Resistance of high power and small resistivity

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How to build by myself a resistance of high power and small
resistivity ?

I need to measure accurately a continuous current
( roughly over 20 Amps and probably more later )
but my amperemeter is limited to 2A.
So I will pass this current thru a very small
resistance, and measure the votlage across.

Using the graphite of a lead pencil ?
I actually measured 1 ohm on a distance of several millimeters :
using some of them in parallel, one can get as low as 0,1 Ohm.

Using a thin aluminium sheet ?
Since:    R = rho * L / S
I can cut it in order to change L and then get R as small as I want.

Any suggestion that may help ?

Re: Resistance of high power and small resistivity
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You could alternatively use a clamp on magnetic sensor around the cable.

A shunt resistor 20A over 0.1 ohm then I^2R is 40W of waste heat.
thermal effects will alter the load resistance unless you have a good
way to keep it cool. A smaller value resistor would help.

0.1R 10W is a nominal stock item in hobbyist shops like Maplin
http://www.maplin.co.uk/Module.aspx?moduleno21%81
Two of those in parallel on a good heatsink would just about handle your
20A (and cost less than 1).
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Why make your own when you can buy them off the shelf?

Regards,
Martin Brown

Re: Resistance of high power and small resistivity
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Yes, I would if I could - but I don't have one.

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Of cours, this was just an example.

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I know that already.
I saw  0,01 Ohm 100W  resistors somewhere.

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Because the shelf is miles away.

Plus - I like the fun of doing things by myself.

Re: Resistance of high power and small resistivity

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I use stainless steel wire for such purposes. I recently needed a  load to
test a 70 Amp, 14 Volt power supply. That's 980 watts. I made a resistance
element winding a few feet of stainless wire, in two parallel paths on a
junk 50 watt resistor. The resistor was a few hundred ohms and was used only
as a ceramic support for the stainless wire. It's resistance was
insignificant. The wound wire totaled  0.2 Ohms and was carefully spaced to
avoid shorts between turns of the bare stainless. The wire ends were screwed
to the resistor terminals to provide a solid electrical connection where
copper leads were attached. The wound resistor was placed in a 3 lb coffee
can filled with water. The water provides heat sinking for the resistor and
easily dissipated the 980 watts produced. The water got warm but did not
boil during the few minutes of the test.

If you don't have stainless,  steel bailing wire, nichrome, nickel or other
junk wire can be used. The thing is to get rid of the excess heat if there
is sufficient power dissipation.



Re: Resistance of high power and small resistivity
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Hey, that is useful for my inspiration - thanks for your help.

Re: Resistance of high power and small resistivity

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Regular, commercial meter shunt resistors
are available, of course.

Resistance of stainless steel is relatively
well behaved, and shim stock (thin sheets)
is a common mechanical-trade item.

A square of .002" ( 0.005 cm) thick in alloy
#304 stainless steel has

Sheet resistance 3D% 72 *10**-6 ohm-cm / (5*10-3 cm) 3D%  0.360 ohms per
square

So a square chunk at 20A will dissipate a little over 100W.
Several squares in parallel, or thicker material, can do
the job.   I'd allow at least a square inch of surface per
watt of energy dissipated.

It has to be soldered (at the edges) to a better conductor (sheet of
copper?)
to make a good electrical connection to the wires, and has to be
supported for good airflow (and so it isn't a shock hazard).

Re: Resistance of high power and small resistivity
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Thanks.

Re: Resistance of high power and small resistivity
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   Graphite you get from almost anywhere is very unreliable in
resistivity and rather unstable to say the least.
   I do not know what could be worse.
   Copper wire is better because then it is very easy to solder on
kelvin measuring taps; just be aware of the temperature coefficent..

Re: Resistance of high power and small resistivity
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Really ... can you tell me why ?

Re: Resistance of high power and small resistivity

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   Graphite from lead pencils have a lot of clay and are porous, from
Leclanche (old fashioned "zinc") cells have less clay but are also
porous, from welding supplies are better but still porous, from arc
lamps ditto.
   Maybe if you ground up a supply and then compacted the pile one might
have a less reliable and less stable resistivity...
...gotta work hard...

Re: Resistance of high power and small resistivity

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                   ^^^^^^^^^^
Right here is going to be your problem.

Since you have no way already to measure 20 amps, how do you plan to calibrate
your homemade shunt?  Resistors (shunts) for this current magnitude tend to get
hot.  If you make it out some ordinary conductor, copper or stainless steel, its
resistance will change substantially as it gets hot.  A good shunt needs to be
made of some alloy with a very low temperature coefficient of resistance, such
as manganin.  Where will you get the thick sheet of manganin to make it?  And,
once it's made, how will you calibrate it?  Shunts are one thing it's probably
worth while to buy.  I would get this one:

Item # 120342761683 on ebay.

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Re: Resistance of high power and small resistivity
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You're perfectly right. The measure will take less than thw seconds.

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Yes, accurate measure of a very low resistance is tricky.
I think I will use an accurate AC/DC power supply in constant
current mode, and measure the voltage across the resistance.

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Are YOU selling this one ?

Re: Resistance of high power and small resistivity

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get
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its
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How will you know what current the power supply is delivering, since you have no
way to measure currents of the order of 20 amps?

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No, I'm going to keep all my shunts.  I need them to measure high currents of my
own.


Re: Resistance of high power and small resistivity

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get
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its
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no
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my
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   Well, clibration could be done at 200mA or 1000mA...

Re: Resistance of high power and small resistivity

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calibrate
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get
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its
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such
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And,
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no
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my
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He said he wanted to "...measure accurately..."


Re: Resistance of high power and small resistivity

wrote:
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calibrate
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get
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steel, its
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such
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And,
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no
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my
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....and.....???
   Calibration can be done at any level where the output signal is at
least 100X that of noise, for 1% worst case accuracy.....
...With proper instrumentation and setup, 2mA could do the job.....

Re: Resistance of high power and small resistivity
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Sure thing.
The point is, 2mA across 0,1 Ohm 3D% 0,2 mV (or 0,01 Ohm -> 20 uV)
and I guess my voltmeter is not accurate enough on this scale.
That's why I thought 1A will be better : 100 mV (or 10 mV) is OK.

Re: Resistance of high power and small resistivity

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---
If 10mV is OK, then you're saying that +/- 1 digit plus the accuracy of
the meter is OK.

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Re: Resistance of high power and small resistivity
On Fri, 10 Apr 2009 10:17:18 -0500, John Fields

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 A 60" length seems to me that it would traverse more than five turns on
a 1.625" tube.

  I got 11 and 3/4 turns, and I didn't account for the center of the wire
being greater than 1.625, so it would be maybe a tenth or two of a turn
less.

Re: Resistance of high power and small resistivity
On Fri, 10 Apr 2009 08:47:56 -0700, Archimedes' Lever

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Yeah, you're right.

Thanks. :-) I don't know what I was thinking...

JF  

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