Turns ratio to voltage in ignition coil

[owin]

Hello, I'm looking at the specs of an ignition coil and its stating 12v, 70:1 turn ratio and 40000v. My understanding was that you times the voltage by the turns ratio to get the output (if driven efficiently), am I wrong about that? as this would only be 12x70=840v. I've looked up the typical turn ratio for ignition coils and this falls within it between 50 and 200 to 1. How do ignition coild produce such high voltages from such small turn ratios?


Specification:

    These Procomp Electronics coil delivered 300 milliamps and 40,000 V of spark.

    The coil feature efficient E-core windings, brass terminals, and a durable Rynite housing.

    Coil Wire Attachment: Male/HEI

    Coil Style: E-core.

    Primary Resistance: 0.355 ohms.

    Coil Internal Construction: Epoxy.

    Coil Color: Red.

    Maximum Voltage: 40,000 V.

    Turns Ratio: 70:1.

    Secondary Resistance: 4.40K ohms.

    Inductance: 6.9 mH.

    Peak Current: 300 mA.

    Spark Duration: 220 uS.

    Hardware Kit: included.

Material & Color:

    Material: made of high quality Plastic and Metal.

    Color: as picture

Package Included:

    1 X 12V E-Core Ignition Coil with Kits. (100% Brand New Never Used Or Installed)

[langwadt]

Hello, I'm looking at the specs of an ignition coil and its stating 12v, 70:1 turn ratio and 40000v. My understanding was that you times the voltage by the turns ratio to get the output (if driven efficiently), am I wrong about that? as this would only be 12x70=840v. I've looked up the typical turn ratio for ignition coils and this falls within it between 50 and 200 to 1. How do ignition coild produce such high voltages from such small turn ratios?

it is flyback, when you turn on the switch to the coil current starts flowing, when you turn _off_ the switch to the coil the primary voltage increases to try and maintain the current, until the spark, on the secondary side, strikes. This can be hundreds of volt on the primary   

[Buriedcode]

To expand on the above answer - its flyback action.  If you were to connect a load - a resistive load - to the secondary, when when you apply voltage to the primary, current will flow in the primary and the secondary.  But with ignition coils, the only thing ocnnected to the secondary is the spark plug, which is a spark gap - effectively open circuit.  This means the ignition coil is essentially an inductor since current can't flow in the secondary so when voltage is appied to the primary, current rises linearly over time and it builds up a field.

When the primary current is interrupted, this field collapses and induces a current in all windings on the ignition coil - that is both the primary and secondary.  The voltage on either will rise to whatever is required to allow current to flow.  Even without a secondary, a single coil inductor could theoretically reach thousands of volts on the primary.  So the voltage on the secondary will rise to whatever voltage is needed for current to flow - for a spark gap that depends on the distance between the points.  Say it arcs at 10kV.  With a 1:70 turns ratio, with the secondary reaching 10kV the primary voltage rises to 10kV/70 = 143V.

So the turns ratio is more about reducing the peak voltage on the primary (which is usually switched by a transistor that has a maximum stand-off voltage) for a given secondary voltage.

Flyback converters use this action to step up/down voltages over a wide input voltage range with a fixed turns ratio.

Edited to add: The key difference is, flyback transformers are really "coupled inductors" - current doesn't flow in both windings at the same time, instead the core stores energy when the primary is energised, and releases that energy via the secondary.  Standard "transformers" current flows in both windings at the same time - the core isn't mean to store any energy, just transfer it from pimrary to secondary via a magnetic field.

[DavidAlfa]

It's a flyback, not a conventional transformer.
This means the coil is charged up, then released  - aka suddenly opened, left floating.
In this condition the coil has no where to dump its energy, the collapsing magnetic field creates a huge back-EMF spike.
This spike can easily exceed 500-1000V at tthe primary side, so the secondary will push a lot more voltage.

I remember trying this with a small 220V/12V transfomer, driven backwards in flyback mode.
The sparks at the 220V side exceeded 5mm, so it was easily 5KV.
Of course it quickly caused internal arcing, killing it, as it was nowhere designed to isolate such high voltages.

[langwadt]

It's a flyback, not a conventional transformer.
This means the coil is charged up, then released  - aka suddenly opened, left floating.
In this condition the coil has no where to dump its energy, the collapsing magnetic field creates a huge back-EMF spike.
This spike can easily exceed 500-1000V at tthe primary side, so the secondary will push a lot more voltage.

I remember trying this with a small 220V/12V transfomer, driven backwards in flyback mode.
The sparks at the 220V side exceeded 5mm, so it was easily 5KV.
Of course it quickly caused internal arcing, killing it, as it was nowhere designed to isolate such high voltages.

usually the transistor driving an ignition coil will clamp the voltage at ~400V so if there is a misfire or or disconnected ignition wire the secondary won't reach a damaging level

[IanB]

To supplement all the discussion of flyback action, this can be thought about in a more fundamental way.

The basic equation for an ideal inductor is:   v = L di/dt

In other words, the voltage across the inductor is proportional to the inductance, and to the rate of change of current in the inductor.

Now, suppose you have a stable current flowing through the inductor and you suddenly switch off the current. In this case, the rate of change of current approaches infinity, since it has instantaneously gone from something to nothing. According to the above formula, the voltage across the coil will also approach infinite volts, since L is a positive constant.

In reality, the inductor/coil is not ideal, and so "infinite" can be replaced by "a very big number".

Secondly, once you have disconnected the current in the primary, the coil does not "know" which winding had current flowing before (it could have been the primary or the secondary--either way, there is a magnetic field established that has a store of energy).

Since the voltage produced across the coil will be proportional to the number of turns, the secondary side with more turns will experience a bigger "very high voltage" than the primary.

[DavidAlfa]

usually the transistor driving an ignition coil will clamp the voltage at ~400V so if there is a misfire or or disconnected ignition wire the secondary won't reach a damaging level

If you have a snubber, yeah. If you don't....
Ages ago I killed dozens of transistors while playing with TV flybacks, not knowing what a snubber was 

[langwadt]

usually the transistor driving an ignition coil will clamp the voltage at ~400V so if there is a misfire or or disconnected ignition wire the secondary won't reach a damaging level

If you have a snubber, yeah. If you don't....
Ages ago I killed dozens of transistors while playing with TV flybacks, not knowing what a snubber was 

FETs will usually just avalanche so it is a matter of whether it will handle the energy

[xavier60]

Something to be aware of. Some if not all  Coil-On-Plug type ignition coils have magnetically biased cores. They may not perform well if the wrong polarity is applied to the primary winding.
I discovered this by chance while testing some BA Falcon coils.

[langwadt]

Something to be aware of. Some if not all  Coil-On-Plug type ignition coils have magnetically biased cores. They may not perform well if the wrong polarity is applied to the primary winding.
I discovered this by chance while testing some BA Falcon coils.

some ignition coils have a diode in the secondary to reduce the risk of a spark at turn on

[soldar]

Some weeks ago, on a whim, I bought on eBay a "classic style" automobile coil just with the idea that it would be fun to play with making sparks, not for any specific purpose. I received it and forgot about it until I read this thread and thought I should at least test it.

I put about 3 A through the primary and interrupted it by just touching the contacts... and I was getting much bigger and powerful sparks in the primary contacts than than at the HV.

I guess I should try (1) putting a condenser (capacitor) like a car would have and (2) using a transistor to interrupt the current as it can do it faster than I can separate the contacts.

I will have to think of some practical application for it. Maybe an electric fence for livestock... but I don't have any livestock. Maybe a mosquito electrocutor.

[owin]

Ok, thankyou guys for the explanation. That makes sense but a couple more questions;

If the iron core in the ignition coil was replaced with ferrite it would result in a lower voltage correct? reasoning being, ferrite trades the weaker magnetic field for high frequency response?

What is the difference between say 10:1000 turns and 20:2000 turns? assuming same gauge wires?

I put about 3 A through the primary and interrupted it by just touching the contacts... and I was getting much bigger and powerful sparks in the primary contacts than than at the HV.

I don't really get much of anything either from just tapping 12v wires onto the primary contacts, at least not with the old type. The newer 3 and 4 pin coil over plugs respond to this much better as (i assume) they have an inbuilt transistor on the extra trigger pin, tapping power into that results in long arcs. I think the trigger is meant to be 5v though, I've used 12 and haven't broke anything yet.

[soldar]

What is the difference between say 10:1000 turns and 20:2000 turns? assuming same gauge wires?

More turns means more inductance and therefore more energy stored for zapping.

[G-son]

I will have to think of some practical application for it. Maybe an electric fence for livestock... but I don't have any livestock. Maybe a mosquito electrocutor.

High voltage insulation tester.   

[Brumby]

I will have to think of some practical application for it. Maybe an electric fence for livestock...

While a great many members here will already appreciate this, I present the following for those who may not be aware........

A general warning to anyone considering any application where there will be access to such a circuit by animals - both four-legged and two-legged.  There are safety considerations that need to be included.  Too much energy could prove dangerous.

Just be aware.

[Terry Bites]

70x600=?
This is what happens in your (old) car. Whean you break the points you create a damped sinusoid with some serious volts. Its those volts that get stepped up.
It stings alright!

BTW, don't wizz on the fence like I once accidentally did. The shock made me fall and get tangled up in it as a further insult.
My partner laughed uncontrollably as I writhed like a jumping jack.
I have sought therapy but I still cant wizz in the country without hearing her evil cackling in the wind.