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  #1  
Old 08-27-2010, 08:02 AM
Classictrial Classictrial is offline
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Default Capacitor degradation?

I am involved with old off-road motorcycles which are fitted with capacitor discharge ignition systems, and these seem to run an awful lot better after the CDI unit has been changed.

Is the capacitor itself likely to deteriorate over time, and if so how long before this is going to make any noticeable difference to running? Have little or no knowledge relating to these things, so any responses are very welcome.

Thanks

Chris
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  #2  
Old 08-27-2010, 12:42 PM
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SgtWookie SgtWookie is offline
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Hello Chris,

Aluminum electrolytic capacitors can and do fail over time; and are the most likely suspects in any given circuit that's more than 5 years old; particularly if the circuit is unused for extended periods of time.

The capacitors are basically made of two sheets of very thin aluminum foil that's rolled up, placed in a can-like package, and filled with an electrolyte fluid that is alkaline in pH.

Then the capacitors' dielectric is formed by very slowly charging the capacitor up to its' rated voltage via a current-limited source, until the "leakage current" through the capacitor falls to an acceptable level.

Automotive environments are particularly brutal; extreme temperatures, shock/vibration, corrosion, electrical noise all take their toll.

Even if an electrolytic capacitor is simply unused for an extended period of time, the dielectric will degrade; the longer it is not used, the worse the dielectric becomes. The capacitance is reduced, and the leakage rate increases. If the leakage rate becomes excessive, there will be enough power dissipation in the package to cause the electrolyte to boil, rupturing the package forcefully.

There are other failure modes; for example the package may have not been hermetically sealed or become corroded and the electrolyte simply dried/leaked out, or the internal connections to the "plates" may have corroded or separated.

Replacing electrolytic capacitors with fresh ones is a very common practice, and will help improve the reliability and performance of the circuit.

Buy fresh electrolytic capacitors from high-volume authorized distributors, like Mouser.com, Digikey.com, AvnetExpress.com, Newark.com, etc. Don't buy electrolytic caps from surplus vendors or auction sites unless you have the knowledge and equipment to test and re-form them, or you will likely experience failures within a short period of time. "Stale" electrolytic capacitors are not a bargain for a novice.
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Old 08-27-2010, 12:52 PM
jpanhalt jpanhalt is online now
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Do we know that the CDI capacitor is aluminum electrolytic? I have seen polyester/mylar capacitors used that are less than 1 uF and 450 to 600V.

It would be nice to know what the markings are on the capacitor being removed.

John
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Old 08-27-2010, 01:08 PM
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marshallf3 marshallf3 is offline
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Do we know that the CDI capacitor is aluminum electrolytic? I have seen polyester/mylar capacitors used that are less than 1 uF and 450 to 600V.

It would be nice to know what the markings are on the capacitor being removed.

John
Most CDI units use a non-polarized cap for storing the HV that is eventually used to pulse the ignition coil. They were rarely of any quality when new so replacing them with modern equivalents is probably a good idea. They also tended to use ones that could barely handle the required voltage to begin with.

I hate CDIs as they tend to pot the circuits wheree it's near impossible to get to the components and even potting compound has been known to break down over time. If I could easily get into mine I'd replace my output caps without hesitation.

Most large supply houses will have modern replacements, and if you can get them in a high enough voltage the caps they use for speaker crossovers are substantially better but rarely worth the extra $$. http://www.partsexpress.com
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Old 08-27-2010, 01:14 PM
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"Stale" electrolytic capacitors are not a bargain for a novice.
Quite personally I don't think they're a bargain for anyone. I'm a "grab bag" addict, if I'm ordering from a place that offers grab bag assortments I'll usually buy one for the heck of it as I've never failed to get a decent amount of useable components from them just to add to my stock along with a lot of oddball stuff that's fun to figure out and/or play with. The electrolytics that come in these usually aren't worth keeping unless they're high value filter caps of a useable voltage, in which case I'll test them for value and leakage before keeping them. I still wouldn't use them in a mission-critical circuit but for normal power supply filtering they're usually fine.
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Old 08-27-2010, 02:02 PM
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John,
I don't know that they are aluminum electrolytic caps. I merely suspect that they are, as they have a distinct size and cost advantage over other types.

Marshallf3,
We're on the same page. It isn't difficult to check capacitance and leakage vs voltage rating, but it's a pretty good bet that the majority of hobbyists wouldn't have the equipment available to perform the tests, and re-forming the dielectric if necessary.

As you're likely aware, it's not difficult to re-form the dielectric of polarized aluminum electrolytic caps. I don't recall offhand how to re-form polarized electrolyics.

If anyone's interested, I'm attaching a .PDF document from a WWII-era polarized aluminum electrolytic capacitor re-forming instrument. It also has abbreviated tables of acceptable leakage rates vs voltage rating vs capacitance value. It's 4.5MB, so beware if you're on a slow connection.
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File Type: pdf ReformerNo1.pdf (4.48 MB, 33 views)
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Old 08-27-2010, 05:16 PM
Classictrial Classictrial is offline
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Thanks very much for the responses..........the CDI units are potted and very difficult to take apart, but I would imagine the component parts are very low quality, and as some units also include the HT coil then perhaps heat is also an issue?

I think perhaps that performance might be improved using a CDI with higher quality parts, and a separate ignition coil, and this is something I will have to try.
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Old 08-27-2010, 09:38 PM
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Potting compound is used to help provide physical support for the components on the board, which helps with the shock/vibration issues, and also protects the circuit against corrosion and fluid contamination.

You might try using very hot (boiling) water to soften the potting compound.

At a minimum, a circuit board being used in an automotive environment needs to be conformal coated. Conformal coating is a type of clear acrylic lacquer that protects the board/components against contamination and corrosion.

It's tough to remove.

Parts need to be secured to the board so they won't fail due to vibration/physical shock issues.

Modern automotive coils provide a lot more output than the old versions from the early 70's and before. You need to drive the primary side pretty hard though.
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Old 08-27-2010, 09:51 PM
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My experience is limited to hobby versions of CDI. I have never seen an aluminum electrolytic used for the charging (i.e., HV) capacitor, even on cheap ($25) Chinese imports.

John
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Old 08-28-2010, 12:14 AM
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Due to the duration of the pulse and energy required it only made economical sense to use plain old 200V or higher regular caps.

Good luck on the potting compound, some is fairly easy to remove, most is a real pain. Denatured alcohol tends to help melt whatever they used in ours.

Most don't generate their own HV either, it comes directly from the magneto coil through a diode to the cap which is discharged across the coil by an SCR. The SCR is triggered by yet another small pickup coil in the magneto or mounted to the cam if it's a 4-stroke engine.
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