The Stovebolt Site Forums The Shop Area The Electrical Bay Capacitor or condenser polarity?

A question was ask several days ago about a poor running engine and the capacitor. It got me to thinking. I believe the distributor capacitor is an electrolytic type. Most electrolytic capacitor I worked with for many years were polarized. I know there are non polarized electrolytics by using two anodes. Does anyone know if the dizzy capacitor is or is not polarized? I have a 1938 6 volt positive ground car and there are caps everywhere which I would not trust much less the distributor capacitor. Thanks

the value of the distributor cap is about 0.1 - 0.5 uF typically

the distributor and other noise suppression capacitors are most likely PP type
PP = Polypropylene metalized film
these are good for AC pulses and high reliability in harsh environments
the internal resistance is very low
for the size of the capacitor you will achieve the 0.01 - 1.0 uF values with several hundred or thousands of volts rating with relative ease and low cost.

at the distributor the AC ring down when the points open will make that capacitor swing below ground, you won't be able to do that with the traditional electrolytic
not only that, the electrolytic will have much too high internal resistance to be useful and they don't like hot, that is the fastest way to kill an electrolytic
repeated hot environment

electrolytic are for high energy storage where series resistance is not critical
think large 10-100000 uF values, and always with a obvious polarity mark
they do not do well in high temperature applications unless they are very expensive specialized electrolytic
they also cannot be negative biased unless specially designed to do so.
AC electrolytics are meant for power applications like 60Hz motor starting, etc, not usually in frequencies in the KHz or above.

when your computer power supply dies, it's usually one of the big electrolytics that shorted out
because it couldn't even handle being in your warm cozy home
-s

Is there a way to test a distributor capacitor? If so, what kind of equipment would you use?

yes there is a variety of meters that can test a capacitor.
It would be a whole essay to get into all the parameters to check for and which meters to use, there is no one right answer because the various applications for capacitors and all the specifications are a very wide field.

if i was handed an old distributor capacitor from the junk yard i would do the following (if throwing it out was not an option that is).

1) using a generic ohm meter, set it to resistance on highest setting, check the ohms, a good cap will not leak much.
if i see numbers like 10Kohms or less, i would be worried, but not give up yet, maybe it's a really old cap and the paper based dielectrics are known for small but manageable leaks

2) i would check the metered capacitance at 1KHz and 10KHz on my multimeter, I have a DER EE DE-5004 that can check the capacitance and equivalent resistance at both those frequencies as part of the built in functions, it's not much but it really goes a long way in knowing if a capacitors is still possibly good or just junk.

3) do a voltage test, bring the cap up slowly to it's working voltage limit and see if the current settles to a constant small leakage, there are many online resources for estimating acceptable leakage, and if that fails, just try to anticipate if the absolute leakage is ok for your application

I don't know if that helps or is more confusing
-s

Thanks “Two many trucks”. I’m still searching as polypropylene types are relatively new vs our old trucks. I’m going to tear apart one of the old rf suppression capacitors to check its construction. Guess it’s time to hit the old school books as we forget so much over the years.

yes you are correct about the time line and original equipment would not have been PP type
original top candidate types are (wax)paper-foil or metalized paper types

upon looking into who still makes metalized-paper types i see there are common for audio and some high voltage applications (but in a oil bath types), both Kemet and Cornell Dubilier have a line of metalized-paper, but not in the appropriate styles for automotive use.
but that doesn't mean they are not still made for legacy support and simplicity and just not easily source-able directly to us, we would need to dig deeper into who is actually making the automotive ones today.
I believe the top two candidates for the modern capacitors may be metalized-paper still or new PP type.


some history that probably has mistakes:
PP capacitors were only just invented in the early 50s (in the lab) and likely didn't start showing up on the market until the late 50s or into the 60s.
before that, the distributor capacitor was likely either wax paper-foil or metalized-paper type.
the other technologies for capacitors at that time were not suitable
mica and ceramics - not easy to be able to get the large values you would want for the distributor capacitor, over kill for that application and would be too expensive, good for 0.01uF or smaller values
PET types were first introduced in the 1940 but would not have replaced a robust well made paper-foil or metalized paper type capacitor yet, it was too expensive and not well known for reliability at elevated temperature.
Aluminum electrolytic - not suitable too much internal resistance for pulse applications, too delicate and failure rate too high for operation under the hood of a vehicle. They were first developed much earlier than PET and PP types at about the turn the the 1900 in the laboratory and slowly came to market in specialized applications all the way into the 1930s, but they were notorious for being low quality and failing easily. It wasn't until military demands in WWII for better quality aluminum electrolytics drove new innovation to make them generally usable.
So starting just after WWII aluminum electrolytics became a normal component in electronics, then newer different types of electrolytics were gradually developed now that the market for them was established.

I'll see if i can track down who is making the exact suppression caps we use today
-s

Most Sun distributor testers had a condenser checker as optional equipment. The one dad used all through the 1950's and 60's had one and he used it a lot. I even got educated about grabbing a charged condenser when he handed me one and asked "What's wrong with this?" That only happened once- - - -I'm a quick study on stuff like that!
Jerry

Thanks once again “Too many trucks”. The reason for my original question was to find out if the capacitors we use in the older vehicles with points, are polarity sensitive as I have a positive ground 6 volt car. If go to a FLAPS for a points cap, and I’m thinking most of today’s caps are made for negative ground systems, will they work with a positive ground. We might be digging deeper into this subject then we need to for this forum. Good intel all the same.

Ignition condensers are not polarity sensitive. If you watch an oscilloscope pattern you'll see a condenser actually switch its polarity several times while it's discharging between the "points open" and "points close" portions of the pattern. That diminishing-amplitude sine wave is the condenser dissipating the voltage spike it initially absorbed to protect the points from arcing and transferring metal as they just barely came open. They will also usually work on 6 or 12 volt ignition systems without any issues.
Jerry