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'[EE] where to put the fuse'
2006\06\16@085749 by alan smith

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Battery operated device, with wires leading from the battery pack (2 AA) to the main board.  The wires were pinched and eventually shorted causing the insultion to melt from the pinch point to the board, and of course the batteries got VERY warm (couple of NiMh) causing the plastic to distort as well.
 
 So, client said...wow...thats bad (uh huh) can we add a fuse or something?  Sure...what I figured is best is to put the fuse right at the battery clip connections thus if the wires were pinched again, it would protect the entire run of wire.  But I was thinking, does it really matter where the fuse is placed?  Next turn of the board he wants the fuse on the board (makes sense) rather than soldering it inline with the power wire.  It would make sense if the wire was shorted again, the lowest path of resistance would first be the wires themselves, and then the path to ground on the board after it melts all the insulation and seperates themselves and finally the batteries (don't they usually have a high internal resistance?).  Just looking for comments.  He really doesnt want this to happen again causing a fire or the like.
 
 From the Duracell website, it states the AA has a capacity of 2.85A*hr, so a set of fresh batteries could have the potential of quite abit of current if shorted directly.  So by rule of thumb, the fuse should have a rating of 125% of typical flow, or 3.56A.  PicoFuse of 4A is the closest thing.
 
 
 

               
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2006\06\16@091109 by David VanHorn

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On 6/16/06, alan smith <spam_OUTmicro_eng2TakeThisOuTspamyahoo.com> wrote:
>
> Battery operated device, with wires leading from the battery pack (2 AA)
> to the main board.  The wires were pinched and eventually shorted causing
> the insultion to melt from the pinch point to the board, and of course the
> batteries got VERY warm (couple of NiMh) causing the plastic to distort as
> well.


The best place to put the fuse, is inbetween the two cells in the pack.
Still won't protect you from internal shorts and catastrophic runaway, but
there really isn't anything you can do about that.

2006\06\16@101458 by alan smith

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hmmm....ok.  Since they have to solder a wire in there, might as well solder a pico fuse instead.
 
 Thanks

David VanHorn <.....dvanhornKILLspamspam@spam@microbrix.com> wrote:
 On 6/16/06, alan smith wrote:
>
> Battery operated device, with wires leading from the battery pack (2 AA)
> to the main board. The wires were pinched and eventually shorted causing
> the insultion to melt from the pinch point to the board, and of course the
> batteries got VERY warm (couple of NiMh) causing the plastic to distort as
> well.


The best place to put the fuse, is inbetween the two cells in the pack.
Still won't protect you from internal shorts and catastrophic runaway, but
there really isn't anything you can do about that.

2006\06\16@102832 by David VanHorn

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On 6/16/06, alan smith <micro_eng2spamKILLspamyahoo.com> wrote:
>
> hmmm....ok.  Since they have to solder a wire in there, might as well
> solder a pico fuse instead.


If at all possible, I use a thermal and a current type fuse inbetween the
cells of a pack, exactly for that reason. Also, silver-teflon wire on the
battery leads.  It's not that expensive, and much more resistant to heat.

2006\06\16@103814 by Howard Winter

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Alan,
On Fri, 16 Jun 2006 05:57:48 -0700 (PDT), alan smith wrote:

> Battery operated device, with wires leading from the battery pack (2 AA) to the main board.  The wires were
pinched and eventually shorted causing the insultion to melt from the pinch point to the board, and of course
the batteries got VERY warm (couple of NiMh) causing the plastic to distort as well.
>    
>   So, client said...wow...thats bad (uh huh) can we add a fuse or something?  Sure...what I figured is best
is to put the fuse right at the battery clip connections thus if the wires were pinched again, it would
protect the entire run of wire.  But I was thinking, does it really matter where the fuse is placed?  Next
turn of the board he wants the fuse on the board (makes sense) rather than soldering it inline with the power
wire.  It would make sense if the wire was shorted again, the lowest path of resistance would first be the
wires themselves, and then the path to ground on the board after it melts all the insulation and seperates
themselves and finally the batteries (don't they usually have a high internal resistance?).  Just looking for
comments.  He really doesnt want this to happen again causing a fire or the like.

It matters absolutely where the fuse is placed!  A rule of thumb relating to fuses: They only protect against
faults that are downstream of themselves (further from the power supply).  So putting a fuse on the PCB will
do nothing whatsoever to protect against the very fault that you had!  As David said, the best place is in
between two of the cells, perhaps by cutting the link in the battery case and soldering wires to it, and
putting the fuse on the end of those.  This is as "upstream" as you can get - actually "within" the power
supply (electrically speaking)!  :-)  But it may be very ugly and/or impractical depending on the structure
and layout of your device.
   
>   From the Duracell website, it states the AA has a capacity of 2.85A*hr, so a set of fresh batteries could
have the potential of quite abit of current if shorted directly.  So by rule of thumb, the fuse should have a
rating of 125% of typical flow, or 3.56A.  PicoFuse of 4A is the closest thing.

Unfortunately the Amphour rating of a battery gives you no idea what its maximum current may be - the battery
chemistry and structure has much more to do with it.  A 30Ah car battery can easily give 400A when shorted
(but don't expect it to do so for 30/400 hours!).

Hang on - are you saying that you're drawing 3.56A from AA cells?  Or are you taking the Ah rating and
multiplying by 1.25, to give the current it can supply?  If the latter, then even if you're right (and I don't
know where that 1.25x comes from) then your fuse will never blow as the battery can't supply the current to do
so.  What you need to do is find the max current your device draws, and calculate the fuse from that.  That
way it will pass the current that it's supposed to, but a fault will blow it.  You may want to consider the
self-resetting "polyfuse" devices that will remake the circuit after the fault has gone - I think Wouter sells
some of them.

Cheers,


Howard Winter
St.Albans, England


2006\06\16@105605 by VULCAN20

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Howard Winter wrote:

{Quote hidden}

As I understand the short occurd in wires that ran from the batteries to
the board.  If thats the case
I think fuse placement on the battery clip is the best if  the same
short appears.  The fuse on the board would have done nothing to stop
the short that occurd.

{Quote hidden}

2006\06\16@113013 by Mike Hord

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>   From the Duracell website, it states the AA has a capacity
> of 2.85A*hr, so a set of fresh batteries could have the
> potential of quite abit of current if shorted directly.  So by rule
> of thumb, the fuse should have a rating of 125% of typical flow,
> or 3.56A.  PicoFuse of 4A is the closest thing.

Not quite.  That's the capacity of the battery, given in time at
a constant discharge rate before the output voltage droops
below a certain level.  "Typical flow" is something YOU must
determine, based on your application.  How much current does
the application normally pull from the battery pack?

Also, NiCad and alkaline batteries frequently have different
capacities (I think; I know NiMH AA cells are usually less than
2.85 Ah), so although that has no bearing on the solution to this
problem, be aware of it for the future.

Mike H.

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