Coil voltage

[Craven]

I will leave this as clearly you have no idea what you are talking about. There will be no voltage across the capacitor when the points are close therefore no current.

[mbell]

Hi Baggy,

A couple of years ago I converted my car back to a ballast setup. I knew I needed to get coil and ballast resistor as part of that. Based on the "9V coil" comments I'd seen on here I spent quite a bit of time trying to find a coil with suitable voltage rating and suitable ballast to go with it, with out really thinking about it. I failed and when I dug past a lot of the voltage based descriptions around the internet, I did find more information on what was happening. So was able to identify I needed 1.5 ohm coil and ballast to achieve the correct current flow.

So I understand and appreciate your aim is to provide a broadly understandably explanation of what is going on but in my experience the voltage based explanation was probably a bit of an over simplification. So I wanted to try provide a explanation that is a bit closer to want is happening and allows people to understand enough to select or confirm they have the right coil components in there car.

Hence me banging on that its about current flow not voltage....

Hi Craven,

Your absolutely correct and a valid point that that its more complex that the descriptions here but like Baggy I am just trying to provide an explanation that might be understandable to people who aren't engineers designing electrical circuits on a daily basis. I suspect that the percentage of electronics engineer on this board is very much higher than the general public, but the vast majority are not electronics engineer and are looking for information they can understand.

Personally I find the

armature electronic wizards

comment I little derogatory and unnecessary. I fully admit that I very quickly forgot my electromagnetic theory after resitting the final exam for that module and I haven't felt the urge to re-learn it since. But you have no way to know what I do for a living so you have no idea if I am armature, or professional intentionally oversimplifying things. If you want to add to or correct information that adds value and I welcome being corrected. But IMO its best to stay away for from personal assertions that could be interpreted negatively.

Cheers,

Mark

[Craven]

I understand you try to explain in terms suitable for the average forum reader but you bang on about current flow, current flow is the result of potential difference ie voltage. Here we have a constant voltage applied to an inductance ( coil ) result a varying current not a constant one and not easily calculated not just the value of the inductance but varying engine speed. I think stating a 12volt or 9volt coil is perfectly sound.

[Baggy2]

Technical talk from armature electronic wizards doesn’t help, but just to add to this, a ballast system comprises of a resistor in series with an inductance ( Coil ) so the function is not a simple equation Rt = R1 + R2 but an impedance one Z = R1 + XL. reactance of coil. Value of XL is engine speed dependent. R1 part is a fixed efficiency loss in a ballasted points system

Thats why I said the DC voltage - didnt want to complicate things - I'll get back to my armchair and stop trying to help.

[rgh0]

I will leave this as clearly you have no idea what you are talking about. There will be no voltage across the capacitor when the points are close therefore no current.

I think you should read more closely the link I attached

The points have some, though low resistance and the coil is earthed and charging up with an inital high current flow so there will be some voltage across the capacitor charging it to some degree also but as i said its impact can be ignored until until the points open as the voltage, capacitance and current flow is small compared to the current flow charging the coil.

like i said just my 2 cents worth..... no need for personal comments like "clearly you have no idea what you are talking about" . Better phrasing would be "I disagree with you for these reasons..."

If I said "clearly you have no idea what you are talking about" to many members who post here about twin cam engines I think they would be offended. I try to help people not insult other members.

cheers
Rohan

[Craven]

There you go again, quote ‘coil is earthed and charging up with an inital high current flow’ , the coil is an inductance, low initial current.
mbell
wizard is a complimentary term.

[rgh0]

Yes you are right.The current takes about 10 milliseconds to build up to maximum as the coil is charged due to its inductance. The voltage drop across the points will build up to a maximum in 10 miliseconds but the voltage drop is small and the capacitor in the circuit has no signficant impact when the points are closed which was the point i was trying to make originally

cheers
Rohan

[Craven]

My point is people that know what they are talking about would not make fundamental mistakes.

[rgh0]

The mistake was a loose use of the english language in post about a trivial point that was not signficant to the overall thread in any case... I said sorry. I am not sitting for an electrical engineering exam and even when I did I never got 100%... I doubt you did also

whats your beef.

cheers
Rohan

[Phil.C60]

Well, at the risk of getting involved in someone else's squabble, here's my two pennyworth!
I spent a few years in the early eighties as a mobile engine tuning guy when these things were current (pun intended), and then another ten years or so in an auto electrical business.
The ballasted ignition system is quite straightforward if it is checked correctly. It is designed to boost the coil supply voltage while cranking to get around voltage drop caused by the starter motor current draw when cranking.
Most of the previous information is correct, some of it isn't quite, and some of it is a bit over complicated.
Before I start: I have been criticised on the forum before by someone for having a somewhat dogmatic manner in my prose. It is not intentional: it just seems to come across like that when I try and write it all down. If it does, I apologise in advance.
Here goes nuthin'.....

All of the following assumes that the car is negative earth, the voltage given is the static voltage: I am aware that when running the voltage across the circuits will rise to the charging voltage (14.5V if you're lucky....less if you're not or all the lights, heater, fan etc. are on!) and drop to the cranking voltage (10.5V if you're lucky, if it get's down to 9V you have other issues to resolve...lol). I am also aware that I have made no allowance for losses, but this is about the principles after all.

In response to mbell's post, most coils are marked for voltage, usually stamped on the bottom, to differentiate 6V coils (early Beetle, motorcycles etc.) from 12V coils which may have the same primary resistance (what we're interested in here) but I seem to think will have a different secondary resistance..... I suppose it's worth checking!

The standard 12v system has a 3 Ohm coil measured when disconnected. This gives a nominal 4 Amp load across the points but this is mitigated by the dwell angle of the points being a nominal 60* (57-63* is the correct setting) ie the points are shut for only 60* out of 90* or 2/3 of the time reducing the load to approx. 2.7 Amps.
Remove the distributor cap and turn the engine by hand until the points are open (yes, I know, it's a pain but it is best to do it this way which becomes more important later, and you should really make sure they open correctly, but apart from that leave well alone for now...). Turn the ignition on and check the voltage between the coil positive terminal and ground. It should be 12V. It should also be 12V at the coil negative. Turn the engine by hand until the points are closed, the voltage should still be 12v at the coil positive, and should be 0v at the coil negative. If it's not quite 0v there is an issue with the points contacts (dirty or pitted, more of which later....). Do not leave the system in this position for extended periods of time (a few minutes is OK) or the coil will get very hot, and the wiring from the coil to the points may get damaged. Best to turn the engine until the points open again.
So far, so good.
The ballasted system has a 12V feed to a ballast resistor, usually 1.5 Ohms (either a china block type or a resistive wire attached to the harness (often grey: MK2 Escort for instance) and then to the coil which should measure 1.5 Ohms disconnected.
Remove the distributor cap and turn the engine by hand until the points are open. Turn the ignition on and check the voltage between the coil positive and ground. It should be 12V. Turn the engine by hand until the points are closed, the voltage should drop to around 6V at the coil positive and 0V at the coil negative (once again, if it's not quite 0V there is an issue with the contacts which may affect the reading at the positive side). This is because the ballast resistor and the coil are two resistors in series making a voltage divider. As they are both equal value, the voltage measured between them and therefore at the coil positive, is half the input value ie half of 12V which is 6V. This means that the 1.5 Ohm coil runs at 6V under normal conditions giving a nominal load of - you guessed it - the same 4 amps, at the same nominal dwell angle, so the same normal running load of 2.7A.
When the ignition switch is at the crank position, a 12v feed from the solenoid (I'm assuming that as it's an Elan the bulkhead solenoid is still present) on a small wire separate from the main feed out to the starter motor (to avoid a loop effect keeping the starter engaged when the key is released) is fed direct to the coil, by-passing the ballast resistor. The connection can be at the coil, or at the output of the ballast resistor ie the end connected to the coil. This has the effect of doubling the coil voltage and therefore the coil current and therefore the coil output, but it's effect is reduced by the voltage drop caused by the starter current draw so in practice it is more like 10V (the minimum should be 9.5V but that's another subject...). Still a useful boost especially on cold days.
To check this, disconnect the starter output wire form the solenoid (you don't want the engine to turn) and turn the ignition switch to the crank position - remember, the points are shut at this time (as we left them in the previous stage). The voltage at the coil positive should rise to 12v as the ballast resistor is now by- passed. If it doesn't, either there is no output from the solenoid, the resistor connections are the wrong way around or there is a connection issue somewhere (or you don't have a ballasted system, but if you have completed the tests in the order above, you should already know if you do or don't unless the system has been by-passed which does happen...see below).
Lastly, and importantly, This system should not be checked with the engine running! Those who have done, and suggest that the coil operating voltage is 9V are incorrect. The voltage present at the coil is 12v with the points open and 6V with the points shut, leading to an average voltage reading at the coil positive of 9V when running, which is what your meter will show, but this is not the operating voltage of the coil as with the points open there is no current flow, there is only flow when the points are shut, at which point the coil voltage is 6V.
I suppose that checking the coil voltage wile running may tell you if you have a ballast resistor fitted, but this assumes that you have 12V points open and 6V points shut. If the supply voltage is only 10V static due to a connection problem (at the rev. counter, for instance as the ignition supply is routed through it) it could drop to 9V running in any case and assuming you had a ballast system you would fit a 1.5 Ohm coil.....you get the picture.

Common problems are:
People fitting a 1.5 Ohm coil or even a 0.8 electronic ignition coil to a 12V system. This causes the coil current to be doubled (or more than trebled with the electronic type...) which is more than the points contacts are designed to carry (The doubling of coil current while cranking in a ballasted system is mitigated by the fact that it's only for a very short period of time). This causes them to "burn"(quite quickly...) and pit in a characteristic fashion. The moving contact usually grows an obvious spike, and the fixed contact has a corresponding hole in it. The contact faces look burnt and turn a bluish colour due to arcing. Eventually in operation they become "sticky" causing misfires, sometimes extreme enough for the car to not even run, just pop and bang when attempting to start. The cure is to replace the coil for the correct item, replace the points and also the condenser which will have been working overtime trying to control the arcing. You may also need to replace the spark plugs if they have fouled due to the misfiring. This failure can also be caused in a ballasted system by people by-passing the ballast resistor permanently, usually by connecting the coil to a 12V supply because there is something else wrong with the ballasted system that they have not worked out. You could by-pass the system if you wish but you must substitute a 3 Ohm coil!
People fitting a 3 Ohm coil to a ballasted system. This causes low coil output when running, usually leading to misfiring under acceleration when spark plug voltage demand is high (or at cruise on a light throttle when, depending on how lean the fuel mixture is, spark plug voltage demand can actually be at it's highest) and potentially spark plug fouling. This means replacing the coil for the correct value and maybe the spark plugs too. It shouldn't affect the points or condenser.
Fitting a 0.8 Ohm coil to a ballasted system will give you the same effect as fitting a 1.5 Ohm coil to a 12V system.

This is only a guide to how the systems work: it is not an exhaustive guide to trouble shooting in an Elan. Our cars have generally been through a few hands before ours, and electrical work seems to be the thing that is least well done - birds nest wiring, extra cables of various colours joined in (often randomly by someone trying to solve a problem they didn't understand which is why I haven't offered any wiring colours, but white is usually ignition feed (unfused) white with yellow 12V ballast by-pass, white with black coil to distributor, in English cars), badly fitted crimp connectors, ScotchLoks (the Devil's device), to name but a few. I am more than happy to offer help to anyone with a specific issue, either on here or by PM. It is also important to say that you should follow the steps in sequence. In my experience most folks trip up by trying to shorten the process usually skipping steps or trying to find away around awkward bits like removing the distributor cap etc. It really isn't worth it in the end, you just end up going around in circles. Fault finding is a process: start at the beginning and follow it through normally gets you there. Even if you can't resolve the issue at the end, you will be able to give anyone who tries to help you the information they need to point you in the right direction. And write down you're figures, make notes of the connections. I'm 63 now, and my memory is not what it was when I was a 25 year old hot-shot (or thought I was LOL. The older I get, the less I seem to know....). That way if you decide to double check something or you can't remember what cable went where after you pulled them off, you'll know where you were, if you follow me.
I have read this through few a few times and I think it's all correct, if I have made a mistake I'm sure someone will let me know!

Hope this helps,
Regards to all,
Phil

[Andy8421]

Phil,

A good, clear summary. Thank you for pulling it all together.

In the spirit of completeness, just a couple of pedantic extra details. Don’t bother reading if you want an overview, Phil’s summary is good. As my old textbooks used to say - “For the interested reader……”

Your calculations of current flow for a static system are correct, but when the engine is running, things are more complicated. Weighting the current flow by the mark/space ratio of the points dwell is only part of the story.

Firstly, your comment about battery voltage is well taken. While everyone talks about a 12v system, it is actually a 14.5v system. When considering current flows, cable sizing, loading etc, the system is designed for 14.5v - which is the battery voltage when the engine is running and the battery is floating on the dynamo / alternator. Those 12v bulbs you buy are really designed for 14.5v. Any current flow should use 14.5v for analysis of a system in normal operation.

Secondly, as mentioned in some of the comments above, the coil / points / capacitor (condenser for those born in the early 1900s…) form a resonant circuit. When the points are opening and closing, the circuit isn’t running on DC anymore, but pulsed DC / AC. The current flow through the coil is a complex oscillating waveform and not the simple mark/space (dwell) weighted average you describe. The DC (heating effect) equivalent of the current flow is likely to be much less than the static value multiplied by the points mark / space ratio you mention.

Andy.

[Craven]

Coil is an inductor units Henry
Resistor units R ohms.
There is no H in Ohms Law.

[Phil.C60]

Andy:
Thanks for the kind words.
I do appreciate that the coil operation is a bit more complex, but as you say that doesn't need to concern most people.
With regard to the lessening of the heating effect ie DC equivalence, I've always felt that sizing the cables on the DC equivalence when working them out for voltage drop etc. on longer runs when wiring cars from scratch is the best way: if it's less it's fine, but if it's more it's potentially an issue. I was only trying to give people an understanding of the issues involved with incorrect coil types using representative numbers, rather than a guide to designing a system from scratch.
As you say, I was only trying to explain how to check the system for correct components, wiring and installation. After that it's probably time to get the 'scope out - preferably analogue at that!
I do know the whereabouts of an old Bosch MotorTester with a working scope and complete with all the leads for not a lot of money that's got my name on it.....when I can find the space!
Regards,
Phil

[AshleyPark]

Great posts Phil, very informative!
Thanks.

David

[Billmack]

No worries. There are a whole slew of defective condensers out there. I wont use any new production with orange wire. I search for new old stock