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'[OT] Resistance'
2000\03\24@141825 by Andrew Kunz

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I need to make a shunt for up to 100A max, 20V max (usually much lower) as light
as possible, in about a 2"-3" length.

Anybody have a CRC handbook?  I looked online yesterday but only found Cu
tables, none for nichrome or similar.

I need the resistance to be .0005 to .005 ohms in a 2" length.

Thanks.

Andy

2000\03\24@152852 by Thomas C. Sefranek

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----- Original Message -----
From: "Andrew Kunz" <spam_OUTakunzTakeThisOuTspamTDIPOWER.COM>
To: <.....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU>
Sent: Friday, March 24, 2000 2:16 PM
Subject: [OT] Resistance


> I need to make a shunt for up to 100A max, 20V max (usually much lower) as
light
{Quote hidden}

They are usually done in brass.


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 |  __O    Thomas C. Sefranek  tcsspamKILLspamcmcorp.com
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ARRL Instructor, Technical Specialist, VE Contact.
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2000\03\24@154348 by Andrew Kunz

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For the 100A 480V ones we use at the office, yes.  But brass lets the electrons
pass too easily for what I'm doing, I think.

You have a table?

Thanks.

Andy











"Thomas C. Sefranek" <.....tcsKILLspamspam.....CMCORP.COM> on 03/24/2000 03:26:01 PM

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Subject: Re: [OT] Resistance








{Original Message removed}

2000\03\24@160332 by Thomas C. Sefranek

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Andrew Kunz wrote:

> For the 100A 480V ones we use at the office, yes.  But brass lets the electrons
> pass too easily for what I'm doing, I think.

A standard shunt does not care what voltage is in the circuit.
A 480 volt shunt is just as effective in a 12 volt circuit.
It is a resistor made out of brass that creates a voltage drop across it
in proportion to the current through it.
The bulk resistance is related to the dimensions of the brass.
Width, length, and thickness.

> You have a table?

No.

--
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 |  __O    Thomas C. Sefranek  @spam@tcsKILLspamspamcmcorp.com
 |_-\<,_   Amateur Radio Operator: WA1RHP
 (*)/ (*)  Bicycle mobile on 145.41, 448.625 MHz

ARRL Instructor, Technical Specialist, VE Contact.
hamradio.cmcorp.com/inventory/Inventory.html
http://www.harvardrepeater.org

2000\03\25@091805 by Thomas McGahee

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<x-flowed>Andy,
For current shunts in the 100 amp range you would generally not
use nichrome wire, but instead make the shunt from flat
copper bar stock. You want to maximize the exposed surface area
so you do not get too large a temperature rise. Some hobby
shops carry various metallic bar stock. Fabricate your shunt
so that it's initial resistance is slightly LOWER than you
actually want. You then increase the resistance by filing
notches along the edge until the reading is perfect.

Best results are obtained when the current lead-in wires and the
sensor pick-off wires are soldered directly to the bar stock
before calibration. This type of attachment is preferred to the
use of compression connections (via screw), since they will
not change ohmic value over time. A good hybrid approach is to
clamp the wires on using a compression fitting and then solder
over the fitting.

Fr. Tom McGahee

At 02:16 PM 03/24/2000 -0500, you wrote:
{Quote hidden}

</x-flowed>

2000\03\25@121206 by Donald L Burdette

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The latest Digi-Key catalog has 0.005 ohm 1% resistors for $1.49 each.
There are also surface mount ones available (Digi-Key) down to 0.002
ohms, for $2.45 each, or to 0.010 ohms for $1.31.

If space is a real concern, take one or several parallel resistors and
solder the leads directly to your conductors.

For my money, I'd take two metal blocks (copper or brass) and attach 10
resistors between them, using screws or a torch for soldering.  Use a
value 10 times higher than you want, bolt main leads and sense leads to
the blocks, and you're there!  BTW, brass is MUCH easier to machine than
copper.

If you want, send me some specs and I'll build you one or a few for
minimal cost or quote you for volume.

Unless you want to buy this thing COTS (Commercial Off The Shelf).  I
can't help you there except to say that such things are available.


Don

2000\03\25@162446 by Tom Handley

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  Andy, I've been looking into this for my PIC-based DAQ project. If you
want precision, check Caddock:

     http://www.caddock.com/

  They sent samples for my voltage dividers and I'm about to specify the
current shunts for up to 100A. I have relevant books in my library if you
really want to cut wire and make a 4-terminal shunt but that's not conducive
to mass production... You might also want to check Analog Devices' AD77xx
family of power measurement/energy meter IC's and related app notes. Another
handy little chip is the MAX472.

  - Tom

At 02:16 PM 3/24/00 -0500, Andrew Kunz wrote:
>I need to make a shunt for up to 100A max, 20V max (usually much lower) as
light
{Quote hidden}

------------------------------------------------------------------------
Tom Handley
New Age Communications
Since '75 before "New Age" and no one around here is waiting for UFOs ;-)

2000\03\27@072228 by Andrew Kunz

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Understood.  Our 100A shunts weigh half a pound.  I need something LIGHT!

Andy










"Thomas C. Sefranek" <KILLspamtcsKILLspamspamCMCORP.COM> on 03/24/2000 03:59:20 PM

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Subject: Re: [OT] Resistance








Andrew Kunz wrote:

> For the 100A 480V ones we use at the office, yes.  But brass lets the
electrons
> pass too easily for what I'm doing, I think.

A standard shunt does not care what voltage is in the circuit.
A 480 volt shunt is just as effective in a 12 volt circuit.
It is a resistor made out of brass that creates a voltage drop across it
in proportion to the current through it.
The bulk resistance is related to the dimensions of the brass.
Width, length, and thickness.

> You have a table?

No.

--
 *
 |  __O    Thomas C. Sefranek  TakeThisOuTtcsEraseMEspamspam_OUTcmcorp.com
 |_-\<,_   Amateur Radio Operator: WA1RHP
 (*)/ (*)  Bicycle mobile on 145.41, 448.625 MHz

ARRL Instructor, Technical Specialist, VE Contact.
hamradio.cmcorp.com/inventory/Inventory.html
http://www.harvardrepeater.org

2000\03\27@093321 by Michael Rigby-Jones

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part 0 5229 bytes
<P><FONT COLOR="#0000FF" SIZE=2 FACE="Arial">Trouble is that the low temp co. alloys suitable for making shunts are quite dense.&nbsp; The shunt needs to be physically massive in order to be able carry such high currents without a significant temperature rise.&nbsp; You could use a block of aluminium alloy which would certainly be much lighter, but the temp co. wouldn't be very good.</FONT></P>

<P><FONT COLOR="#0000FF" SIZE=2 FACE="Arial">Mike</FONT>
</P>
<UL>
<P><FONT SIZE=1 FACE="Arial">{Original Message removed}

2000\03\27@094220 by Andrew Kunz

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part 0 2798 bytes content-type:text/html;Since I will only be storing the reported ADC value and the temperature, it
stands to reason that the post-processing on the PC can handle the calculation
to determine actual amps knowing the temp, ADC of (false) amps, and some
calibration data from EEPROM.

We are looking at 1W max dissipation, more likely only about 1/4W due to
worst-case vs. typical operational parameters.

Andy











Michael Rigby-Jones <RemoveMEmrjonesspamTakeThisOuTNORTELNETWORKS.COM> on 03/27/2000 07:52:37 AM

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Subject: Re: [OT] Resistance








Trouble is that the low temp co. alloys suitable for making shunts are quite
dense.  The shunt needs to be physically massive in order to be able carry
such high currents without a significant temperature rise.  You could use a
block of aluminium alloy which would certainly be much lighter, but the temp
co. wouldn't be very good.

Mike

> {Original Message removed}

2000\03\27@102922 by Spehro Pefhany

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At 01:52 PM 3/27/00 +0100, you wrote:
>        You could use a block of aluminium alloy which would certainly be
>much lighter, but the temp co. wouldn't be very good.  Mike   -----Original

The main problem is that his power dissipation will be 50W to 5W depending
on the resistance, so that requires something physically fairly large, as
you say, to keep it from getting too hot. At the lower end of the scale,
5W isn't too bad if not much accuracy is required, and/or if a heatsink
surface is available.

Maganin has a relative resistivity of 26, tempco is about 20ppm/K IIRC
Brass has a relative resistivity of 3.9, tempco is about terrible,
about 2000ppm/K
Constantan has a relative resistivity of 28.45, tempco is about 200ppm/K

The above resistivities are relative to copper at 1.72 E-6 ohm-cm

Assume you have a 6 mm diameter wire of maganin (strip would be better),
to get 0.0005 ohm would require:

L = 0.0005 * (.3 * .3 * 3.141)/(26 * 1.72E-6)
 = 3.2 cm  (If I didn't make a mistake!)

That has a volume of 0.9 cm^3 , specific gravity is about 8, so
weight should be about 7 grams.

I'll leave working out the temperature rise of a 6mm diameter x 32 mm
rod dissipating 5W as a (necessary) exercise. It will get rather hot
without heatsinking, I think.

Hope this helps.

Best regards,


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2000\03\27@105008 by Thomas McGahee

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<x-flowed>Andy,
I have always found it useful to use an opamp to amplify the voltage
drop across the current sensor. That way you can use the entire
4.096 volt range of your ADC. You use a multi-turn trimmer in the
feedback loop for calibration. For example, arrange for 100A=4.000V.
Since your sensor is on the ground side you can get by with a
single supply rail-to-rail opamp. Connect it as an inverting amp,
with (non-inv) connected to the OUTPUT side of the sensor, and the input
resistor of the (inv) of the opamp to the INPUT side of the sensor.
That way the output of the opamp will go positive when current flows.

Calibration can be done via the trimmer.

Fr. Tom McGahee

At 09:40 AM 03/27/2000 -0500, you wrote:


{Quote hidden}

> > {Original Message removed}

2000\03\27@105216 by Andrew Kunz

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Spehro,

You made my mistake obvious.  I mis-calculated I2R losses to be 1/2 watt!

Now referring to http://www.rencousa.com/wiretable.htm I see that the 12AWG wire
already in the system will be perfect for what I'm trying to accomplish!

Thank you so much!!

Andy










Spehro Pefhany <RemoveMEspeffKILLspamspamINTERLOG.COM> on 03/27/2000 10:21:41 AM

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Subject: Re: [OT] Resistance








At 01:52 PM 3/27/00 +0100, you wrote:
>        You could use a block of aluminium alloy which would certainly be
>much lighter, but the temp co. wouldn't be very good.  Mike   -----Original

The main problem is that his power dissipation will be 50W to 5W depending
on the resistance, so that requires something physically fairly large, as
you say, to keep it from getting too hot. At the lower end of the scale,
5W isn't too bad if not much accuracy is required, and/or if a heatsink
surface is available.

Maganin has a relative resistivity of 26, tempco is about 20ppm/K IIRC
Brass has a relative resistivity of 3.9, tempco is about terrible,
about 2000ppm/K
Constantan has a relative resistivity of 28.45, tempco is about 200ppm/K

The above resistivities are relative to copper at 1.72 E-6 ohm-cm

Assume you have a 6 mm diameter wire of maganin (strip would be better),
to get 0.0005 ohm would require:

L = 0.0005 * (.3 * .3 * 3.141)/(26 * 1.72E-6)
 = 3.2 cm  (If I didn't make a mistake!)

That has a volume of 0.9 cm^3 , specific gravity is about 8, so
weight should be about 7 grams.

I'll leave working out the temperature rise of a 6mm diameter x 32 mm
rod dissipating 5W as a (necessary) exercise. It will get rather hot
without heatsinking, I think.

Hope this helps.

Best regards,


=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
Spehro Pefhany --"it's the network..."            "The Journey is the reward"
KILLspamspeffspamBeGonespaminterlog.com             Info for manufacturers: http://www.trexon.com
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2000\03\27@150722 by Chris Eddy

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Andrew;

If you are getting into this kind of current and complexity, why not use a hall
effect transducer?  The ones from Energy Science Corp are good for 0-100kHz.  That
one is the CS50, for about $25.  And that is not the smallest one they make.  By
the time you add up the hastle of machining and hardware and connection points for
the shunt, you are better off passing a wire through the hall sensor and being
done with it.  I used it on a 3PH brushless controller and it works fine.

Chris Eddy

Andrew Kunz wrote:

> Understood.  Our 100A shunts weigh half a pound.  I need something LIGHT!
>

2000\03\28@110454 by Andrew Kunz

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Just a word of thanks here to Tom McGahee for his outstanding help on this
subject.

He sent me a PDF with a schematic and explanation of how it works (does that
ever help!), along with suggestions on what to change (and why or why not) if it
didn't fit exactly with my needs.  Not being very strong on analog, this is
_exactly_ the kind of help I needed.

I urged him to post the PDF on his site, since it is _very_ helpful.  I'm sure
others with similar problems will find it a useful resource.
The next one is to Chris Eddy for his input on the _hows_ of connecting it up.
Chris made some good suggestions on what to do, and they will be helpful as well
when it comes time to the actual implementation.

Thanks again, both of you!

Andy


'[OT] Resistance'
2000\04\07@145733 by Andrew Kunz
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Thanks to all who helped on my shunt/current measurement problem.

I built a prototype based upon inputs from several of you - Chris, Tom, Spehro,
Michael, Russel, ... - and it was successful.

What I ended up with was 2" of AWG 12 for the shunt (it's actually just a
section of a lead which would normally be there), an AD626 configured at
gain=100 per the data book, a single cap for the LPF, and several bypass caps.

Works great!  Thanks!!

Andy

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