Re: [Gumstix-users] Super Cap

[<tim...@co...>]

Hi Dave,

I, like you, are a professional programmer, but I moonlight as a hardware eng.

> A low ESR cap looks like even more of a short than a high-ESR cap, and
> therefore I would suspect that it's even more of a candidate for

As I understand it the real problem is the cap blowing up.  A high ESR
would cause heat build up in the cap.  Well what I think anyway.
The current limiting solution prosented by cap-xx is a very engineered
solution.  My proposal of using a NTC resistor is not a eligent, but does
the job.  NTC (negitive temperature coefficent) resistors work by increasing
resistence at higher temperature caused by the in-rush current.  After the
in-rush current subsides the temperature lowers and the resistence goes
down.  Actually just adding a 1 ohm standard resistor is enough.  I did this
on a power supply that could not handle the in-rush current and it fixed
the problem (cmos corruption).  So it's a trade off, the resistor would cost
battery life as it induces power loss.   The MOSFET solution would cost bucks.

Anyone else have an opinion on the matter?

Tim

 -------------- Original message ----------------------
From: "Dave Hylands" <dhy...@gm...>
> Hi Tim,
> 
> On 3/23/07, tim...@co... <tim...@co...> wrote:
> > This is the reason why you need a low esr
> > ESR is the internal resistence of the cap
> > It is my opinion that no protection is needed with a low ESR cap
> > With that said a good solution would be a NTC resistor of about 1 ohm would 
> work
> > Place it in series with the battery
> > Thoughts?
> 
> My feeling is that the opposite would be true (bear in mind that my
> training is not in analog electronics - so I have a fairly shaky
> foundation here).
> 
> A low ESR cap looks like even more of a short than a high-ESR cap, and
> therefore I would suspect that it's even more of a candidate for
> protection.
> 
> Lower-ESR = more instantaneous inrush current.
> 
> This PDF document describes several circuits which can be used to
> limit the initial inrush current.
> http://www.cap-xx.com/resources/app_notes/an1002.pdf
> 
> -- 
> Dave Hylands
> Vancouver, BC, Canada
> http://www.DaveHylands.com/
> 
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