EE question..., capacitor voltage |
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EE question..., capacitor voltage |
Mueller |
Jun 16 2005, 10:37 AM
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#1
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914 Freak! Group: Members Posts: 17,146 Joined: 4-January 03 From: Antioch, CA Member No.: 87 Region Association: None |
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??? |
redshift |
Jun 16 2005, 10:55 AM
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#2
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Bless the Hell out of you! Group: Members Posts: 10,926 Joined: 29-June 03 Member No.: 869 |
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.. (IMG:http://www.914world.com/bbs2/html/emoticons/unsure.gif) M |
lapuwali |
Jun 16 2005, 11:00 AM
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#3
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Not another one! Group: Benefactors Posts: 4,526 Joined: 1-March 04 From: San Mateo, CA Member No.: 1,743 |
The voltage is just an upper limit to what it will take w/o blowing up. Where do they suggest adding this cap?
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Mueller |
Jun 16 2005, 11:07 AM
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#4
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914 Freak! Group: Members Posts: 17,146 Joined: 4-January 03 From: Antioch, CA Member No.: 87 Region Association: None |
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??? |
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redshift |
Jun 16 2005, 11:13 AM
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#5
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Bless the Hell out of you! Group: Members Posts: 10,926 Joined: 29-June 03 Member No.: 869 |
Looks like they are using it per Lucas instructions... magic smoke pressure regulator.
(IMG:http://www.914world.com/bbs2/html/emoticons/laugh.gif) ... or a check valve.. Don't use electrolytics. (IMG:http://www.914world.com/bbs2/html/emoticons/smile.gif) M |
TonyAKAVW |
Jun 16 2005, 11:57 AM
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#6
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That's my ride. Group: Members Posts: 2,151 Joined: 17-January 03 From: Redondo Beach, CA Member No.: 166 Region Association: None |
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 |
redshift |
Jun 16 2005, 12:33 PM
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#7
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Bless the Hell out of you! Group: Members Posts: 10,926 Joined: 29-June 03 Member No.: 869 |
Radio Shack still has parts there? Here, they have cell phones.
(IMG:http://www.914world.com/bbs2/html/emoticons/smile.gif) M |
scotty914 |
Jun 16 2005, 01:30 PM
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#8
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suby torque rules Group: Members Posts: 1,525 Joined: 20-July 03 From: maryland, the land of 25 year Member No.: 924 |
well you could go check out you local car audio shop, just get the smallest cap they sell for amps
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Flat VW |
Jun 16 2005, 01:43 PM
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#9
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Illegal Soapbox Derby Racecar Group: Members Posts: 1,692 Joined: 13-November 04 From: Phoenix, AZ Member No.: 3,105 |
(IMG:http://www.914world.com/bbs2/html/emoticons/agree.gif) Sometimes I will pick a "larger size" componet partially based on its "wire guage" legs to be compatible with other connections or componets. John |
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mskala |
Jun 16 2005, 06:24 PM
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#10
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R Group: Members Posts: 1,925 Joined: 2-January 03 From: Massachusetts Member No.: 79 Region Association: None |
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. |
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