INSTRUMENTAT ION by Ralph H. Mullet
Solid-state substitutes for sensitive galvanometers and thermostats have many practical applications
rriHE exacting and unusual demands of A modern science and technology are accounting for a large variety of new instruments and devices. They are also rendering several time-honored de vices obsolete. Long ago it was found that sensitive galvanometers and thermostats are un wieldy and relatively useless in aircraft. Electronic equivalents had to be found and the substitution is still going on. One of the latest of these innovations is a substitute for the standard cell. Before we outrage the physical chem ist, who well knows that the Weston cell is the international standard of electromotive force, we hasten to add that this cell will continue to serve in this important capacity for many years to come. What has come, as a very significant advance, is a very rugged electronic equivalent which is almost, but not quite, as precise, as a voltage reference source. With relatively sim ple associated circuitry, the Zener di ode accomplishes this task. If a voltage is applied to a semicon ductor junction iu its high resistance, nonconducting, or "unallowed direc tion," very little reverse current will flow until some voltage in the region of 3 to 20 volts is applied. At some criti cal voltage a very large current will flow. The critical potential marks Zener breakdown and at a slightly higher potential an "avalanche" occurs, somewhat akin to Townsend multipli cation. At the Zener breakdown volt age the current may increase over sev eral decades for a very slight increase in potential. Zener diode is a term embracing voltage regulators, stabilizers, and ref erence diodes. Their operation in volves the low slope resistance of a sili con diode in the well defined breakdown region. In behavior, these solid-state devices are similar to gas-discharge reg ulators in which the potential drop across the tube is independent of cur rent over a wide range. The temperature coefficient of volt
age changes from negative to positive as the breakdown voltage is increased and goes through zero in the 5- to 6-volt region. For this reason manufacturers strive to produce diodes with a break down in the region of 5 volts, at which the slope resistance is also low. The Zener diode is essentially a sili con rectifier. By judicious doping of the silicon with an impurity a low breakdown voltage can be obtained. The currents which can be tolerated are between 20 and 500 ma., although larger values, 1 to 2 amperes, can be made, especially if the mechanical de sign is such as to permit heat dissipa tion to a well conducting sink. The Zener diode is a tiny device resembling ordinary pigtail resistors. In the large powers, a massive stud constitutes the body, and it can be bolted to a panel for heat dissipation. Use a s R e f e r e n c e V o l t a g e
Sources
One of the most useful and important applications has been the development of reference voltage sources of this kind in recording potentiometers. Several manufacturers have made this impor tant step and with such voltage refer ence sources, a recording potentiometer no longer requires a standard cell, dry cell, and electromechanical standardiza tion mechanism. Furthermore a re cording is no longer interrupted by the jitter and jiggle of the standardizing process. Through the courtesy of Lou Schultz of the Albuquerque office of the Minne apolis-Honeywell Regulator Co., we are able to describe the regulators or con stant voltage units which will be a standard feature of all their recorders. Several hundred recorders in our insti tution are being modified as rapidly as possible to use these units. In our own laboratory we have had the opportunity to examine two units for their perform ance characteristics. In the M-H unit, which is some 2 1 / 2 Χ 2 3 / 4 χ 4 inches in dimensions and which weighs some 5 to 6 ounces, there
is a stepdown transformer from the line which feeds a silicon rectifier and smoothing capacitor. The first regula tion stage consists of a series resistor and a pair of Zener diodes. The regu lation at this point is ±0.5%. This voltage is fed to the second stage, which is a bridge network containing another Zener diode. The output from the diagonal of this bridge contains a series resistor of copper to provide tem perature compensation for the Zener diode. The diodes are operated to pro vide a base temperature of 85° F. At a rated value of 1.029 volts, these devices are guaranteed an over-all ac curacy, with any combination of ±10volt variation from 117-volt line supply and ±55° F. variation from base tem perature of 85° F., of ±0.15'/·. The guaranteed accuracy, based only upon ±10-volt variation from 117-volt line supply, is ±0.03%. It is stated that these figures are conservative and in a location protected from large tempera ture variations the accuracy should be much better than 0.03%. Specific
Applications
This is borne out by our experience with these units. In one test, the con stant voltage unit was connected in series opposition to a 0- to 25-mv. Brown recorder. The average drift over a 16-hour recording was 25 μν. when compared with the standard cell voltage of 1.01952 volts or about ±0.025%!. Somewhat larger drifts were observed toward the end of each automatic standardization period, indi cating that the conventional recorder, using standard cell, working dry cell, etc., had drifted in the interim. After each automatic standardization the system returned to the stated value within the 25-μ.ν. error range. Zener diode systems are by no means limited to the potential without loss of precision. There are further advan tages in using Zener diodes in series and series-parallel arrangements, particu larly if high currents are desired, beVOL. 32, NO. 8, JULY 1960 ·
69A
INSTRUMENTATION
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cause the heat dissipation can be distributed among several units, and if the heat factor is minimized, larger useful life will result. Experience has indicated that life expectancy doubles with every 10° C. drop in junction temperature. This discussion has been confined to Zener diode uses in recording potentiometers and has ignored their other countless uses. Since the voltages which are useful do not go much above 30, the use of these diodes is not as great in vacuum tube circuitry as it is in transistorized devices. Here the Zener diode has many uses, for regulation, stabilization, and as a reference potential. It is possible to establish such diode units at fixed and critical values and as such they can trigger circuits in a very destructive and critical manner. This is something which can be done by conventional circuitry but only with much more elaboration. Crystal diodes and transistors have man}' advantages over a conventional tube; their small size, extremely low power requirements and immediate response, requiring no warm-up time, are among the principal advantages. However, notable advances are being made in extending these devices to higher power ratings. The chief factor to be overcome is heat dissipation ; this is achieved by providing bases of good thermal conductivity which can be bolted to a panel. Four electrode devices, akin to a thyratron, have been developed for fast switching service. Westinghouse has recently developed a solid-state tetrode which can switch 300 amperes at 50 volts.
ANALYTICAL CHEMISTRY
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Astonishing progress has been made in photosensitive solid-state systems. Most of them are effective in the infrared and have found extensive use in spectrophotometry and in missile guidance systems. Indium antimonide has numerous interesting possibilities, not only as a phototransducer but in its magnetic properties. When employed for its unusually high Hall coefficient it is an extremely sensitive detector for feeble magnetic fields and, conversely, as a means of detecting very small mechanical displacements. There are about 100 commercial sources of these solid-state devices in the U. S., some 40 in Great Britain, and others in some 15 other countries. Japan is particularly active in this field and from the Japanese has come the new tunnel diode. Progress has been so rapid that much of the information is not to be found in even the most recent textbooks.
