RPMS the engine started missing like it had a rev limiter. (Not enough dwell?) I have ran the engine over an hour on a 1200mha battery, don't really know how long it will run on a single charge? I think I will install the ignition on a ryobi or Mac32 and see how it does. May also try two magnets to increase the dwell angle, but don't think I will really want to exceed 7500 RPM's anyway.

RE: Scratch Built Ignition???
 

Congrats on the ignition system. Sounds like you are running out of dwell near 8000 rpms as you suspected. It will start missing like you said when it doesn't have enough dwell for the rpms occurring. More dwell will increase your battery drain, but that doesn't sound like a problem if you're getting an hour or more out of a battery.

You won't believe the difference in power if you go to the Mac 32!!

Oh, and someone asked if that's me in my avatar... Nope, that's my beautiful wife and I'm a lucky dog!

Take care,
av8tor

RE: Scratch Built Ignition???
 

Hi Sukhoi_One,

AFAIK, the Jerry Howell (JH) ignition has fixed ignition timing unlike the CH ignition that has automatic advance/retard. It sounds like you have found an advance angle that allows the engine to start easily, but the angle may be too retarded at the top end to produce good power. This could be one reason for the "really weak" performance. Of course, the engine itself is a factor, as mentioned by av8tor1977.

You may want to check the advance angle on the JH ignition. As I recall, the CH ignition, when properly set, changes the advance angle from around 4 degrees when starting, to 28 degrees at top end. If your advance angle is significantly less than 28 degrees, then the top end power could be suffering.

Also, what coil did you use?

Thanks,

Bruce

RE: Scratch Built Ignition???
 

The 18cc Featherlite is a pretty lame engine no matter whos ignition is used. The engine is small enough to be run at full advance with out any timing control. So just crank up the timing to a full 30 deg BTDC and go for it. The engine may idle a little faster, but thats what the throttle is for.
I think the best we ever got one to run was 7600 on a 14x8. The lower end is the same as the 21 or 24cc, so the easiest hopup is more cubic inches.

RE: Scratch Built Ignition???
 

The CH Ignition only has the automatic advance you speak of if you buy it for extra. The small weekwhacker conversions they sell don't have automatic advance as it is not needed. These engines are small and low compression and by setting them at a full advance of about 28 degrees they still start without kickback and idle slowly enough. The automatic advance is recommended for larger and/or higher compression engines.

Take care,
AV8TOR

RE: Scratch Built Ignition???
 

Can somebody post a few photos of the Tm6 ignition circut ? Thanks Capt,n

RE: Scratch Built Ignition???
 

Hi CaptinJohn,

The JH Website shows pictures of the circuit board, components, and coils at:

URL> jerry-howell.com/ignition.htm

Bruce

RE: Scratch Built Ignition???
 

Unfortunately, www.jerry-howell.com is not currently available (I hope it's only temporary). [sm=crying.gif]

The good news is I have another site bookmarked with circuit diagrams of the Tim4, Tim6, and Tim7 ignition circuits, based upon 12V. By changing the component values, it should work with lower battery voltages. This person is using them for a homemade radial engine.

By the way, he also documents how he built his own spot welder... Scary stuff [sm=stupid.gif]

RE: Scratch Built Ignition???
 

Care to share the link?

RE: Scratch Built Ignition???
 

Hi strato911,

I had the link up around 6:30 PM but can't raise it now. It must be a temporary Problem.

Bruce

RE: Scratch Built Ignition???
 

[link]http://www.5bears.com[/link] will get you the schematics and a whole bunch of other neat stuff.

dave

RE: Scratch Built Ignition???
 

Sorry, I thought I had. I should have re-read my post before clicking "OK"

Zagnut has already posted it for me. http://www.5bears.com/tim4.htm has the ignition circuits.

_____________________________
I try to learn from other people's mistakes...
I won't live long enough to make them all myself.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ ¯¯¯¯
Dennis aka Strato911
MAAC 43408

RE: Scratch Built Ignition???
 

Just got back from a 3 day trip!!!! Thank you ALL for the return data. I am very interested in scratch buit ignition systems. Keep up the good work. Smooth landings.... Captinjohn

RE: Scratch Built Ignition???
 

I have a copy of of the RCM article and also have an ignition system built from a kit by an outfit called Fuji Max. It was given to me by a friend who is an electronics guru. I have not tried it as yet, but was assured it worked fine. Seems like the only thing that is hard to do is to wind the Toroid coil.

RE: Scratch Built Ignition???
 

My first partner wound the toroid coils, took 2 to 3 hours....Then we bought some, $12.50 each...Then I found some for $6.50..Easier to buy a circuit from C&H...:)
That wimpy little coil in the picture won't last as long as a real one...Weak insulation....They were/are made for about 3 volts, the CD circuits put about 300 into the primary, put out 20,000+, enough to spark over 1/2 inch in dry air, depends on the current draw, somewhere from 150 to over 300 MA.....At some point the spark jumps from the output wire about 1 inch along the coil, looking for a ground....I have seen a bakelite carb insulator with a 1/8 inch deep track burned into it when the owner of the engine grounded the coil to the carb bolt..It's insulated, so the spark just followed the path of least resistance, the bakelite block...Usually the sensor goes out first, but this one didn't..Wish I could find some sensors like that..:)

RE: Scratch Built Ignition???
 

Hi
RCIGN1 would you mind shareing your source for $6.50 coils would be apreciated

RE: Scratch Built Ignition???
 

The $6.50 coils are the toroids, NOT the spark coils...I have about 300 toroids, how many do you want ?
I buy my SPARK coils from C&H......

RE: Scratch Built Ignition???
 

can you explain the diffrence between the two It was my understanding that the torids were being used in the ch type of ingition you now have me very curious I must have gotten in the middle of the conversation

RE: Scratch Built Ignition???
 

The toroid coil is used on the circuit board, along with some other components, to step up the 4.8 volts to somewhere around 300 to zap the spark coil...The circuits used on the other ignition are not CD ignition, and only control the voltage to the Modelectric spark coil..These coils were used about 50 years ago for the spark ignition engines with points...The coils work fine when used as designed, but the insulation won't hold up for long to the 300 volts from a modern CD ignition system....

RE: Scratch Built Ignition???
 

Ok let me see if I have this right the coils like the modeltec work with igntions of the I think its called a keting type like howells but the toroid are used in the cdi much as a capacitor in a points type ingnition would be used I hope I have this in my mind correctly and am explaining myself I'm still trying to get my head around this ign. stuff thanks for your patients RCIGN1

RE: Scratch Built Ignition???
 

The capacitor in a points type ignition, connected across the points, is used to supress the sparks that are made by the points opening and closing..Modelectric coils are made for points ignition where the voltage to the coil is less than 12 volts...I think the insulation is some kind of wax material that doesn't stand up too well..The CD coils are made to take the 300 or so volts from the CD ignition...There is a relatively large capacitor in the final stage of a CD ignition, it takes the 20,000 or so volts from the CD circuit and sends it to the spark coil when the circuit is triggered by the Hall sensor...
I don't quite understand it either, but it works.. :)

RE: Scratch Built Ignition???
 

The Howell type is a Kettering or saturated coil type of ignition. The current flows through the coil (dwell time) and when it is stoped the magnetic field collapses making a spake. High rpm or short dwell time reduced spark out put.
The CDI uses the toroid as part of the dc to dc converter need to charge the HV cap.
The converter takes 4.8v dc and converted it to 300v dc and charges the HV cap with it. When the ignition is triggered the energy in the HV cap is dumped into the coil. The coil sparks as the magnetic field builds up. The coil is acting like a transformer rasing the 300v pulse to a 25,000v pulse making a spark.

RE: Scratch Built Ignition???
 

Yes I'm starting to get a better pic. on this I read the Bob Short book and quite frankly since my stroke this stuff gets in my thick head slowly. In his book Short does in fact use wax in his coils but I wonder if one could sub. very thin epoxy or is this not a good insulator? thanks guys

RE: Scratch Built Ignition???
 

I made my first prototype of a CDI ignition today, from parts cannibalised from an old UPS. It is based on the RCM schematics, but i made some changes to make it fit the components i used.

I put some info/pictures on my web;
[link]http://home.online.no/~jon-mj/cdi_ignition.htm[/link]

Sorry for the crude construction - i will make a PC board when i have the electronic advance system finished.

There is no schematics yet, all i have at this time is a hand-made schematic.
I will put the schematics on the web next week.