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Fat Capacitor Question

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mjurotich(at)hst.nasa.gov
Guest

Posted: Sun Jan 14, 2007 6:35 pm Post subject: Fat Capacitor Question

I am thinking of putting a Fat capacitor between the E bus and ground.  Idea is



if the E bus switch is open

and the battery is suppling power through the diode

and the voltage sags for a short time (300 milliseconds is what I 

remember) when first engaging the starter



the capacitor would keep the voltage above 10 for the voltage 

sensitive stuff that may reset.

2 questions:

Would this work?

It seems to introducing a single point failure for the shorted 

capacitor case. Are todays electrolytic caps sufficiently robust to 

mitigate the risk?



Thanks

Matthew M. Jurotich



e-mail   mail to: <mjurotich(at)hst.nasa.gov>

phone   : 301-286-5919

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nuckollsr(at)cox.net
Guest

Posted: Mon Jan 15, 2007 6:59 pm Post subject: Fat Capacitor Question

At 09:33 PM 1/14/2007 -0500, you wrote:



 	  Quote: 	
		  

<mjurotich(at)hst.nasa.gov>



I am thinking of putting a Fat capacitor between the E bus and 

ground.  Idea is



if the E bus switch is open

and the battery is suppling power through the diode

and the voltage sags for a short time (300 milliseconds is what I 

remember) when first engaging the starter



the capacitor would keep the voltage above 10 for the voltage sensitive 

stuff that may reset.

2 questions:

Would this work?


	


    Sure. In fact it's been done. The quick-n-dirty design

    tool takes notice of the fact that a 1F capacitor discharged

    at 1A will fall at 1V per second. So, assuming the system you're

    protecting draws 3A. Assume further you want to keep the voltage

    from falling below 10v with a start from 12v. Assume further that

    you want to support the device for 1/2 second.



    A/F = V/S



    3/F = 2/0.5



    3/F = 4



    F = 0.75 Farads



    That's a FAT capacitor. You can buy 1F caps popular with the

    folks who like to install ear-crusher audio systems in their

    vehicles. See:



http://tinyurl.com/tvrtx



    The capacitor can get small by reducing supported

    current, reducing support-time or allowing a greater voltage

    drop during inrush transient.



    If your system will benefit from some form of transient isolation

    system, I think you'll find that a small battery is a much better

    energy storage device. The smallest of SVLA batteries can be isolated

    from the rest of the system during a few seconds of cranking time.

    The installed weight and volume will be smaller but there will be

    a preventative maintenance issue for the AUX battery. In this case,

    AUX battery capacity is not a big issue ASSUMING that its only

    task is to mitigate cranking inrush brownout. This means that you

    could run it  until it fails to support the protected system.



    I've used some FAT caps for similar tasks but they have some

    special integration issues. You can't just hook these critters

    to a battery to charge them . . . charging currents are very

    high and tends to burn switches. You need to develop a special

    current limited charging circuit to avoid the stress.



    Super caps are going to REPLACE batteries in our airplanes

    in the foreseeable future. But using a FAT cap to replace

    a small sealed lead-acid battery in this application is

    problematic.



    Bob . . .

      ---------------------------------------------------------

      < What is so wonderful about scientific truth...is that >

      < the authority which determines whether there can be   >

      < debate or not does not reside in some fraternity of   >

      < scientists; nor is it divine. The authority rests     >

      < with experiment.                                      >

      <                            --Lawrence M. Krauss       >

      ---------------------------------------------------------

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