Resistance amplification? 3

[timeline1968]

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.

 

[itsmoked]

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

 

[Skogsgurra]

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

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[IRstuff]

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

FAQ731-376

 

[stevenal]

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

 

[waross]

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

 

[IRstuff]

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

FAQ731-376

 

[waross]

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

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

 

[IRstuff]

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

FAQ731-376

 

[OperaHouse]

"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.

 

[itsmoked]

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

 

[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?

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.

 

[MacGyverS2000]

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

 

[IRstuff]

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

FAQ731-376

 

[ijl]

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?

 

[zeusfaber]

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.

 

[timeline1968]

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.

 

[timeline1968]

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.

 

[melone]

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

 

[unclebob]

Brainstorming results:

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

 

[timeline1968]

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.