Numpty Question!

[Jon Green]

Hi Guys,

 

Can anyone tell me what a condenser on the points actually does in a car ignition system?

 

[Longboarder]

The condenser/capacitor serves a dual function. When the points are opening the majority of the current in the primary coil circuit flows into the condenser until it is fully charged. When fully charged it discharges back in a reverse flow through the primary coil circuit giving a more rapid collapse of the coils induced magnetic field. This gives a greater and more rapidly occuring secondary voltage for distribution to the plugs. i.e. more volts more times per second than without a condenser. It also serves to reduce the spark that tries to jump across the opening points gap thus reducing electrical pitting of the contact breaker faces but not eliminating it completely.

 

Nigel

[Jon Green]

Cheers Nigel

[peter_m7uk]

Just a small correction: When the condenser is fully charged, the coil's magnetic

field has already completely collapsed. The reverse flow just reduces the voltage across the coil back to zero, ready for the next time the points close.

 

It's a bit of a trade-off as well - The smaller the condenser, the faster the collapse of the magnetic field, but the more contact burning. With a large condenser, the contacts will be fine, but the magnetic field collapses more slowly, giving a weaker

spark at the plugs.

 

I can create some graphs of the voltages and currents in the ignition system

if you like?

 

Pete

[Longboarder]

I don't agree Pete. The condenser is there to assist by giving reverse current flow through the coil primary when the points open to give a more rapid drop of the coil primary current. More rapid drop in current gives a more rapid collapse of the magnetic field in the coil and thus induces a greater rise in induced voltage in the coil secondary and a bigger spark at the plugs. If the magnetic field were already collapsed by the time the condenser produced this reverse current flow it would be without purpose as the coil would have already fired. There is no benefit in 'reducing the coil voltage back to zero' in preparation for the points closing, only benefit in reducing the coil voltage from 12v to zero as rapidly as possible on the points opening. Thus condenser discharge has to take place as rapidly as possible after the points open otherwise it serves no purpose other than to reduce points pitting which is very much a secondary purpose.

 

Nigel

[Guest Andy Rathbone]

i just thought it was to stop your stereo going zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz in time with your revs, and to stop you spark erroding your points, just shows every days a school day

[peter_m7uk]

I don't really want an argument on this friendliest of boards, but...

I can't resist a good discussion! I attached a picture of a 'typical ignition system',

except that I'd have a resistor in series with L1 and C.

 

What I reckon is happening on the primary side is that when the switch is closed, L1 conducts current, establishing a magnetic field with +12V at the top of inductance L1. When the switch opens, the energy in the magnetic field creates a positive spike in voltage at the bottom of L1, which would arc across the switch but instead charges the capacitor/condenser, C. That is the points protection part. The sudden release of energy from L1 into C collapses the magnetic field, inducing a huge positive voltage spike at the top of L2, which causes the plug to spark.

 

In fact, come to think of it, there is no reverse flow, in the sense that current flows

from C back through L1. Is that what you meant by reverse flow, Nigel, or perhaps we're at cross-purposes?

 

When the switch closes again, current flows out of the condenser and through the switch, grounding the bottom of L1 and re-establishing the magnetic field. There must be a resistance in series with the switch, though, or else you would short-circuit the

capacitor!

 

Or maybe Andy is the one talking sense here?

[Guest Andy Rathbone]

maybe Andy is the one talking sense here?

 

theres a first time for everything.

 

the way i understood this system was...

 

Step 1 the battery charges the coil, (points closed) creating a magnetic field around it.

Step 2 the battery supply is dissconnected ( points open)

Step 3 the mag field around the coil colapses back into the coil creating a large voltage, known as a back EMF pulse.(EMF = Electro Motive Force) this is sent to the spark plug by the distibuter.then the rotar moves to the next plug.

Step 4 goto step 1 and repeat, lots and lots of times very quickly.

 

therefore thats why i think the condensor/capacitor just stops the points arcing (the buzzing on your stereo) and it may also help to debounce the points a little, but i dont think a car ignition system is delicate enough to be worrying about that.

 

If ford fitted it, it probably needs it.

 

Should i go and hide now

or just put my HND in electronics and telecomms notes away next to my city and Guilds radio amature exam notes.

[peter_m7uk]

Yep, that's basically what I said, but a bit more technologistically, like!

 

Now I'll put away my notes from my degree in electrical and electronic engineering and shut up

[Guest Andy Rathbone]

I knew there was some educated people on here.

 

If you hold on to the end of one of the plug leads and get your mate to turn the key it dont half make your eyes light up. i did it to a mate of mine, what he said afterwards would definatly be censored.

he was a little less educated then but definatly knows more now.

 

yep notes away, i'm going to the garage to grease my nipples.

[Longboarder]

Of course we can have a discussion of this without rancour. (Both of us are men. If one was a woman that would be different and I'd shut up ) I have to confess a lack of knowledge of electronics and electronic components. Maybe my explanation suffers because of this. I certainly don't know how electronic components work individualy. I did mean the capacitor produced a flow of current in a reverse direction back through the primary windings on separation of the points to accelerate the speed of collapse of the magnetic field. That is how I understand it. I include a pic below to show the effect on the primary coil circuit. My understanding of why this happens may be wrong. In practical terms you can remove the capacitor from a dizzy and drive the car. Performance will be very poor for the few miles you get before the points are shot.

 

Nigel

[peter_m7uk]

Hi Nigel. My analysis could be wrong, as I've only thought about this electrically in terms of the primary windings , in which case there definitely wouldn't be reverse flow. I guess the secondary winding could somehow induce a reverse flow in the primary, but I'll have to think a bit more about that one. Where did you get your diagram from?

 

Cheers,

 

Pete

[Guest Andy Rathbone]

if you stick the key in and twist it, and it makes that brum brum noise does it matter what it does??? as long as all the electrical bits keep hold of their magic smoke everything will be fine

[Longboarder]

I did a City & Guilds college course on classic car restoration when I retired a few years back. Amongst the topics covered was ignitions such as magnetos and points dizzys and this diagram was in part of the printed lecture notes along with the explanation. So its at least third hand and its origin is probably some text book from the fifties or sixties. The old guy who taught us could perform magic with the infernal combustion engine having worked in proper garages for a lifetime but has sadly retired long ago. He didnt have a clue about modern engine diagnostics, electronics, fuel injection and all the other electrickery on modern cars but give him a proper car and it would purr. Enabled me to sort the magneto on a friends bullnose morris which had been misbehaving.

 

Nigel

[Guest Andy Rathbone]

sounds like you gained some very valuable knowledge