Lotus Elan

Another coil question.

PostPost by: HCA » Tue Jun 30, 2020 9:28 pm

2cams70 wrote:Yes exactly. See the diagram of the Kettering system in the link. HT side of the ignition coil - one side connected to the spark plug and the other side connected to ground. In the case of a metal can type ignition coil fitted to a metal bodied car the earth pathway for the HT supply is through the coil's metal can housing and not the negative terminal on the coil that is connected to the points.


I sympathise - sometimes the schematics drawn by our betters are not illustrative of what really happens.

If though you think about it, those who have the secondary winding earthed, how do they think the spark is produced - the high induced voltage would take the psth of least resistance to earth..

Back to the terminal - I really cannot see it is an earth or a ballast and not have it's use embossed on the coil end. If this terminal were incorrectly wired - in either of these cases, there would be some trouble ahead.. :D
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PostPost by: persiflage » Tue Jun 30, 2020 10:04 pm

HCA wrote:There is a manufacturer's logo on the coil body - post a close up piucture of this? Also a close up of the bakelite top.


As requested.
Attachments
20200630_224850-1-1-1.jpg and
20200630_225020-1-1.jpg and
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PostPost by: 2cams70 » Tue Jun 30, 2020 11:43 pm

Craven wrote:2cams70You are just so wrong. Please read it fully, the primary winding provides the HT return path in a practical operating system not as in an illustrated schematic.


That's what I like about this forum. It gets the brain juices flowing. Please look at the second link provided by HCA and read it. My understanding is that a coil is wired internally as per the type "B" shown but I see there are a couple of different variations possible.

Thank you HCA and also the original poster for expanding our knowledge base and broadening our horizons.
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PostPost by: 2cams70 » Thu Jul 02, 2020 3:13 am

Ok I see now that the information in HCA's second link and also my previous understanding is wrong. A typical automotive coil is actually an autotransformer construction and not a conventional transformer construction where the primary and secondary windings are independently isolated.

I'm still having trouble visualising how the HT path connects to ground. At the point when the spark occurs it can only ground via the points condenser or the +ve terminal of the coil (which seems counterintuitive). It can't ground through the points themselves because they are open circuit at that stage. Perhaps in the context of around 30,000V on the HT side it can still ground at the +ve coil terminal because the potential here at 12V is still well below 30,000V so it in effect can become a ground.

It would be appreciated if someone could clarify. Let's try and keep it nice and not take who is right and who is wrong personally however. It's all just for the purpose of gaining a greater understanding!
1970 Ford Escort Twin Cam
1972 Ford Escort GT1600 Twin Cam
1980 Ford Escort 2.0 Ghia
Peugeot 505 GTI Wagons (5spdx1) (Autox1)
2015 Honda City 5spd.
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PostPost by: HCA » Thu Jul 02, 2020 7:49 am

I have edited my original response as I since came across a Youtube clip that explains it all far better than I attempted to do!

Enjoy!

https://www.youtube.com/watch?v=lWyn_eV-DzM
Last edited by HCA on Thu Jul 02, 2020 8:47 am, edited 1 time in total.
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PostPost by: HCA » Thu Jul 02, 2020 8:26 am

I am trawling through the internet as it is a bit grey outside trying to find an answer to the third terminal part of this thread and came across this:

http://www.da7c.co.uk/History%20Section ... se%203.pdf

I give this 100+% rating - maybe a bit aged in the tooth now the way it is written, but certainly worth downloading if you hobby service the older ignition systems. Explains the theory simply followed up with the testing procedures.
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PostPost by: 2cams70 » Thu Jul 02, 2020 10:09 am

Thanks HCA. That old Lucas technical information especially is really good stuff!!
Regarding the OP's original question as to the mystery of that third terminal do you think that the coil has been made with 2 voltage taps on the primary side rather than the more usual 1?

By making the coil this way you could in effect have the advantages of a system designed around a ballast resistor without actually having the resistor - i.e one lower voltage primary tap is powered during cranking only and the other powered during normal running? Has some advantages because you aren't wasting energy by heating up a ballast resistor during normal running.
1970 Ford Escort Twin Cam
1972 Ford Escort GT1600 Twin Cam
1980 Ford Escort 2.0 Ghia
Peugeot 505 GTI Wagons (5spdx1) (Autox1)
2015 Honda City 5spd.
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PostPost by: Craven » Thu Jul 02, 2020 11:46 am

Youtube clip is again is a simple explanation and how generally most people envisage high voltage is obtained. However an ignition coil has around a turn’s ratio of 80:1. 12v x 80 = 960 volts so clearly, what’s going on, welcome the capacitor.
Unless those trying to understand in detail have a good grasp of magnetic induction, AC theory and pulse techniques the answer will I’m afraid elude them.
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PostPost by: HCA » Thu Jul 02, 2020 4:50 pm

2cams70 wrote:Thanks HCA. That old Lucas technical information especially is really good stuff!!
Regarding the OP's original question as to the mystery of that third terminal do you think that the coil has been made with 2 voltage taps on the primary side rather than the more usual 1?

By making the coil this way you could in effect have the advantages of a system designed around a ballast resistor without actually having the resistor - i.e one lower voltage primary tap is powered during cranking only and the other powered during normal running? Has some advantages because you aren't wasting energy by heating up a ballast resistor during normal running.


Dunno - I still harp back to thinking that if the third terminal is for dual voltage, ballast, earth etc, it it too important for there not to be a mark as to its use. Note how prominently the CB and SW terminals are marked...
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PostPost by: Slowtus » Thu Jul 02, 2020 5:03 pm

HCA wrote:
I give this 100+% rating - maybe a bit aged in the tooth now the way it is written, but certainly worth downloading if you hobby service the older ignition systems. Explains the theory simply followed up with the testing procedures.


If memory serves we used this document or similar back in the early '70s as part of Electricity and Electronics as an O level course/exam, we were guinea pigs or beta testers as we now refer to such unfortunates as this was a completely new subject and, importantly, no previous exam papers to refer to.

I was in the RAF at the time and all the electrical/electronic techy types wanted to do this as it was obviously going to be easy given our intensive, broad-based training and lengthy experience on all types of things electrical (and 'electronic').

Twas a humbling day when all but two in a class of 18 failed! The abiding thing was that it (re) taught me was to never assume, start with the basics and ALWAYS use "head before hands". :D
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PostPost by: bob_rich » Tue Jul 07, 2020 1:27 pm

Hi All

sorry a bit late looking at this thread. By and large the way things should work seems to have been sorted but the coil with the 3rd 1/4" fast on(Lucar) terminal is unusual.

getting back to the original post the picture showed a few resistance measurements that I dont fully understand
the SW and CB terminals should measure around 3.5 ohms with a 12V coil but the high voltage output should read to either SW or CB around 4.5k Ohms (4500 Ohms). and what does the 3rd unmarked terminal actually read, and to which if any, of the other terminals.

To be safe I like to ground the body of metal cased coils just for added safety and minimising radio interference.

hope this helps best of luck

Bob
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PostPost by: persiflage » Wed Jul 08, 2020 9:01 am

Hi Bob,
Switch to C/B. 4 ohms
Switch to Unknown. 8 ohms
C/B to Unknown. 4.6 ohms
Secondary to all 3 low tension terminals 7k
All connections isolated from the case.
I've shown the coil to an aqaintance, now in his seventies who has been in the motor business all his working life. The last thirty five years or so his work was building, servicing, maintaining and competing in rally cars, both here and abroad .... He's not seen a coil like it before.
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PostPost by: HCA » Wed Jul 08, 2020 9:41 am

Here is a response from someone who works in coil production with SMP who really does know his coils:

Hi Hal,

That’s really interesting. I thought I knew my coils – but this is a new one on me! I know we’ve never produced a coil with this 3rd terminal, but I will ask some industry veterans and see if anyone knows what the function is! I will come back to you, I’m eager to find the answer myself now!

Thanks
Pete

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Senior Product Manager
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PostPost by: persiflage » Wed Jul 08, 2020 1:30 pm

Who would have thought that the original simple question would have travelled this far.
I'll be really interested to see any answer that Peter Lowery can supply. :)
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PostPost by: Lyn7 » Wed Jul 08, 2020 4:13 pm

Hi Guy's. I'll throw this into the mix! I mentioned this coil to a friend who happens to be a boat enthusiast as well as car. He suggested it was for a boat and had seen them before. The third terminal being for the rev counter??
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