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Resistance amplification?
3

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.

RE: Resistance amplification?

The standard way of doing such measurements is to let a known (and constant) DC current pass through the specimen and show the voltage drop across it. If you use 1 amp, your reading vill be 1 V = 1 ohm. If 1 A is too much current, you can use less current and then use a convenient constant (like .1 or .01) to read resistance from voltage drop.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Resistance amplification?

If you connect your different samples in series and maintain a relatively constant current the voltage across each sample will be proportional to the resistance of that sample.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Resistance amplification?

You can use either longer wires or a micro-ohmmeter:
http://www.ietlabs.com/IET/LOM-510.html
http://www.testequipmentdepot.com/AEMC/MicroOhmmeters/6250.htm

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?

A cheap way is to use a 7805 regulator in current mode producing .5 to 1 amp.  That requires using a five or ten ohm resistor from the output pin to the common pin. Review  regulator app notes for details. Requires a lot of heat sinking, but that will make the voltage readable by a 200mV digital meter.  Then just calculate resistance.

RE: Resistance amplification?

Get hold of a 3.5 digit DPM module with differential reference and signal inputs.

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?

(OP)
First off, thanks for all the replies. Second: I'm not an electrical guy and this is going to be used to show students the different resistances of materials, theoretically by mounting short lengths of wires on a board. I need the simpler methods such as running a current across the wires or maybe adding the regulator/resistor in order for this to work for me. This is not an industrial application where we can procure expensive equipment or use complicated methods.

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?

The 1 amp regulator will provide 40 mV not a lot of resolution.  I have used a toroid (donut style) AC power transformer to create high currents.  Even a small 40W transformer can create currents of 10-20 amps at under 1/2 volt for these tests.  I break out the usual epoxy center so I can loop a couple turns of #12 wire.  Down side is ac measurements with many voltmeters are very poor.

Adding a couple regulators in parallel can increase the DC current and make readings easier.  

RE: Resistance amplification?

Perhaps very fine wire.  40 ga is about 1 Ohm/ft,

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?

Don't use short wires, use large rolls.  For example, measure the resistance of the entire spool of wire.

RE: Resistance amplification?

I assume you have the same gage wire for all of your samples.
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?

(OP)
@ Operahouse: Too complex and costly

@ 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?

(OP)
The series option didn't work. Not enough resistance.

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?

Well #24 copper wire has a resistance of 25 Ohm per 1000 ft. 6 ft would be 0.15 ohms. You won't see much with an analog meter, but you should get a good reading with a digital multi-meter.
Have I slipped a decimal place here? How long are your wires?

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Resistance amplification?

(OP)
The wires are 60cm long.

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?

Too expensive!!!  I can assure you that many here would like to have a low cost method to measure low ohms.  Sometimes the best lesson learned is that it can't be done easily.  I finally got a nice HYPATIA 309 that will easily calculate those resistances for any current from 1-100A.  For years I used the  methods I mentioned which cost less than $3.  Clearly you should look for some other physical property to demonstrate.  Calculating how a lamp goes through an nearly 10:1 resistance change as it warms up is more practical.

RE: Resistance amplification?

timeline1968,

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?

(OP)
I'm sorry, I didn't mean to offend anyone. Maybe a little deeper explanation regarding the use of this may be in order.

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?

I think that if you had given the full story in your original post the whole thread might have taken a different direction, and may well now.  People can only respond to the information given.  I don't have a solution to offer, but I'm sure that the creative folks here will not take the new set of givens and come up with something creative and probably something that produces a far more interesting result than just different numbers on a meter.  Besides a DMM doesn't meet your definition of inexpensive.

RE: Resistance amplification?

A D-cell will put out in excess of 10 A into a dead short.  However, it's not a constant current, so it'll fluctuate while you're doing your experiment and give you variable results.  

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: http://www.allaboutcircuits.com/vol_1/chpt_8/9.html

In fact, the whole subject of using Kelvin connections is probably worth a lecture alone.

TTFN

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RE: Resistance amplification?

How about designing a cheap little board that provides the Kelvin connection and gain on the current reading that could be read by a cheap meter.

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?

If there is no power supply and no DMM available, then I can understand the problem. But it is difficult to understand what such a demonstration would contribute to understanding of nature in a school without these basic devices.

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?

The OP has, apparently, at least one Fluke 189, which has a basic performance of 1 uV resolution or 10 milliohm.  However, the 10 milliohm is sort of moot without Kelvin connections, since the probe resistance dominates.  But, the DMM can be used with a separate current source and should be able to make the measurement.

I'll have to try this at home and see what I get.

TTFN

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RE: Resistance amplification?

Aluminum is commonly used for wiring. Let's not give the kids false information.

RE: Resistance amplification?

My suggestion of powering all the samples in series was meant to avoid the complications of Kelvin connections and supplying equal currents through different resistance samples.
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?

Actually, Bill, your suggestion IS Kelvin connections.  I get the impression that there's some disagreement as to what Kelvin connections are, from the link I gave previously:

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?

Thanks for the clarification and support, IRstuff.
I like your sketch.
Respectfully

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Resistance amplification?

Thanks, Bill, I can't take any credit, except for linking to it.  However, it clearly illustrates what we've been talking about.  

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 http://www.testequipmentdepot.com/fluke/pdf/189.pdf can only source 100 uA max in resistance mode, and only 1 mA in diode mode, which is why it would have trouble measuring the resistances in question accurately.

TTFN

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RE: Resistance amplification?

"don't have access to equipment other than a d-cell battery and some wires."

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?

Good point Opera;  You can buy isolated regulated UL wall-warts for less than D batteries these days.

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Resistance amplification?

A duracell D cell will provide at least 15Ah that means that it will last at least 10 hours with around 1 ohm load, is this not long enough?

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?

Quote (bogeyman):

A duracell D cell will provide at least 15Ah that means that it will last at least 10 hours with around 1 ohm load, is this not long enough?
I don't see you getting anywhere near 10 hours of use with a 1 ohm load out of a D-cell.

Dan - Owner
http://www.Hi-TecDesigns.com

RE: Resistance amplification?

Discharge characteristics are fairly complex.  Assuming that we need to keep the voltage above 1.1V, Duracell's alkaline D-cell has 7.5 Ah with 0.5-A discharge, at 1-A, it drops to 3 Ah, and less than 1.4 Ah at 2-A discharge.

TTFN

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RE: Resistance amplification?

If you are interested in the relative resistivities of different metals, not the absolute values, then a sliding Wheatstone bridge might do the job, see http://www.lhup.edu/~dsimanek/scenario/labman4/wheats.htm , for example. One leg of the bridge would be made of metal A, the second leg of metal B. The third and fourth legs would be a continuous wire with a sliding contact in the middle. If an oscillator is used as the voltage source, the galvanometer could be replaced by earphones, or maybe even by a speaker.  But one problem remains: Is the workig principle of the bridge obvious enough for this purpose?

RE: Resistance amplification?

I can't help worrying that all these elegant schemes risk losing the simple concept that Timeline's trying to demonstrate in a tangle of elegant, but distracting circuitry.

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?

(OP)
zeusfaber: You're entirely correct. I'd just about given up on this thread since it has some esoteric solutions and seems to have gone off on tangents here and there. As I said, this is for kids. Young kids, maybe 6th - 8th grade or younger. It is only meant to show that different metals have different resistive properties, a concept that some might not understand because to them a metal is a metal, and why wouldn't the electrons move the same way?

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?

(OP)
Oh, one other thing: the idea behind the 60cm board was to measure resistances at different lengths. We have another product utilizing the same board, with a pre-printed scale on it so the thought was to just combine the two and use the same board. This was before I actually carried out the calculations for the resistance on strands of 0.5mm wire.

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?

Why do you have to use non-insulated wire?  You realize insulated wire comes in drastically different sizes (gages), right?

RE: Resistance amplification?

Brainstorming results:

Heat the wires at an higher temperature...
Use a 400 Hz source...

RE: Resistance amplification?

(OP)
melone: I don't have to use uninsulated wire. I originally thought it would be beninfical if I did because then resistances could be measured along the entire length of the wire (instead of just the ends) and a graph could then be drawn showing the linear nature of resistance. The problem is obtaining insulated sources of eight different materials with all the same wire diameter. If you know of such a place, I'm all ears.

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?

Without highly sensitive equipment or custom circuitry, I doubt you will be able to see any real difference between those materials at room temp.

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?

So, how about using an instrumentation amp: http://www.analog.com/en/prod/0,,759_782_AD524%2C00.html

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://www.analog.com/en/prod/0,,759_782_AD620%2C00.html
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?

Think I got it! Why not use a micro-ohmmeter! Service guys call it a ducter. With a sufficient lenght of wire, 'bout
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?

Instrumentation amp!  Any crummy op amp would do it. Just need to adjust the zero pot every minute.  Circuit board would cost more than the parts.  Again, not practical for a run of 100.  

RE: Resistance amplification?

(OP)
Forgive my lack of knowledge regarding IA's, but it appears as though they need a voltage supply to work, in the first case 18V? Is this correct?

RE: Resistance amplification?

It's possible to run the current source from a D-cell, and the gain stage from a couple of 9V batteries.  The minimum supply voltage on the IA is +/-6V

TTFN

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