Coil voltage

[gjz30075]

Hi all, I know this has been discussed ad naseum, sorry. I recently lost ignition due to the
condenser failure. A new condenser got me back on the road but a friend of mine who is
much better versed in this sort of stuff said I need to look deeper.

S3 SE is the subject car. I recently changed it from positive ground with Pertronix to negative
ground with points. The condenser failed within 40 miles. My friend suggested I'm missing the
ballast that knocks the voltage down to 9v, or whatever, and this is causing a full 12v to reach the
distributor, hence the failure.

An S3 never had an external ballast so a coil with 3 ohms or so is used. I had also installed a
new Lucas coil that measures 2.9 ohms across + and - but I just measured the voltage, while ]
running, at the + side of the coil and it is 12v. I also measured my Sprint that has an external
ballast and it measures, while running, 9v.

So the question is, why do I still have 12 v at the coil on my S3? Could this be the reason the
condenser failed?

[mbell]

Long answer: The voltage isn't important its the current flow that is important. The ballast setup allows the use of a 1.5 ohm coil but keeps the correct current flow. It's not there to lower the voltage.

In the standard setup you have a 3ohm coil. For a ballast setup you have 1.5 ohm coil and 1.5 ohm resistor, so you get 3 ohms total. Meaning the current flow (4-5A) into the coil and via points is the same for both. The ballast does also cause a reduce voltage to be seen at the coil on average (really switch between 12v and 6v) but that not really important.

However the ballast system allows the the full battery voltage to be applied to the coil when starting. This increases the current flow through the coil when starting, even if the voltage has dropped due to the current pull of the starter.
For example 10V/1.5 Ohm would give ~6.6A during starting. Making a good powerful spark to help start the car.

If you just ran a 1.5 ohm coil with no ballast it will overheat on 4 cylinder, as the power going in due to much higher current (~9A) and there isn't enough power dissipated via the sparks. So the coil heats up until it gets too hot. But a 1.5 ohm coil on a V8 can be fine as there are more sparks generated and you need higher current to charge the coil in the reduced time period due to twice the frequency of sparks.

Short Answer: Your problem is not coil voltage or resistance, its likely poor quality condenser.

[gjz30075]

Thanks Mark, but confused still. In your first paragraph, you say the external ballast is not there to
lower the voltage, but your second paragraph you say it does cause a reduced voltage to be seen
at the coil.

I was under the impression the ballast was added for points longevity.

Yes, my Sprint bypasses the ballast for 12v starting but in the 'run/on' key position, it reverts to running
voltage through the ballast, as I see 9v at the coil while running.

So, ultimately, running 12v at the coil is ok?

[mbell]

Thanks Mark, but confused still. In your first paragraph, you say the external ballast is not there to
lower the voltage, but your second paragraph you say it does cause a reduced voltage to be seen
at the coil.

The purpose of the ballast is to reduce/limit the current flow. It also does reduce the voltage but that's more of a side effect than the primary purpose. Unfortunately I've long forgotten the electromagnetic theory I learnt at uni to be able to explain why it's the current that matters not voltage.

I was under the impression the ballast was added for points longevity.

The current through the points is determined by combined resistance of coil and ballast. They both configs total 3 ohms so there no change in current. It's likely reducing the current that would improve points longevity, so don't believe it has any effect.

So, ultimately, running 12v at the coil is ok?

Yup. What matters is to have the correct total resistance, to get the correct current flow through the coil (and points). The voltage doesn't matter, you can't buy a 9v coil but you can buy 3 or 1.5 ohm coils.

For elan you need a total of 3 ohms. I believe both your cars have that and the condenser failure wasn't related to lack of ballast resistor.

[gjz30075]

Thanks Mark, appreciate the explanation.

[Baggy2]

Hi
I have another explanation of the operation of the ballast resistance that may help. This is not specific to Lotus . just ballast resistors in ignition systems in general.
During normal operation the supply voltage in the car is a nominal 12 volts. However when starting the car the starter motor draws a very high current and the supply voltage around the car (including ignition circuit) drops to around 9 volts so at the very time you want a fat spark to start the car the ignition system is operating under non-ideal conditions. To get round this a coil designed to operate with a ballast resistor operates at the lower voltage present when the car is starting . However once the car has started the general supply around the car will be back to 12 volts - too high for the lower voltage coil. The solution is the ballast resistor. This drops the nominal 12 volts present during normal running to the lower voltage present whilst starting. When starting the car the Ballast resistor is shorted out (by a relay or similar) and the full (reduced) battery voltage is applied to the coil so the coil is happy operating at 9 volts or so - the car starts - the ballast resistor is re-inserted into the circuit and again the coil is happy running at 9 volts or so.
I hope I havent confused things - its a simple process but putting it into clear sentences is more of a challenge.

To answer your first question "why am I seeing 12 volts at the coil" - if the points happen to be open you will see 12v. There is no current flowing through the ballast resistor so no volts drop .

Cheers
Baggy

[h20hamelan]

I had changed over to Napa echlin condensers a few years ago, any the seem to fail resistor or not.
The echlin ones look identical to those of your last post
https://simonbbc.com/lucas-25d-points-c ... rotor-arm/
Do tell if there is some quality condensers and points available in n america please-anyone

[gjz30075]

Thanks Baggy, I am seeing 12v at the coil while running. The car is an S3 so no external ballast. The
explanation makes sense for my Sprint, which has an external ballast, and shows 9v at the coil while
running.

[Craven]

Standard setup for a S3 Elan is a 12volt coil and points. 12volt coil is just a generic term for a non- ballast ignition coil that also happens to have a DC resistance of around 3 ohms. A complicated function for the condenser, its working voltage is around 350 volts, so as mbell suggestion likely Just a faulty component.

[mbell]

the ballast resistor is re-inserted into the circuit and again the coil is happy running at 9 volts or so.

I don't believe that to be true.In a ballast system you will likely measure ~9V at the coil but that not the voltage the coil is running at.

For a ballast system you should have 1.5 ohm ballast resistor and 1.5 ohm coil. This means when the points are closed and current is following you have ~6V across both the ballast resistor and coil. When the points are open there is no current flow, this means there is no voltage drop across the ballast resistor and you will measure 12V at the coil as you stated.

Hence when the engine is running the voltage across the coil is changing between 6V and 12v. Which will average at ~9V, which is the reading your getting. Bu the coil is actually running at 6V.

When selecting a coil you're doing it based on getting the correct current flow through the coil not the voltage* it will see. So in a ballast system you use a 1.5 ohm coil so that the total is ~3ohms as that give the correct current flow for a 4 cylinder on 12v. If you had 6V electrics you'd need a 1.5 ohm coil with no ballast to get the correct current flow. If you paired 3 ohm coil with 1.5 ohm ballast you will get lower current flow but higher voltage across the coil. So if the system was voltage driven that would likely perform better, but believe it won't and your likely to start to have running issue due to weak spark especially at high RPM.

* how many coils are stamped with a voltage rating?

[Baggy2]

Hello mbell
my post was intended to indicate the general operation of an ignition system incorporating a ballast resistor - I said that at the start. Your right the dc voltage across a 1.5 ohm coil in series with a 1.5 ohm resistor will be around 6 volts (of course) - all my comments were meant to be approximate but the principle was right and I got the impression the original poster didn't understand the principle.
I've spent a lifetime in electronics having got my degree in electrical engineering about the same time my +2 was built - 1968!

[Craven]

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

[rgh0]

The impedance effect of the capacitor is small during the closed points state and can be ignored in terms of current flow for most purposes. Its main function is to suppress the arc across the points when they open and to create the resonance high frequency AC flow between the condenser and the coil primary windings that generates the high voltage in the secondary windings and thus the spark after the points open.

A nice write up of the full electical engineering / physics of the kettering points ignition system

https://ttypes.org/conventional-ignition-systems/

just my 2 cents worth

cheers
Rohan

[Craven]

Quote Rohan
‘The impedance effect of the capacitor is small during the closed points’. Think about that the capacitor is short circuit in the points closed state.

[rgh0]

Yes.... but the current required to fully charge the capacitor it is very small compared to the current flowing through the points.

Hence it can be ignored for most purposes

cheers
Rohan