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914World.com _ 914World Garage _ EE question...

Posted by: Mueller Jun 16 2005, 10:37 AM

Upon talking to the techs at http://www.link-electro.co.nz/home.html, they recommend a .5 to 3.0 microfarad capacitor on the +/- of the MSD CDI box.

For 16vdc caps, in the range I need, all I can find are little caps that are no bigger than a dime, would I be better off lets say getting a 100vdc cap that is the larger "can" style to have more reserve????

Now if I can just use one of those tiny board mount types, film or tantalum???

Posted by: redshift Jun 16 2005, 10:55 AM

Get mil spec stuff if you can find it, and I don't see any reason to use tantalum, or germanium.. or..

Unless you want it to sound like Jimi Hendrix..

M

Posted by: lapuwali Jun 16 2005, 11:00 AM

The voltage is just an upper limit to what it will take w/o blowing up. Where do they suggest adding this cap?

Posted by: Mueller Jun 16 2005, 11:07 AM

QUOTE (lapuwali @ Jun 16 2005, 10:00 AM)
The voltage is just an upper limit to what it will take w/o blowing up. Where do they suggest adding this cap?

in // with the positive and negative from the battery to the MSD box......they are calling it a "suppressor", mabey for noise only and not a voltage dip compensator???

Posted by: redshift Jun 16 2005, 11:13 AM

Looks like they are using it per Lucas instructions... magic smoke pressure regulator.

... or a check valve..

Don't use electrolytics.

M

Posted by: TonyAKAVW Jun 16 2005, 11:57 AM

There's no problem with using a physically small capacitor on the input of the MSD box. A 0.5-3.0 pF capacitor stores very little energy so doesn't have to be very big. Any old little ceramic cheapo capacitor will do the job. Radio Shack has plenty of these.

Putting a single capacitor on the input however is not the best way to go. What you really want is at least two capacitors. A small ceramic capacitor is good for filtering out high frequency energy from getting onto the power supply. I reccomend a tantalum capacitor of perhaps 10 uF would do an excellent job of getting rid of the lower frequency components. Put them in parallel and place them across the input. A 16v rating is fine, but if you are really concerned go ahead and get caps with a 25v rating. Make sure that the capacitors are as close to the housing of the MSD as possible. If you have two feet of wire between the capacitors and the chassis of the MSD you'll be picking up a lot of noise.

If you are still having problems with noise, try a ferrite core and run the cable through it a few times.

Electrolytic capacitors are not all bad. Tantalums in general are electrolytic, and they have extremely low series resistance, making them excellent for filtering applications. The regular old can style electrolytics are not particularly good for filtering in an application like this however.

-Tony

Posted by: redshift Jun 16 2005, 12:33 PM

Radio Shack still has parts there? Here, they have cell phones.

M

Posted by: scott thacher Jun 16 2005, 01:30 PM

well you could go check out you local car audio shop, just get the smallest cap they sell for amps

Posted by: Flat VW Jun 16 2005, 01:43 PM

QUOTE (TonyAKAVW @ Jun 16 2005, 09:57 AM)
If you are still having problems with noise, try a ferrite core and run the cable through it a few times.

Sometimes I will pick a "larger size" componet partially based on its "wire guage" legs to be compatible with other connections or componets.

John

Posted by: mskala Jun 16 2005, 06:24 PM

In addition to what everybody else said, you really should have
a higher voltage cap than 16V. At work, we frown on using
anything rated for less than 3 times the expected voltage.

Now this doesn't mean 12x3=36V rating, either, because you
should really consider 'load dumps' that can happen in any
automotive system using an alternator/battery. These can be
much much higher than 12V.

I would use a 100V rated part.
Luckily, as you've found, the relatively capacitance you need
doesn't make it Foster's can size or anything.