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Nic Jarman
Joined: 05 Oct 2008 Posts: 1031 Location: Stoke by Clare, Suffolk
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Posted: Sun Nov 21, 2010 3:05 pm Post subject: What a condenser is for |
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As I have been having a few problems with the Morris ignition system I thought I would have a look at what is really going on in this simple circuit. As the coil is open circuit on the secondary side it is acting as an inductor and what exactly is that condenser (capacitor) doing. It is helpful to consider the spark as an arc that has an amount of time.
2 laws that we must bear in mind are 1; You cannot instantly change the voltage across a condenser, 2; An inductor does not like change in the current flowing through it.
Let’s turn the key and see what happens (points closed).
The current flow saturates the core of the coil (inductor) and continues to flow to earth ( 0v) , so node A is at 12v and node B is at 0v. The points now open, I will quote “The art of electronics” This is without the condenser fitted. “When the switch is opened, the inductor “tries” to keep current flowing from A to B, as it had been. That means that terminal B goes positive relative to terminal A. In a case like this it may go 1000 volts positive…….
But it is not an inductor and we have a condenser across the switch (points).
The coil tries to push up the difference in voltage between the terminal A & B (potential) but the condensor will only let it raise at a certain rate and will limit the voltage rise. As there are far more turns on the secondary winding of the coil there is far more potential across it’s output (sparkplug) and up it goes until the voltage is enough to bridge the gap (spark). This is “seen” as a load on the primary coil and it can “nick” some energy from the condenser. I guess this is what people refer to as the “field collapse”.
So the condenser determines the rate at which the potential rises and helps to give a big fat and long (in electrical time) spark.
OK so who is still awake? If anyone has a different concept or would like further info, please post.
PS electronic ignition also has a condenser across the output transistor otherwise it would get fried. _________________ 1936 Morris 8 Series 1
1973 MGB roadster
1977 MG Midget 1500
Dax Rush
Last edited by Nic Jarman on Mon Nov 22, 2010 11:22 am; edited 2 times in total |
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Phil - Nottingham
Joined: 01 Jan 2008 Posts: 1252 Location: Nottingham
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Posted: Sun Nov 21, 2010 4:38 pm Post subject: |
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Or more simply the condensor, or properly the capacitor, soaks up the voltage that would try to jump the points (as its easier) as they separate and discharges it via the primary winding giving an extra kick to the secondary (HT) so it will make the already high voltage spark jump the plug tip gaps which needs higher voltage as its under compression rather trna open air.
It in effect smooths the process to relase the energy where its most needed and stops the points arcing and burning .
Camera flashes store up (deadly) voltage in a capacitor to make the lamp flash quickly all at once _________________ Rover P2
Rover P4
Rover P5 & P5B
Land Rover S2 & S3
Morris Mini Traveller Mk2 |
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Nic Jarman
Joined: 05 Oct 2008 Posts: 1031 Location: Stoke by Clare, Suffolk
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Posted: Sun Nov 21, 2010 9:05 pm Post subject: |
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Phil, that is what I thought but it just does not work that way. See law 1. The cap holds the voltage down while the points open to stop them arcing. If the cap is soaking up charge it would increase the voltage at node B too quickly, while the voltage is rising the cap is trying to dissipate it to earth. More current is going through the coil than the cap can handle. You also may have noticed that when the points are open the current flow is the opposite way to the voltage differential, ie Ohms Law does not work here.
The cap will hold the voltage to around 300 volts not around 1000 and with approx 1:100 turns ratio that gives 30,000 volts across the electrodes. _________________ 1936 Morris 8 Series 1
1973 MGB roadster
1977 MG Midget 1500
Dax Rush |
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Peter_L
Joined: 10 Apr 2008 Posts: 2680 Location: New Brunswick. Canada.
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Posted: Mon Nov 22, 2010 1:22 am Post subject: |
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Nic has introduced what is perhaps one of the less complex and yet so often misunderstood parts of the motor car.
I found this subject, with diagram and text which together with Nic's postings, may prove useful to all present and future forum members.
http://www.britishcarforum.com/tikiwiki/tiki-index.php?page=Ignition+Systems
A detail of this system is the fact that the opening and closing of the points, is creating a psuedo AC circuit in the primary windings, which allows the secondary circuit to provide the very high voltage needed to jump the gap at the spark plug.
Last edited by Peter_L on Mon Nov 22, 2010 3:50 pm; edited 1 time in total |
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Nic Jarman
Joined: 05 Oct 2008 Posts: 1031 Location: Stoke by Clare, Suffolk
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Posted: Mon Nov 22, 2010 12:26 pm Post subject: |
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OK lets throw this into the mix. When electricity flows through an inductor the current will lag behind the voltage and through a cap the voltage lags the curent ( CIVIL ) so balancing the values of the coil and cap is quite important to get the voltage and current back into phase and thus transfering maximum power. I believe it was Mr Kettering that designed the points/coil system - clever chap! _________________ 1936 Morris 8 Series 1
1973 MGB roadster
1977 MG Midget 1500
Dax Rush |
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whiteshadow
Joined: 24 Jan 2011 Posts: 78
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Posted: Tue Apr 05, 2011 6:07 pm Post subject: Bouncy |
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I've seen a few forums where this is debated...
It can be over analysed....
The Voltage from the coil is dependent on how quickly/cleanly the current is cut via the points (dV/dt). The points are like any switch contact, and actually 'bounce' - having a capacitor in parallel cleans up the bounce making a cleaner turn off of current (therefore higher voltage) and reduces the amount of arcing so increases the contact life.
The reason it will arc so much at the points is the back emf from the primary side of the coil. If you charge up a coil then break the current, you can get a massive Back EMF voltage....the capacitor (condenser) shorts this transient voltage safely to ground and not across the points.
Try a back EMF yourself .....but not with a weak heart....they hurt. It's the reason solenoids (and other coils) often have a 'flywheel' diode to short it out) |
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victor 101
Joined: 03 Apr 2009 Posts: 446 Location: East Yorkshire
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Posted: Tue Apr 05, 2011 6:45 pm Post subject: |
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Hmmm, thats strange, I thought the condenser was for converting that little spark over there to that big fat spark over there. |
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Peter_L
Joined: 10 Apr 2008 Posts: 2680 Location: New Brunswick. Canada.
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Posted: Tue Apr 05, 2011 6:49 pm Post subject: Re: Bouncy |
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whiteshadow wrote: | I've seen a few forums where this is debated...
It can be over analysed....
The back EMF from the coil is dependant on how quickly/cleanly the current is cut via the points (dV/dt). The points are like any switch contact, and actually 'bounce' - having a capacitor in parallel cleans up the bounce making a cleaner turn off of current (therefore higher voltage) and reduces the amount of arcing so increases the contact life. |
........ but because it is a DC voltage applied to the Primary, there is no voltage at the secondary, as long as the primary sine is flat. The points don't physically bounce, but the arcing caused without a capacitor would flatten the primary sine to a point where there is no induced secondary voltage.
Connecting a diode across a solenoid does nothing but stop ugly arcing at the switch and therefore protects those contacts from useless arcing which will reduce their life. |
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whiteshadow
Joined: 24 Jan 2011 Posts: 78
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