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	Tue, 15 Apr 2014 12:25:55 -0700 (PDT)
In-Reply-To:	<20140415195425.37808583@richard-laptop>
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Date:	Tue, 15 Apr 2014 15:25:55 -0400
Message-ID:	<CAOFvGD7y3a85_8O3HVuMhDe=kQfY+S1ADUBbtS+tYU3q47TSqA@mail.gmail.com>
Subject:	Re: [geda-user] printed meter shunts
From:	Jason White <whitewaterssoftwareinfo AT gmail DOT com>
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A quick Google for "PCB trace resistance calculator" shows many nifty
calculators like

http://circuitcalculator.com/wordpress/2006/01/24/trace-resistance-calculator.

But that does not take into account temperature rise form the high
currents, so a better calculator exists in this case the author called
it a "PCB Trace Width Calculator." It follows the IPC-2221 technical
standard for calculating current capacities of PCB traces. For your
task this will should be sufficent as it gives both the required trace
width and trace resistance for a specific current. If it is not
accurate enough I would recommend taking the data directly IPC-2221 to
figure it out mathematically.

Make sure to leave room for a >10% manufacturing tolerance in copper
thickness (and thus resistance). I have a feeling that a *lot* of
trimming will be required. Make sure the trace is wide enough so that
is doesn't heat up significantly as resistance changes with
temperature

 http://circuitcalculator.com/wordpress/2006/01/31/pcb-trace-width-calculator/

Good Luck!
Jason White

On Tue, Apr 15, 2014 at 2:54 PM, Richard Bown <richard AT g8jvm DOT com> wrote:
> Hi,
> I thinking of playing with the LTC1645 using its for over current shut
> down,
> it requires only 50mV across a shunt resistor to trip, at 10A thats
> 5milli ohm.
> And those resistors in TO-220 packages are expensive, so I was thinking
> why use a length of wide track, needs to wide as handling up to 10A .
> Are there any tables to give track resistance if the thickness , width
> and length are known.
> In practice its better to go for a lossier track and tap the voltage
> across it with a potential divider, so target around 10 - 20 milli ohms,
> cant use anything bigger as the board will get hot, 10A through 0.02 R
> is 2 W, so better around 0.01 R
>
> Thanks
>
> --
> --
> Best wishes /73
> Richard Bown
> Email : richard AT g8jvm DOT com
> HTTP  :  http://www.g8jvm.com
> nil carborundum a illegitemis
> ##################################################################################
> Ham Call G8JVM . OS Linux Mint 16 x86_64 on a Dell Inspiron N5030 laptop
> Maidenhead QRA: IO82SP38, LAT. 52 39.720' N LONG. 2 28.171 W
> QRV VHF 6mtrs 200W, 4 mtrs 150W, 2mtrs 400W, 70cms 200W
> Microwave 23 cms 140W, 13 cms 100W, 6 cms 0W & 3cms 5W
> ##################################################################################
>



-- 
Jason White

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