Can some one tell me the capacitance value, the working voltage and the type of dielectric which was used in the capacitor and fitted which were in the distributer, across the points in 1950/60 cars. Some folk might know these caps as condensers, one and the same. John

Can't tell you the value but I have been told by old timers that all points condensers are the same value and can be interchanged, provided they match 6 or 12 volt and positive or negative earth . Never tried it myself though.

Gosh it is so long since I looked at one of those. Not sure they listed the value but probably something like 0.1mfd. But would need to be high voltage of course. It is only for spark suppression so value not critical and any modern one would do.

According to my Haynes Automobile Electrical Manual "A capacitor used in an ignition circuit has a capacitance of about 0.2uF." (That's microfarads)

It doesn't provide answers for dielectric or working voltage, but at a guess (but I'm not an expert) any voltage greater than about 20v and probably paper. Probably not critical.

Rob

My 1952 copy of "Motor Repair and Overhauling volume IV" says "Quite satisfactory performance may be obtained with condensers which vary in capacity between as much as 0.15 to 0.40 mfd". It goes on to say "...cars of high speed operation are usually fitted with condensers which have a capacity for lower limits ; for cars with normal slow speed performance , the condensers which give the best results are those with capacities near the higher loads"

Which seems to imply the capacity depended on the vehicle. It goes on to extensively describe the method for selecting the right size by running the vehicle for a few thousand miles, and then seeing where the pitting occurs. Pitting in the contact point of a negative earth vehicle shows the condenser is under capacity, whereas pitting on the arm suggests over capacity

I would assume they would be paper capacitors and rated for thousands of volts but the book doesn't say.

Yes, about 0.1uF, ±150%. There's probably an optimum value that suits a particular induction coil best, but they're all similar. They increase the power of the spark, stop the contacts arcing, and reduce radio interference. Not needed if an electronic ignition is fitted.

The originals were paper capacitors, rather unreliable, with ±20% tolerance. The electrical duty isn't demanding but they work inside a hot dirty vibrating engine bay. I think any modern non-polarised 0.1uF capacitor of more than 100V working would do. Car condensers are special in that they come in a metal can with a lug and lead sized to fit securely inside the distributor; you don't want an ordinary capacitor coming loose when the car is in motion!

Dave

Happy to be corrected, but I'm not seeing the need for a very high voltage rating as the condensor is on the low voltage side of the coil? Appreciate that the inductance might cause things to spike higher than 12v though.

Rob

Yes some are rated at a few hundred volts so there must be some spiking due to inductance or some such electrickery. Definitely not for the 25,000 volts or whatever on the high tension side of the coil.

I think if you get a condenser from one 1950s car it should work OK on another. Plenty of that old Lucas stuff is still available, and a lot of it is still being made. Ditto the American points ignition stuff. But trying to find a match from an electronics supplier relying on specs could be tricky and may not be up to the heat and vibration as SOD says above.

The capacitor needs to be at least 100V rated. Higher is better. A plastic film one will work well. Personally I'd go for a 150nF 250 or 275 Volt AC rated capacitor intened for mains suppression e.g.

https://uk.rs-online.com/web/p/film-capacitors/1908545

Epoy it to a plate and pot the lead with epoxy.

Robert G8RPI.

Didn't the cap sit on the moveable contact plate and the lug on the dizzy body? Otherwise you have a lot more dismantling before you can get at the bob weights

If you contact these guys, they can probably sell you the exact condenser your car needs LINK

One size seems to fit many different cars LINK

Edited By Hopper on 16/02/2023 13:47:14

strangely I was reading this last week link

http://www.da7c.co.uk/technical_torque_articles/capacitors.htm

H

Lots of different types available here.
Never dealt with the company, just see lots of adverts for them.
Rob

Links to **LINK**

https://www.gsparkplug.com/ignition/condensers 

Edit - didn’t work as expected, doing this on a phone for the first time

Edited By Grindstone Cowboy on 16/02/2023 13:48:58

Pedant alert, if the value was - 150% it would be negative, never seen a negative capacitor

The capacitor/condenser in an ignition circuit is there to collapse the magnetic field in the coil's primary winding as quickly as possible. They balance the reactance of the coil with a 180° out of phase reactance of a capacitor to get a vector sum as close as zero as necessary for a good spark to be generated.

Martin C

I have! An electrolytic went bang, filled the room with acrid smoke, and ruined a brand-new carpet. When house-proud young wifey arrived to start a frank and open discussion about the catastrophe I left them to it...

Dave

What vehicle? The Lucas 25D4 range all used the same cap, and still readily available.. DO NOT forget to oil the distributor shaft, the felt pad in the middle, 2 drops ! Noel.

300V Would be my guess. These are to suppress sperks - and you don’g get sparks like these at low voltage, as it is the induced back-emf from the coil windings.

These are the ones I use in my Mallory twin points dizzy on the Rover 3.5 V8
https://www.holley.com/products/ignition/distributors/distributor_accessories/distributor_components/parts/400

28MFD 600V

Bill

Must exist, after all negative resistance exists..