Coil Relocation

[Alec]

Hello Rob,

what I meant was if the windings were connected to the case they would be energised continually, i.e the points would do nothing.

Alec

[Alec]

Hello Rob,

the HT connects to earth at the spark plug body, i.e. the prominent electrode, after it has jumped the gap.

The primary and secondary windings are connected together in the coil.

Alec[frame][/frame]

[Rob_Jennings]

thanks

so indeed the coil is held at 12v above earth and the complete loop for the high voltage does include the battery! (not that 12 extra volts will make much difference)

Cheers

[bmcecosse]

It generates a charge that is held in the condenser/inductor circuit of the coil - and this then discharges to earth through the spark plug when the points open. So yes - in a sense since the power must come from the battery - it is in the circuit.

[PAULJ]

where is the earth path then? is it only via the negative terminal on the coil?

[PAULJ]

Now I know this one, Its amazing and a bit hard to believe. 12v go in the coil and go round and round a small wire wound loads of times around a soft iron core then to earth via the points which will be closed to make a circuit This makes a big magnet making a magnetic field. This is the low tent ion side. When the points open the field collapses and as it passes more windings many 1000's it induces a voltage in every winding. It is instant and if there is no path to earth the voltage disperses. if however you supply a path to earth it will go that way, say down a large HT lead. This HT voltage is so high 17000v plus so high it will jump a gap. The one in your spark plug. it will try to go back and jump the points and so a condenser is fitted to help stop this. The reason for fitting it the right way around is to assist in the magnetic field build up
A basic explanation but to the point I think

[mike.perry]

Just back tracking a bit. If the early coils are marked IGN & CB, when you change the polarity of the car and change the coil leads over then IGN will be CB and CB will be IGN.????????????
Excuse me if I missed something somewhere

[Alec]

Hello Mike,

no you are correct. I assume the earlier markings were done as the convention was positive earth and the markings refect this for ease of connection. So a change to negative earth means those markings are misleading and should be swopped over.

The reason polarity is important is that heat makes the electrons more active (remember thermionic valves which had heaters for that very reason) so the hotter centre electrode carries the current, not the earth which it would be with the wrong polarity.

Alec

[mike.perry]

So if the polarity of a coil is important I can assume that a CB IGN coil is poss earth, how do I know if a coil marked + & - is a poss or neg earth coil?

[PSL184]

For a neg earth car just wire the negative side of the coil to the points.....

[katy]

For coil polarity checking, see this old thread:
http://www.mmoc.org.uk/viewtopic.php?f= ... ty#p329100

HTH, Ken

[jamest344]

yes... I know but just want to solve for myself where the 'earth' for the HT side of the coil gets back to the block

1: coil is turned on 12v one side, earth via dizzy/contacts the other
2a: when a spark is needed points open in dizzy and HT build up in coil
2b: dizzy directs 'output' to correct spark via rotor arm but...

where is that HT coil tied electrically to the car body
a: does it jump back through the now open points gap to earth? (20k through them so often they would wear out in no time)
b: through the condensor? (I understood that was there to cause the coil to oscilate and so keep the spark going longer)
c: or via the only path left which is the positive side of the coil and via the battery

I have been having these exact thoughts and have concluded that the HT circuit completes via the condenser with displacement current. So when a condenser fails the points spark strongly because the HT circuit finds a path there instead. Even when working normally there is a small spark at the points (from LT current), which the condenser does not prevent. I believe this is the only answer to the rather obvious question of how does the HT circuit complete. Publications seem to just sidestep the issue, they draw the coil as an empty cylinder with 3 connections, or they assume the secondary coil earths to the coil casing (even when some coils are supplied painted!) On the wikipedia talk page there is a massive argument about the role of the condenser, and people are tying themselves in knots with mathematical equations, yet all agree that it is a vital component (presumably by dint of experience). This forum is the only place on the entire internet where I have found another person having similar thoughts to me on this!

I think that a battery produces a potential difference and current cannot flow through it. Neither can current flow through a condenser, but displacement current appears to be an established 'thing' whereby a charge goes in, causing a charge to come out the other end momentarily which would complete the circuit back to the secondary coil.

[oliver90owner]

Condenser has nothing at all to do with the HT spark, electrically. It is needed to suppress any back emf induced in the primary when the coil primary magnetic field collapses due to current interruption.

The HT is induced in the secondary coil in 'the coil', so again not connected in series with the primary coil windings. The 'coil' simply acts like a transformer (it sees a change in magnetic field but only in one direction, not like an A/C transformer which changes polarity 100 times a second for a 50Hz supply), producing a high voltage in proportion to the primary windings count, but as close as equal to the current and voltage product (the system is never 100%efficient , of course)

The primary windings are fed and interrupted from the battery through the contact breaker points (note nomenclature). The eventual coil current might settle at around 4-5A if simply allowing current to go to earth potential (points closed) however, the current only rises exponentially, initially, due to the inductance of the windings. This is why the dwell angle is important - the higher the current when suddenly interrupted, the larger the magnetic field collapse, if done quickly and without arcing at the points (hence the condenser wired across the points). Both primary and secondary clearly have a common earth - the car structure, back to the earth side of the battery.

Hope that helps to explain it a bit. It is a simple ignition system which works well for most older engines but breaks down at very high rpm, or too many cylinders, (too much time the contacts are open, so less current/energy) per primary cycle). Taken care of by muliple coils in modern ignition systems, basically.

[jamest344]

Both primary and secondary clearly have a common earth - the car structure, back to the earth side of the battery.

This is the part that I am interested in - happy to take the other points as read for now. I think that this drawing correctly shows the arrangement of a standard points ignition system.[frame][/frame]

What do others think?

[philthehill]

The coil mounting bracket pictured above is a - bracket dynamo rear Pt No: 2A758.

[oliver90owner]

You need to read up on reluctance, tuned circuits and ringing with LC circuits (inductor capacitor circuits). Only the primary inductance will be anywhere near related to the capacitance value. That 'ringing' will basically lengthen the HT spark period. One of the problems with early capacitive discharge ignition systems was spark duration. My early system required twice the capacitance at start up, but then was lacking in invertor power for high revolutions; OK for 'cooking engines', but not when the rev range was considerably beyond the norm.

Just think about it a little and accept that the HT supply energy goes across the rotor and spark plug gaps to earth potential. It is not dissipated in the condenser. Those condensers are generally rated at only a few hundred volts (less than a thousand). Think about it in another way if you are stll befuddled - in resitive terms the connection you are thinking of would be at battery potential of 12 -14 volts when the points are open. Just does not happen with LC ringing circuits. At only 500rpm there will be a spark generated every 60ms, and one every 6ms at 5000rpm. Points may only be open half of the time, too....

[jamest344]

Thank you for your kind reply, I will endeavor to read further.

For the avoidance of doubt, could you possibly say:
- Is the diagram I posted correct?
- What is the path taken by the HT current? (In particular from the engine block back to the coil).

[jamest344]

i guess the HT 'earth' must be running back through the 12v feed through the battery and to the car/engine earth otherwise a high voltage at the coil with no earth reference would not create a spark gap.

Rob is in good company here as Hillier agrees with him. Or at least he did in 2005 (Ed. 5): "A return path from the plug, via the earth electrode, passes through the battery..." (p.178) Presumably Hillier thinks that the ignition switch is part of the HT circuit too.

Hillier seems a little uncertain though because in 1996 (Ed. 2) he has this to say: "Each circuit must be complete to give current flow, so the end of the secondary winding in the coil is earthed. This is achieved either by connecting the winding to an LT coil terminal... or to an additional coil terminal linked by an external cable to earth." (p. 167)

Here Hillier begins by correctly describing the normal arrangement within an ignition coil, however neither of the LT terminals are connected to earth in any direct or permanent sense. The second option he mentions seems to describe an arrangement like an ordinary transformer with no physical contact between the 2 coils. This was undoubtedly used on very early systems.

At least Hillier is clear that each circuit must be complete.

The use of the term 'earth' could be a source of confusion. I think it is really used here as a shorthand for 'battery negative'. (Or battery positive on certain vehicles). I don't think that having a connection to the engine block or chassis is equivalent to being 'earthed' in the sense of a connection to, for example, a conductive stake hammered into soft ground.

[jamest344]

I wonder if anyone has ever seen an alternator being function tested by disconnecting a battery terminal whilst the engine is running? Never done this myself but I gather that it may have been a common practice if we go back a few decades.

If the engine continues to run with the battery disconnected, how then do we explain the HT return path? Through the alternator? The voltage regulator? The glove box light?

[oliver90owner]

I am wondering if you reside under a bridge? Even alternators are earthed at ''zero'' potential through the engine, onto which it is mounted. Do take a few books under your bridge and do something with them. ''Zero'' being a very arbitary figure, of course. It could be different frlm the real earth below us. Some alternators were 12V positive ground, of course.