Ignition points current

[NMNeil]

I'm checking out a circuit for a CDI ignition from a 1970 copy of Wireless World. In the article it says to have 250mA through the points to keep them 'clean'.
Anyone help on what keeping the points 'clean' means as I only intended to run about 10mA through the points, I can't work out why the need to make it any higher.

Many thanks for helping with my confusion.

[HighVoltage]

You have always some oil / water/ dirt mist inside the distributor and this likes to lay down on the breaker point switching surfaces. If the current is low, then the resistance at the breaker points can increase so high that switching is problematic. A rule of thumb for the OEM ignition systems of those days was to have a 56 Ohm power resistor over 12 V, so about 200 mA to "burn" the oil and dirt away with little sparks when the points closed.
 

[NMNeil]

I forgot about the inevitable layer of oil, thanks for that.

[Benta]

250 mA in car terms is not very much. If you want to keep the points working correctly, heed the advice on pulling some current.
A next step could be replacing the points with an optical switch, where this limitation is irrelevant.

[SeanB]

Careful with the opto switches, they really do not like the oil which is an almost inevitable environment inside the cap of the distributor, along with condensed water in humid climes. There is a good reason Bosch uses a metal vane with slots and a bias magnet and a coil in the VW Golf/Rabbit/Sirocco/Passat/Seat vehicles with distributors, as that will work till metal fragments bridge the gap totally. The optical ones are finicky, and stop working when the distributor gets wet, or the lower oil seal wears slightly and it picks up oil mist.

[Gyro]

Yes, it takes a lot to beat the original points with decent wetting (or should that be drying) current. A decent ignition circuit should take out the contact bounce and they will last a very long time compared to directly driving the coil.

One tip I got from the instructions of a kit I build years ago... Do not fit new points. By all means clean up the contacts, but the heel on a used set of points will have bedded in to match the cam. A new set of points will initially wear very quickly, prematurely throwing the timing out. Just use the existing ones and put a smear of grease on the cam.

Wow, that takes me back! 


P.S. One other thing, if you keep the original points you can implement a changeover switch to revert  back to conventional iginition if the electronics fail on you!

[David Hess]

Small signal relays have the same problem and require a minimum current for reliable operation *unless* they are specifically designed for low or no current switching.  In the case of points, a minimum current needs to be switched for reliable long term operation.

[Brumby]

250 mA in car terms is not very much. If you want to keep the points working correctly, heed the advice on pulling some current.
A next step could be replacing the points with an optical switch, where this limitation is irrelevant.

Not optical.  Magnetic.  A hall effect device would be the way I would go.

[David Hess]

250 mA in car terms is not very much. If you want to keep the points working correctly, heed the advice on pulling some current.
A next step could be replacing the points with an optical switch, where this limitation is irrelevant.

Not optical.  Magnetic.  A hall effect device would be the way I would go.

Or a reluctance sensor which is what they used before hall devices became available.  They are even more rugged and reliable.

[N2IXK]

Yes, a reluctance sensor was what Chrysler used on the first electronic ignition systems introduced on production vehicles here in the US. They were VERY reliable systems, used in more or less the same form from the early 1970s up until the advent of computer controls in the late 80s.

Even without the problems of contact resistance due to contamination, mechanical breaker points suffer from drift in timing and dwell angle caused by mechanical wear of the distributor cam against the rub block on the movable arm of the points.

[Brumby]

Yes, there is always the reluctance sensor, which is also magnetic in nature.  I just like Hall effect goodies, because they are cool.

[CatalinaWOW]

Yes, a reluctance sensor was what Chrysler used on the first electronic ignition systems introduced on production vehicles here in the US. They were VERY reliable systems, used in more or less the same form from the early 1970s up until the advent of computer controls in the late 80s.

Even without the problems of contact resistance due to contamination, mechanical breaker points suffer from drift in timing and dwell angle caused by mechanical wear of the distributor cam against the rub block on the movable arm of the points.

The reluctance system is very reliable.  I have to take some exception with the use of that word with Chrysler reluctance based systems.  As a Jeep owner left with a dead engine at the far end of nowhere on more than one occasion.  The problem wasn't the reluctance sensor, but the connectors and harnesses used in implementing it.  And to be fair that Jeep travelled many miles before the problems developed.

[HighVoltage]

The reluctance sensor was by far the best, it was also implemented at Mercedes, BMW, and many other German cars.

At one time I was confronted with a big problem on a 1976 Cadillac that started perfectly but as soon as one stepped on the gas, the engine died. Since no dealer in California could fix this problem I bought the car very cheap.

It turned out to be the magnetic reluctance sensor that was installed wrong. The two output cables had been switched and this caused the polarity to be wrong and this caused the engine to retard the timing dramatically, as soon as the engine speed increased. Well, it did take me a full day or so, to find this problem.  After that, I drove this car for many month before I sold it for a good profit.