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Should 22K uF capacitor be fused?

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rparigor(at)SUFFOLK.LIB.N
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Posted: Fri Apr 13, 2007 11:28 pm Post subject: Should 22K uF capacitor be fused?

Hello Bob



I was reading a post you made from 01-2000 about putting a 10 amp fuse on

the 22k uF capacitor of a Rotax internal generator.



Do you still advocate fusing the capacitor? I plan on using a 22K uF 35V

with a 105C rating.



Thx.

Ron Parigoris



Below is some of 2000 post you made:



The general rull of thumb for 60 Hz power supplies is 1,000

   uFd per amp of power supply capacity. Applying the same rule

   to the 18A Rotax alternator puts the 22,000 uFd device right

   in the ballpark.

The capacitor does it's job by looking as

   if it were a dead short to the AC voltages on the system.

   ANY device connected in series with the capacitor raises

   it's equivalent series resistance value and degrades it's

   ability to do the job. A 10A fuse is a good idea . . . capacitors

   rarely short but it's not impossible. Better to open a fuse

   that blow the end out of the capacitor . . . they stink

   and are really messy.



 Capacitors rated for operation at 85 degrees C are standard.

   Further, they don't have to be big either. A 33,000 uFd,

   16 volt device can be had with an envelope of 1.4" diameter

   and 2.13" long. You want a computer grade device with 10-32

   screw terminals. These have the lowest internal resistance

   and are most suited for attaching the fat wires used to wire

   the alternator system.

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nuckollsr(at)cox.net
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Posted: Sat Apr 14, 2007 8:58 am Post subject: Should 22K uF capacitor be fused?

At 07:15 AM 4/14/2007 +0000, you wrote:



 	  Quote: 	
		  



Hello Bob



I was reading a post you made from 01-2000 about putting a 10 amp fuse on

the 22k uF capacitor of a Rotax internal generator.



Do you still advocate fusing the capacitor? I plan on using a 22K uF 35V

with a 105C rating.'


	




   It doesn't hurt but it's also unlikely to

   be a big help either. The fault currents that

   can flow in a self destructing capacitor can

   stay below fuse blowing levels.



   Better to treat this capacitor as an expendable

   like batteries and tires. Replace it every so often,

   probably 3-4 years is fine. Even then, you're sandbagging

   a really rare event.



   Bob . . .

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