quick one about rev counters

Pos side is always 12v

Neg side switches, so this is your pulse.

sorted :)

Why isn't neg side always 12v? isn't it 'grounded' to the chasis that way?

sorry i'm still learning little by little.

Thanks.

But there's still enough 'dirt' on the positive side - reasonably close to the coil - to trigger a pulse counter.

The batteries negative post and the chassis are connected together. The positive 12v dc gets connected to the coil when you switch on the ignition... The negative coil connection "used to be" connected to the cars "points". When the points are shut the coil is charged up... When the circuit is broken (when the points opened) the electrical field in the ignition coil collapses and this results in 25,000v at the coils output.

The points are now replaced with your "engine management" which does the switching but it still works like this.

what happens if you turn off the engine, one of the points rests in the 'closed' position and then you turn the key to the 'on' position later on, wouldn't that burn out the coil because 12v constant current is supplied to the coil?

btw as I said before my car is a carb engine, no engine management hehehe.. still using contact points.

Possibly, if you left it for long enough. The DC resistance of the coil is, from memory, a matter of ohms so you could easy cause a few watts of heat dissapation in it, whether this is enough to burn it out is a question for its design & spec.

No due to the high resistance and no emf at that time the current is low. Just warms the coil slightly...

Well get rid of the points they truly are crap! Swap to Luminition, or get a set of bike coils and pickups??

What?!! That is TOTALLY wrong!! That fact that it is a DC current not switched (AC of sorts) means that it will have NO EMF opposing the current in the primary, thus its DC resistance will be very low, and its extreamly possible that the coil will burn out.

Here try this get a 240V transformer of any kind, then get a bridge rectifier and put it across the mains to make 240DC now put this into the 240V transformer, stand well back turn it on and watch what happens to the coils..

Ed

But we are talking about what 10 ohms? at typically nine volts via its ballast/start resistor. They just get warm... Try it. Your battey will go flat in the end but the coil is very unlikely to burn out...

OK I calculate it...

Nope, a typical points ignition system has a primary coil DC impedance od 1.3-1.5 ohms, that means about 8 amps of current would flow when turned on this would easily burn one out..

Ahh yes I do agree here they do but can get quite warm, I have have seen this happen to the extent that the coil did break down.

I have personal experience of the opposite, :) Land rover SIIA had funny ignition left it on accidentaly over night next morning flat battey and dead coil!!

See above typical DC resistance of 1.3-1.5 ohms in either case means a min of 8 amps = 96 watts :)

Measure the resistance of one if you dont believe me :)

Done, 3.45 ohms (my VR6 electronic ignition vans stock coil...) plus 2.5 ohms ballast resistance. A bike coil (aftermarket high output one double ended blue plastic thing) is marked 3 ohm, measures 3.8 ohms... no ballast used.

I have seen some coils used for racing, and some coils used with electronic ignition (and a ballast resistor) that are as low as .8 to 1.5 ohms. But the ballast used with these drops the total back down again??? So on 12v they would get fairly hot, but the ballast reduces this by approx 2 to 3 ohms, so maybe not...

I suspect points coils are higher than this though. But don't have one to measure.

If points ignitions really were switching 8 amps they would last about ten mins! So must find an old points coil to measure... But I dont have one. Seems that electronic ignition ones draw about double the current...

The contacts are similar in size to a standard 30 amp car relay.

From memory I think a coil does have a form of back EMF.

Initially there is a current inrush but as the magnetic field builds up in the core this reduces the current flow but not enough to stop a coil possibly burning out if left on for a long time.

The points survive because of the condenser quenches the spark cause by braking the circuit. This is why the points fail just after the condenser.

A normal coil is about 1 to 1.5 ohms a coil from and electronic system is about 0.8 ohms as the solid state switch can stand the extra load. This give a stronger more reliable spark needed to jump the wider plug gaps needed in lean burn engines for reliable firing.

Regards

Barry snipped-for-privacy@dircon.co.uk

Of course it does. That's how the high voltage is induced in the secondary winding.

As has been pointed out, this is a secondary function of the capacitor. Its first one is to form a tuned circuit with the inductance of the coil primary - increasing the efficiency.

Nope, that's the wrong way round - the initial rapid change in the flux density in the coil induces a back EMF, and so limits the initial current. As the rate of change of flux density drops, the back EMF drops and the current increases. If you apply a voltage across a coil and look at the current wrt time, it starts at zero and rises asymptotically towards an end value given by the resistance of the coil.