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Model T buzz coil ignition from Coil on Plug module

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DustyB

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Here is another COP ignition question. I show a condenser (capacitor) in my attached drawing but the COP doesn't actually work like regular Kettering ignition coil and points. Should this capacitor be a diode instead, to stop the points from arcing? What should the diode rating be? I first thought a capacitor would do it but now am having second thoughts. The relay makes/breaks the COP switch circuit at high frequency to generate a continuous spark like an old Model T buzz coil. The relay points carry very little current since the switching is done by the coil's transistor. According to my DVM the other set of points will carry 4 amps.

Thanks,
 

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Hi DustyB,
No you cannot place a diode in this circuit instead of the condenser..... if you do, (for example with its cathode to ground) your relay coil will be energised all the time. If you try again with the diode reversed (ie with anode to ground) then the switching transistor will have no path to ground to conduct to, as the diode will block the conduction path.

Forgive me for perhaps a stupid question here..... what are you trying to achieve here with this COP circuit? .... and is this your own circuit design or someone else's that you are trying to modify for some purpose?

hope I can help,
Rotarymaker
 
The concept for this circuit isn't mine however I've never seen anyone using it with this type of ignition coil. Eighty plus years ago gasoline engines frequently used multi spark ignition systems such as the Ford Model T also used. I have an old antique gas engine of this type missing its complete ignition system. To get it running again with parts I had laying around I got help here last week building one system with a hall sensor and this type of coil. I also wanted to try one similar to what others have built using a relay as a "vibrator". I assembled one of these yesterday and it seems to work well, producing a continuous stream of sparks. Kettering ignition systems used a capacitor to protect the points and to enhance the spark which got me to thinking I might need to protect my points similarly. I don't think I need one to enhance the spark because it seems quite lively already. I've also seen diodes used to snub an automotive air conditioning clutch coil when its circuit is opened which made me think I might be able to do the same.
 
Ahh DustyB,

I see what way you are thinking now.
The issue you are probably trying to prevent is the back emf voltage generated by the relay coil going open circuit, which may possibly destroy the transistor input stage of the COP.
I would suggest you try a 1N4007 diode across the relay coil ....the anode of diode to point 86 of the relay coil.... and the cathode (bar marked on diode) to point 85 of the relay coil.

try that and let us know how things go,
Rotarymaker
 
If you want continous sparks like a Ford coil, try driving the COP with a NE555 ic like in the circuit. The 10K var resistor varys the spark rate. Very simple & efective circuit & you will get good strong sparks.
 

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Are the wires at NE555 #2 and #6 parallel? To start and stop this circuit for spark timing do I just break it from +12?

Thanks for your help last week. My goal is to get both circuits installed and tested soon.
 
Yes pin 2 & 6 go to the same position. It would be better if you could break the circuit betwean the 1K resistor & B on the COP.
 
Did you use a 555 IC back then? I suppose you drove a conventional coil with the circuit. Did you use a power transistor for switching? The COP has one built in and heat sinked which is quite convienent.
 
I did use a 555 and, i think, a 2n3055 plus a small low signal transistor like a 2n3904, 3906 at the time.

Actually, I dual purposed a solid state vibrator supply to run the coil. It probably ran about 300 Hz.
 
If you want continous sparks like a Ford coil, try ...
Mr. Debe, if you get a chance please look at the diagram as I re-drew it to the approximate pc board layout. One question I still have, something I have not seen in similar circuits, the capacitor at pin 5, does it just provide some protection rather than leaving pin 5 floating? I assume it is a ceramic capacitor however my local Radio Shack store does not have that size available.

Thanks,
 

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Hi Dusty, Pin 5 can be left floating or use .01uf cap probably not realy necesary. The rest looks ok to me.
 
What circuit modifications would I need to complete the connection between 555 pin 3 and coil B when the ignition points are closed? I need sparks when the points are closed.
Thanks,
 

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What circuit modifications would I need to complete the connection between 555 pin 3 and coil B when the ignition points are closed? I need sparks when the points are closed.
Thanks,

Hi DustyB,

Try this (see attachment) ..... you may wonder why have so low value of resistor for keeping transistor off when points are open ??..... it's to allow for a reasonable current (~66mA) to flow thru points when they close, as points tend to tarnish / get dirty in an engine environment..... a little trick I learned years ago with designing points ignition systems.

Just a thought about your circuit Debe..... shouldn't the LED be connected the other way around to light / pulse (ie cathode / bar to resistor)?? ..... also, is 560K resistor to the LED not way too high a value to allow LED to light / pulse? My thoughts on its value should have been approx 330 ohms to 470 ohms .... not kilo-ohms

hope this helps,
Rotarymaker
 

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You are quite right the LED is around the wrong way, the resistor i used was 560 ohm & the LED was bright enough.
 
Yes, the LED lights fine and when it didn't light at first I turned it around. The 10k variable resistor doesn't seem to adjust the frequency very much but I think it will be fine. I'm glad I picked up three coils since I just smoked one. I'd be happier if I had taken all six. I left the power on and sparking for about a minute or so when it quit. The coil's heat sink was very hot. We'll see if it recovers after it cools down. Somehow I doubt it.

Thanks for the circuit, Rotarymaker. I'll give it a try.
 
Yes, the LED lights fine and when it didn't light at first I turned it around. The 10k variable resistor doesn't seem to adjust the frequency very much but I think it will be fine. I'm glad I picked up three coils since I just smoked one. I'd be happier if I had taken all six. I left the power on and sparking for about a minute or so when it quit.The coil's heat sink was very hot. We'll see if it recovers after it cools down. Somehow I doubt it.

Thanks for the circuit, Rotarymaker. I'll give it a try.

Not a problem DustyB, hope it works for you.... if not get back to us.

Another thought on your coil burning up.... make sure you have a sparkplug in the coil with a max gap of about 1.2mm.
If you are just arcing it over a longer distance to earth/ground, that'll surely cause coil failure, as the high voltage will always take the path of least resistance and arc and carbon track its way thru the internal coil windings.
Also make sure that your max frequency output of the 555 is within the range of the coils design
ie normal car, say max rpm = 7000,
at firing once every 2 revolutions (4 stroke) ..... = max 3500 sparks per minute
.....divided by 60....
= approx 58 hertz or 17millisecs period

.....hope I've got the sums right!
Nevertheless, too high a frequency will cause failure of the COP internal driver circuit.

Rotarymaker
 
I was using a spark gap of 6-10mm while testing and it had a good solid spark. I don't have a way to measure frequency so I just listen to the spark and I'm not sure what frequency the Ford Model T coil would have operated at either. Back then they used a set of vibrating points on the coil to generate the pulses unfortunatly I don't have a working T coil to compare it with. My old gas motor has a top speed of 500 rpm at which point the exhaust valve latches open until the rpm drops back down. The ignition system is also disabled while the engine is coasting.

Thanks,
 
Last edited:
Here is another COP ignition question. I show a condenser (capacitor) in my attached drawing but the COP doesn't actually work like regular Kettering ignition coil and points. Should this capacitor be a diode instead, to stop the points from arcing? What should the diode rating be? I first thought a capacitor would do it but now am having second thoughts. The relay makes/breaks the COP switch circuit at high frequency to generate a continuous spark like an old Model T buzz coil. The relay points carry very little current since the switching is done by the coil's transistor. According to my DVM the other set of points will carry 4 amps.

Thanks,
The breaker points may need at least 0.3A through them to keep them clean.
 
On the old engine I'm working on the points are completely open and exposed to the weather. The points are a 25mm wide metal strap that contacts a pin on the cam gear. The pin slides along the metal strap and is in contact for several degrees of rotation. I think they should stay pretty clean but the minimum current to keep a more normal set of points clean is valuable info.
 
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