Resistance amplification?
Resistance amplification?
(OP)
I have a project that I am working on that is used to show, very simply, the difference in resistance between different materials. I'm using wires of a relatively small diameter, however I'm seeing that the resistance is going to be so small that it will not be measureable using standard multimeters (~.04 ohm).
Is there some way that I can amplify the resistance for all these wires so that it will be measureable, and distnctly different by the same amount for every wire?
Any assistance would be greatly appreciated.
Is there some way that I can amplify the resistance for all these wires so that it will be measureable, and distnctly different by the same amount for every wire?
Any assistance would be greatly appreciated.





RE: Resistance amplification?
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Resistance amplification?
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Resistance amplification?
http://www.ietlabs.com/IET/LOM-510.html
ht
They're not cheap, since you're paying for a substantial reduction in electronics noise. You could probably rent one for a week or so. Don't forget that with low resistances, you need a Kelvin connection to eliminate the probe contact error.
TTFN
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RE: Resistance amplification?
RE: Resistance amplification?
Use a D ell as the power source with a series resistance of 1.2 ohms to limit the source to about 1A.
Connect a 0.1 ohm across the DPM ref input and the test sample across the signal input. Connect the DPM common to ref lo.
Power the ref hi from the +1.5v 1.2 ohm source and link the ref low to the sample resistance, return the other end of the sample resistance to -ve of the 1.5v source. Use 4 wire connections (one wire each end for the current and one each end for the DPM input)on the ref and sample resistances.
This will measure 0 to 0.1999 ohms. You can scale the values and test current for other ranges.
The display reads the ratio of the ref and sample resistances.
RE: Resistance amplification?
I tried to just run a voltage across the wires (from a D-Cell battery) however I am seeing no drop at all (I'm using a Fluke 189 DMM to read the voltage). I suspect this is because the wires are so short (about 60cm long) and there just isn't enough resistance to matter. Will the regulator method gain me anything in this case?
RE: Resistance amplification?
Adding a couple regulators in parallel can increase the DC current and make readings easier.
RE: Resistance amplification?
37 ga about 0.5 Ohm/ft, 34 ga about 0.25 Ohm/ft. Every three gage size decrement halves the resistance.
RE: Resistance amplification?
RE: Resistance amplification?
I would use a board (1"x6", or 1" x 8") as long as you feel is reasonable in the class room. The length may be 3Ft., 4 ft., 6 ft., or even 8 ft.
Run your samples the length of the board. Connect alternate ends together so that all samples are in series.
Now run 1 amp through the series arrangement. The higher resistance of the series arrangement may make the current easier to stabilize.
Measure the voltage across each section with a mill-ammeter.
With 1 amp flowing, the voltage will equal the resistance. 40 ma will infer 40 milli-ohms.
A tip o measuring voltages;
There will probably be some non linear voltage drops in the connections between the different wires.
You may avoid this error by measuring the voltages just beside the connections.
For example, consider an iron wire connected to an aluminum wire connected to a copper wire. When measuring the voltage drop across the iron wire, position your meter probes so that they both contact the iron, just beside the splices. When measuring the aluminum wire, move the probe to the other side of the connection so that it contacts only the aluminum.
Good luck!!
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Resistance amplification?
@ Sreid: I've thought of this, however since students will be using these themselves, I'm afraid they will break the wires. Maybe a possibility if I can somehow protect the wires.
@ melone: I can't use big spools of wires because not all types of wires come insulated and measuring the resistance across the ends of a big spool of uninsulated wire won't get me where I need to go :)
@ waross: Yes, all wires are the same gage, all around 24. I will experiment with this idea, however I still think that the the resistance will be too small. Thanks for the detailed explanation. I'll start futzing with this now.
Thanks to everyone. Please keep the brainstorming ideas coming!
RE: Resistance amplification?
How valid would this be:
Connect a D-Cell battery in series with a multimeter. Set DMM to measure amps. Put one lead on far side of power circuit. Measure amperage at short end, then at long end. There seems to be enough to measure when it comes to amps (I get differences of at least 0.04).
Then have them use the V=IR equation at both ends of the wire, subtract the differences in R to get resistance of the wire and compare that value to the theoretical value?
Plausible?
RE: Resistance amplification?
Have I slipped a decimal place here? How long are your wires?
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Resistance amplification?
Yes, you're right, the copper and silver wires in the set will have very little measurable resistance, however some of the other materials seem to work well using this method(iron, constantan, Ni-Chrome, stainless steel). I can just use the low resistance wires (copper, aluminum, silver) to show how little resistance there is and maybe make references as to why aluminum used to be used for wiring and no longer is standard.
RE: Resistance amplification?
RE: Resistance amplification?
I am somewhat puzzled to see how you reject all these solutions to your "selfinflicted" problem. My answer, for one, is a perfectly valid solution. All you need is a power supply with settable current limit and a cheapo digital voltmeter. Not anything you need to buy, but exists in plurality in every technical school worth the name.
What is the reason you make this simple (and, if I may express a personal meaning) rather uninteresting lab so difficult?
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Resistance amplification?
I work for a company that manufactures basic science equipment for physics labs (and classes) anywhere from elementary school up to the college level. We specialize in the middle school area though, and rarely are our products used at the college level.
When it comes to making demonstration equipment for use in public schools, cost is the number one driving force. Certainly I could make a demonstration unit that would cost $100+ or more, but no teachers would purchase it. We have 50+ years of experience showing this, especially nowadays. These demos must be inexpensive ($20-30) and simple since many of the teachers using them do not have expertise in every area of physics, and don't have access to equipment other than a d-cell battery and some wires.
All this is meant to accomplish is to show young kids that different metals have different resistances and maybe to expose them to some initial formulas regarding electricity. I highly doubt that a technical college or advanced university lab would pick this up. That's not our primary focus.
Thanks again for every-one's input, and I again apologize if I offended anyone with any of my remarks.
RE: Resistance amplification?
RE: Resistance amplification?
You're basically doing something wrong somewhere. A Fluke 189 has a 1 uV resolution. You should be able to use a constant current LED supply, ala http://www.national.com/pf/LM/LM2755.html which puts out 30 mA, and be able to measure the voltage drop across a 1 milliohm resistance with plenty of resolution. Or any of a variety of approaches. A DC wallwart with 5V output and 200-ohm series resistance will give you a relatively constant 25 mA current source, since all your test resistances are less than 1 ohm.
You should review Kelvin connections, which may be why you're having such difficulties in the measurement: h
In fact, the whole subject of using Kelvin connections is probably worth a lecture alone.
TTFN
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RE: Resistance amplification?
Would that cause too much confusion between the result and the material to a student?
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Resistance amplification?
Anyhow, if you need to produce such kits in the 1k+ or 10k+ range, then a battery, a constant current regulator (constant current diode is even simpler) and an analogue panel meter showing, say 0-50 mV would do the trick.
Constant current diodes may be tricky to find in current ranges you need. So a simple regulator as described by OperaHouse is probably better. An analogue panel meter will cost like USD 5-10 in quantities. Can't be done at much lower cost than that.
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Resistance amplification?
I'll have to try this at home and see what I get.
TTFN
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RE: Resistance amplification?
RE: Resistance amplification?
Aluminum is a great conductor and is still much used. The problem in houses was not the higher resistance, which was economically solved by going two AWG sizes larger than copper for the same application. The problem was the mechanical and chemical properties of aluminum which caused serious problems with the connections.
If the aluminum was nicked when the insulation was stripped, the work hardening property combined with the low strength would lead to broken conductors.
Heat expansion and cold flow created problems with connections in circuits with cycling loads.
When aluminum is exposed to the air, it quickly forms a protective layer of aluminum oxide. This layer insulates ad leads to high resistance connections which tend to run hot.
This makes heat expansion and cold flow worse. These problems have all been overcome in industrial applications but the solutions are generally too expensive and/or time consuming to be used in residential installations.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Resistance amplification?
This is a basic Kelvin connection, the separation of the current source connections from the voltage drop connections. There are fancy Kelvin "clips" that mechanize the connections, but they're totally unnecessary.
TTFN
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RE: Resistance amplification?
I like your sketch.
Respectfully
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Resistance amplification?
Kelvin connections are pretty routine in some industries. Van der Pauw's theorem on measurement of bulk resistivity uses Kelvin connections, and process control test patterns in many IC fabs are based on those principles.
Note that the 189 h
TTFN
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RE: Resistance amplification?
I don't think it is a given that the schools will have a FLUKE 189. The design criteria should be based around a standard 200mV digital multimeter meter of the $5 variety.
The D battery idea is a bit simplistic. Someone who knows what they are doing can make a quick measurement, but these kids are going to stare out into space till they have nothing but a pile of dead batteries. We all know a school gets a one time budget for equipment and items like batteries never get reordered. There are low cost switching regulator supplies that can supply 5V and under at 1A that can be paired with a resistor.
RE: Resistance amplification?
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Resistance amplification?
In the interest of safety I would not like some of the ferrel animals that populate our british schools to be given any chance to use mains powered lab experiments.
RE: Resistance amplification?
Dan - Owner

http://www.Hi-TecDesigns.com
RE: Resistance amplification?
TTFN
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RE: Resistance amplification?
RE: Resistance amplification?
Since we're looking at production quantities, wouldn't it be worth investing in some finer wire, and concentrating on a way of packaging it which permits longer lengths while allowing sceptical kids to see that each sample really is the same length, and avoiding the turn to turn contact problem you get with wire on a reel.
I'd be tempted to see how much wire I could wind round a foolscap paper former with a few slots cut down the long edges - and whether I could then get the whole lot (except the tails) to laminate inside one of those plastic pockets.
Once you've got the resistance up to a useful level, you have the choice of measuring it direct with a cheap multimeter, or using a battery with an ammeter and a voltmeter.
A.
RE: Resistance amplification?
Unfortunately our "productions quantities" are only 100 or so, unless a product really takes off, at which point it may become 500 per year.
The whole point of my subject line was that I was looking for a simple way to "amplify" the resistance linearly to a measurable level with a cheap DMM. Not my Fluke 189, but something more like the $40 special we carry, or those $3 ones in Harbor Freight, which are more likely to be readily available at a public school in the USA.
It appears as though the only economical way to do this will be by passing a current through the wires (either with a power supply or a battery) and measuring a voltage drop. I am unable to obtain all the samples at a reasonable price at very small wire diameters, although I guess we could cut back the number of samples. I'm looking more carefully at the Kelvin measurement as well, that may not be too complex and may also give me what we're looking for.
Thanks again everyone for the input.
RE: Resistance amplification?
The other set is all Ni-Chrome wire at different diameters to show how the diameter affects resistance. That set works fairly reasonably.
RE: Resistance amplification?
RE: Resistance amplification?
Heat the wires at an higher temperature...
Use a 400 Hz source...
RE: Resistance amplification?
The materials I was going to use are Aluminum, Copper, Constaintain, Silver, Ni-Chrome, Iron, Brass and stainless steel. Finding them all in an insulated form and all the same diameter is impossible (economically impossible) at lengths less than a custom run from a wire facory.
RE: Resistance amplification?
Heating the material is a very interesting idea. That would increase the resistance, but I'm not sure how much R would rise per degree. Still, it's a good idea!
The only other way to see the difference in Resistivity is to pass a large amount of current through each of your chunks of metal, and use a simple DMM / voltmeter to measure the voltage drop across.
Actually, this may be more desirable anyway. The following experiment will illustrate V=IR, and how each metal has a different resistivity.
1) Make large metal "bars" with each of your desired materials. 2 x 12 inches should work.
2) Attach a bar to a constant current source, and pass 10 amps through it. Obviously, make sure that the circuit is not exposed to the students, etc..
3) Measure the voltage drop across the bar at various points.
4) Calculate the R based on the known I and V.
RE: Resistance amplification?
With a gain of 1000, it'll crank the voltage drop up to something that a typical 3.5 digit DMM can read.
Or:
http:/
Which requires external resistor to set gain up to 10000.
They both list for <$10 in 1000 quantities, but the latter lists at $3.14. There are cheaper ones that AD sells that might also work.
TTFN
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RE: Resistance amplification?
5-6 feet long, it may be sensitive enough for the job. Set at 10 Amps, it can read 10e-6 Omh.
We use it to measure resistance of MV busbar junction
(among other things), where conducting surfaces are large to provide very low resistance...
It should be easy to get, just have to rent one.
RE: Resistance amplification?
RE: Resistance amplification?
TTFN
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RE: Resistance amplification?
RE: Resistance amplification?
TTFN
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