Condensers - Vintage Cars

I'm not sure what you are using to measure the capacitance. Most capacitance bridges allow you to measure an equivalent ideal capacitor either as an ideal capacitor with a parallel leakage resistance or as a capacitor in series with an ESR. E.g. I have an AIM 401 Databridge which is pretty ancient and this has a SER-PAR button to switch between the two measuring modes. Leakyness shouldn't stop you getting a reasonably accurate measurement.

I would imagine the 0.2uF was to some tolerance around +/- 0.03uF. So 0.1u is probably a bit on the small side. But I am not an expert on ignition systems!

Alan

If it's truly NOS, then the capacitace could vary as much as -50% +100%, production tolerances for caps really only tightened in the 60's. I'd probably try to measure using an AC source, perhaps a 12V transformer, looking at the AC current through the cap and a series resistor and the voltage across each, and do some Hard Sums to work out the impdance of the cap and hence its value? Or a proper capacitance bridge (which will use AC for its measurements) as Alan suggests.

Dave H.

If I remember rightly, and it was along time ago, the condenser or capacitor as it is now called is there to reduce arcing across the points, extending their life by reducing pitting on the surface of the points. I think it also allows the magnetic field in the primary coil to collapse at a faster rate thereby increasing the voltage in the secondary hence a stronger spark. I think the capacitance is critical and it generally used to be 0.2uF. Don

Sorry, I didn't answer your specific question, I 've taken to rambling on a bit (old age :-( ) . I wouldn't think that a leakage of a few microamps would make any difference in an old car ignition circuit you will probably get more than that in leakage due to damp. I believe the value of the capacitor is critical in that it can affect the timing. Cheers Don

The former I believe to be correct.

The latter ISTR was for the opposite effect - to increase the time for which the spark existed to improve the chance of causing a bang in the pot. Without the capacitor, the spark can be infinitesimally short and ineffective

YMMV EOE

When I used to run old motorcycles, I fitted any capacitor that was to hand - IMV it is not very critical. I don't think the capacitance will affect timing significantly, an old singer probably revs to what 3,000 RPM max, if it a 4 cylinder, then 200 sparks per second, 50 mS per cycle, whereas the RC circuit will collapse must faster than that (anybody do the maths on that??). And it will have either spring and bobweight auto advance, which will flap around all over the place, or maybe a manual advance lever? If it were me, I would get a modern Lucas condenser and lash that up on the distributor, and see how it runs like that, then try the old one, and see if there is any difference. The main risk is if there is insufficient capacitance, the points will spark and erode very quickly. And upset TV reception .

Good luck.

If it is a waxed paper condenser of some age the fact it is hasn't been used is probably immaterial - it is unlikely to work effectively. Paper is highly hygroscopic and absorbs moisture over time despite the sealing can and wax impregnation. Absorbed moisture degrades performance by increasing dielectric losses and decreasing insulation resistance. Waxed paper condensers generally have a useful life of about 20-50 years.

The more or less standard capacitance value in Kettering (coil/contact breaker/condenser) systems from the 1930's onwards was 0.22uF +/- 20% and 400-600VAC working . Buzz coil ignition tended to be higher values - 0.47uF/500VAC

The capacitor is there to slow the rate of voltage rise as the contacts open to reduce the energy lost in arcing as the contacts open. For an instantaneous current break the peak voltage reached is I root L/C so the peak voltage is reduced if the capacitor is too big..

For typical ignition coils the best compromise is about 0.2uF but it's a very flat optimum 0.15 to 0.25 would be OK

Ignition coil primary inductance is typically 10mH. so the source impedance of the inductive overswing is very low - root L/C = approx 200 ohms. This means that capacitor leakage is unimportant - even 1Mohm leakage would make no significant difference.

Jim