090111-1634 EST
ohmhead:
LVDT is an acronym for Linear Variable Differential Transformer. An LVDT consists of three coils and a moveable iron core. One center coil is the primary, on either side of this is a secondary coil. These are aligned so that iron core can be moved thru the coils. The two secondary coils are connected in series opposition. An AC excitation is applied to the secondary and a phase sensitive detector is connected to the output of the series connected secondaries. We use 10 kHz.
When the core is centered between the two secondaries it couples equally to each secondary. Since the secondaries are in series opposition the output is zero.
As the core is moved off center one way there is more coupling to one secondary than the other and the output voltage increases with a particular phase relative to the excitation.
Move off center in the opposite direction and the voltage increases but with opposite phasing.
We routinely resolve 0.000,1" over a range of +/-0.100" on automotive assembly lines with zero stability very good over long time periods. Full scale is somewhat less stable than the zero point.
On the Wheatstone Bridge I think electricalperson needs to start very simply. At this time I am going to suggest one 1.5 V D cell as the bridge excitation. If a one turn potentiometer, they are not quite one turn, is used I will suggest 25 k. It needs to have a calibrated scale. A 10 turn Helipot and its multiturn knob can be used without calibration if the linearity specification is adequate.
I suggest initially two calibrated scales for a one turn pot. One will be zero to 100%. This can be created by connecting the pot across the 1.5 V battery. Measure the voltage across the pot's two outside terminals which come from the battery. Next connect the voltmeter between the "lower end of the pot", I prefer this to be the negative end and counterclockwise on the pot position, and the "wiper". Adjust for each of the 5% points and mark the scale. Probably a longer mark and value at each 10% point. Neither 0 or 100% will likely be possible.
The second scale is ratio. At the center of rotation the ratio is 1.00, at 25% of rotation from the counter clockwise position it is 25/75 = 1/3 = 0.33. At 75% of rotation it is 3.00. This can be calculated from the voltage scale, or the resistance on each side of the wiper can be measured.
When making measurements on a diode in the forward direction, such as a 1N4148 or 1N4005, the reference resistor will determine the diode bias current. It may be better to view this kind of experiment as a means of measuring the diode voltage drop vs bias current using a potentiometer.
In the early days of electrical experiments the potentiometer, invented by Wheatstone, and in conjunction with a galvanometer would have provided a means to measure either voltage or current.
Note: from
http://en.wikipedia.org/wiki/Galvanometer
Deflection of a magnetic compass needle by current in a wire was first described by Hans Oersted in 1820. The phenomenon was studied both for its own sake and as a means of measuring electrical current. The earliest galvanometer was reported by Johann (Johan Schweigger) of Nuremberg at the University of Halle on 16 September 1820. Andr?-Marie Amp?re also contributed to its development. Early designs increased the effect of the magnetic field due to the current by using multiple turns of wire; the instruments were at first called "multipliers" due to this common design feature. The term "galvanometer", in common use by 1836, was derived from the surname of Italian electricity researcher Luigi Galvani, who discovered that electric current could make a frog's leg jerk.
Originally the instruments relied on the Earth's magnetic field to provide the restoring force for the compass needle; these were called "tangent" galvanometers and had to be oriented before use. Later instruments of the "astatic" type used opposing magnets to become independent of the Earth's field and would operate in any orientation. The most sensitive form, the Thompson or mirror galvanometer, was invented by William Thomson (Lord Kelvin). Instead of a compass needle, it used tiny magnets attached to a small lightweight mirror, suspended by a thread; the deflection of a beam of light greatly magnified the deflection due to small currents. Alternatively the deflection of the suspended magnets could be observed directly through a microscope.
Galvanometers of this age did not produce a deflection proportional to current.
You can build your own with a large number of turns on a coil and a compass. Place the compass at the center of the coil and orient the coil so its principal magnetic axis is perpendicular the the earth's field.
See: D'Arsonval for the basic invention of today's panel meter. Also see Weston.
http://chem.ch.huji.ac.il/history/arsonval.html
Also discussed in the Galvanometer Wikipedia article.
http://www.scitechantiques.com/galvanometer-astatic/
Note: Edison's work on the electric light predated the D'Arsonval and Weston instruments. I believe Edison's work was done primarily with galavnometers that had a non-linear deflection.
My results from two diodes:
Code:
1N4148 1N4005
Temperature 70 deg F.
0.9 MA 0.607 0.594
2.7 MA 0.658 0.646
10. MA 0.728 0.708
15. MA 0.751 0.725
Temperature 24 deg F.
0.9 MA 0.657 0.641
2.7 MA 0.704 0.688
10. MA 0.772 0.750
15. MA 0.788 0.763
The voltage differences
0.9 MA 0.050 0.047
2.7 MA 0.046 0.042
10. MA 0.044 0.042
15. MA 0.037 0.038
Average calculated coefficients MV/deg C.
-1.70 -1.63
An approximation for temperature coefficient of voltage vs temperature
for constant current for a silicon diode is -2 MV/deg C. The difference
in temperature here is 70 - 24 = 46 F or 46*5/9 = 26 C. Using 2 MV/deg C
this is 0.052 V.
See what your experiments show.
You can make one of these diodes a thermometer by supplying it with a constant current.
Other than using the Wheatstone bridge in straingage transducers I think today that other means would be used for resistance measurement. But it is a great learning tool.
Also see:
http://en.wikipedia.org/wiki/Diode
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