EE question..., capacitor voltage Options

[Mueller]

Upon talking to the techs at Link, 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???

[TonyAKAVW]

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