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John L

Heated Coils

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Hi Folks,

one of the posts mentioned 'ringing' in the circuit.

 

To eliminate this possibility place a 1uF capacitor on the 12V contact of the coil to earth. Similar to that used to quieten the radio interference.

 

Roger

 

PS - it may solve the problem but not cure the fault.

Edited by RogerH

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Yep, contained in every set fitted into car for killing radio interference.

5 uF might be good and the FORD EDIS has 25 uF in the feeding line.

Also the alternator often does crazy things and should be treated same way.

 

Most of that make trouble with the radio or when swapped to EFI.

We do not notice because we normally do not have electronics on board.

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We haven't heard back from the original poster John L for a while . From his posts it looks as though the owner has been using only the Lucas 101 coils and no-one has yet suggested a (standard type) alternative as he initially requested. There are a number of other makes he could try though most cost a bit more -presumably for better quality. The Green Sparkplug Co have a selection including Bosch as Andreas mentioned.

 

http://www.gsparkplug.com/ignition/ignition-coils

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The ballast resistor system is used to allow a coil to function "better" when starting.

 

The theory being that under cranking the voltage of the electrical system is less than when the alternator is delivering 14v or so.

Thus you can use a coil designed for 9v to get the full 9v at cranking but restrict the voltage to 9v when running (the ballast resistor being bipassed when cranking) If a coil designed to run at 9v is subjected to full running voltage of 12 to 14v it would be expected to run hot and fail prematurely.

 

The primary (low tension) side of the coil generates a magnetic field field, the opening of the points allows the magnetic field to change and induce a current to flow in the ht circuit. The higher voltage of this being produced as a result of of more windings in the secondary (ht) circuit.

 

In some ways similar to a transformer except the coil has a DC current rather than AC.

 

However if a 12v coil can't cope with a continuous 12v it is probably of dubious quality, a mis-labelled 9 or12v coil or possibly the system is putting out too high a voltage. (Given that the battery voltage won't be much over 12v the latter won't explain it failing when the ignition is on but the engine not running.

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I have the last of the the three listed options (Triumph V8), bought on eBay for £6.

 

They turn up from time to time, as do Bosch blue coils.

 

If you fry a genuine on of those you are definitely doing something wrong.

 

Rgds Ian

 

FWIW there is a S/H one on there now, looks a bit used but would probably clean up. £12.50 at the moment.

Edited by Ian Vincent

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In some ways similar to a transformer except the coil has a DC current rather than AC.

 

Hi Andy,

it is exactly the same as any transformer. In an AC circuit the current collapses to zero before going reversing and starting to rise.

In the CB circuit the current collapses suddenly - but still collapses. Is then restored when the CB opens. It doesn't have to reverse as in the AC circuit.

 

It is the collapsing/building of the magnetic field that induces the current flow in the secondary winding.

 

I think the above was a bit of granny sucking eggs syndrome B)

 

Roger

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Close Roger, but no prize this time I'm afraid.

 

/pedant mode on

 

Its not really like a conventional transformer because in a transformer you normally never 'saturate' the core with flux , they are operated in the linear region. If a transformer is allowed to saturate it draws excessive current, overheats, and the waveform is distorted.

 

In an ignition coil operated with points the core is deliberately fully saturated (i.e. you cannot make it any more magnetic) and is used to store the magnetic flux which will do the work.

The primary current just sets up the flux in the core by Ampere-turns while the points are closed, though as Andreas has explained above the initial rise of the current is slowed by back-emf until saturation is reached. The charging current is a curve which rises steeply at first and then becomes asymptotic to the final maximum value. Once the core has magnetically saturated, the only thing limiting the current is the coil DC resistance and that remains the case all the time the points are closed, with the current maintaining the magnetism in the core. Its during this time that the coil is heating up.

When the primary circuit is interrupted by the points opening, the flux collapses suddenly. Its that collapse which induces a large voltage in the secondary winding, proportional to the change in flux magnitude times the length of the winding in the flux times the speed at which the flux changes. It also induces a smaller voltage in the primary (smaller because the length of the conductor in the flux is shorter as there are fewer turns) which in the absence of a condenser will arc across at the points and cause burning of the contacts. One function of the condenser is to suppress that arcing.

 

/pedant mode off

 

Of course none of that is in any way helpful to the OP with his failing coils....Gosh I need to get out more.

Edited by RobH

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Thanks guys ...Have been away for a couple of days

I like #23 good idea, but I feel he is died in the wool....

Will reread and get back with some more info, I have asked DD for some of his yellow wire condensors

John

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Apologies if it came over as patronising.

 

Reverting to the topic - and we have examples here of coils boiling when the ignition is on but not running I still make the case for a defect. The oil is there presumably to prevent the coil windings overheating and burning the insulation.

 

If a coil draws a notional 5amps at 12v - that's 60w.

A coil may hold say 250g of oil it should take a while for that to boil if we assume the specific heat of oil is approx 1.7KJ/Kg per degree it should take over 20 mins to boil at a conservative 200 degrees C if it started at 20 degrees C. That assumes there is no heat loss at all.

 

So you do wonder if there's an internal short so that more than 5A and thus more than 60w is generated, or the coil lacked oil so there wasn't as much to heat up and ultimately protect the wire's insulation.

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Hi Andy,

sorry, my soppy comment was me telling you about granny, eggs and sucking.

 

Sadly I'm not an expert in auto electrics and ended up with a lesson on saturation and sparky things.

 

I'm glad that somebody knows what they are talking about

 

Learning all the time.

 

Roger

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Close Roger, but no prize this time I'm afraid.

 

/pedant mode on

 

Its not really like a conventional transformer because in a transformer you normally never 'saturate' the core with flux , they are operated in the linear region. If a transformer is allowed to saturate it draws excessive current, overheats, and the waveform is distorted.

 

In an ignition coil operated with points the core is deliberately fully saturated (i.e. you cannot make it any more magnetic) and is used to store the magnetic flux which will do the work.

The primary current just sets up the flux in the core by Ampere-turns while the points are closed, though as Andreas has explained above the initial rise of the current is slowed by back-emf until saturation is reached. The charging current is a curve which rises steeply at first and then becomes asymptotic to the final maximum value. Once the core has magnetically saturated, the only thing limiting the current is the coil DC resistance and that remains the case all the time the points are closed, with the current maintaining the magnetism in the core. Its during this time that the coil is heating up.

When the primary circuit is interrupted by the points opening, the flux collapses suddenly. Its that collapse which induces a large voltage in the secondary winding, proportional to the change in flux magnitude times the length of the winding in the flux times the speed at which the flux changes. It also induces a smaller voltage in the primary (smaller because the length of the conductor in the flux is shorter as there are fewer turns) which in the absence of a condenser will arc across at the points and cause burning of the contacts. One function of the condenser is to suppress that arcing.

 

/pedant mode off

 

Of course none of that is in any way helpful to the OP with his failing coils....Gosh I need to get out more.

That is the best explanation i have ever read, thanks!

 

Until now i didnt understand WHY a condenser is needed, now i do

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Thanks Rob for a very clear explanation.

Is there a related reason for Bosch T coils, where T stands for transformer , being better? The GT40T was once sold by Bosch in Oz, cant find it in UK. But it was listed for the TR5 and 6.

http://www.shoeys.com/docs/Bosch_coils.pdf

 

Peter

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Not sure on that one Peter as there doesn't seem to be much hard data published on how the things differ - I can only find advertising material similar to the pdf you linked to.

A limiting factor with a conventional coil will be the amount of magnetic core plus windings that you can get in the space available within the standard can. I gather that these coils have better magnetic core materials and a more efficient magnetic circuit. That should enable higher flux to be established for a similar size and hence there will be more energy available for the spark.

I can't see that using epoxy insulation rather than oil will in itself increase efficiency, except that you can use all the available space because no air-gap is necessary to allow for expansion. It doesn't look as though there is any fundamental change in the operating principle - its just incremental improvement in the implementation.

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I had similar problems on a TVR a while back burning out two coils and while the dealer insisted the coils were 12 volt non ballasted on further investigation they both turned out to be 9 volt ballasted coils of suspect origin. I did however suffer extreme mis-firing as the coils heated up under normal running so this might be a complete Red Herring!

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If you buy a coil from any of the usual suppliers these days (including ones which I trust more than others) don't expect it to last more than around a week!

I had to take advantage of the breakdown cover I have last Sunday when returning from TRR International at Lincoln. My new coil had covered around 120 miles before total failure- and yes, condenser, points etc were all new (from Distributor Doctor) and correctly set up at the same time.

The supplier in question has apologised profusely and made an acceptable offer of a free replacement. I shall be fitting a second coil close by just in case.

post-9736-0-57074400-1530602304_thumb.jpg

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If you buy a coil from any of the usual suppliers these days (including ones which I trust more than others) don't expect it to last more than around a week!

I had to take advantage of the breakdown cover I have last Sunday when returning from TRR International at Lincoln. My new coil had covered around 120 miles before total failure- and yes, condenser, points etc were all new (from Distributor Doctor) and correctly set up at the same time.

The supplier in question has apologised profusely and made an acceptable offer of a free replacement. I shall be fitting a second coil close by just in case.

attachicon.gifIMAG15281.jpg

Did you use a Dizzy Doc coil?

 

Mine has been in place for 18 months or so with no problems.

 

I understand that Martin keeps an eye on what they are doing and takes appropriate action.

 

Roger

Edited by RogerH

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I think I am right in saying that all suppliers are aware of the quality shortfall of many components these days (and so are most TR users). Its just a shame that nobody seems to be willing to put some of their profits into developing a decent standard coil that is reliable. My experience was highly dangerous- no chance of limping along when the coil fails totally- it just stopped. I shall be mounting a spare in a position a little cooler than the normal mounting and hoping that I get a few more miles out of the new ones. I am aware btw that I was unlucky, but that's not really the point. Other users be aware!

James

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A similar story here. I travelled to Lincoln in convoy with a friend also in a TR3A. He had an old coil fail a couple of weeks ago and had replaced it with an expensive new supposedly bullet proof new one. All ok up to Lincoln, about 200 miles but leaving the show ground Sunday it misfired all the way to our B&B and Monday morning it would not start. Replaced it with my old spare and we returned without any problems.

 

He will be taking it up with the supplier.

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I installed a Beru coil last winter when i fitted 123tune+

 

Was ‘reassuringly expensive’ and so far is working ......

 

Steve

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so what is the best to use ??????????????????????????

Hi Roy

I fitted a flamethrower about 4 years ago, not had a problem so far!

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