INSTRUMENTATION

---

INSTRUMENTATION A new series of d.c. regulated power supplies, dynamically compensated, approaches the goal of perfect regulation

by Ralph H. NEW and inexpensive type of d.c. power supplies of unusual stability is available from the Millivac Instru­ ment Corp., 444 Second St., Schenec­ tady 6, N. Y. A typical instrument is shown in Figure 1. The instrument is dynamically compensated to provide zero or slightly negative internal imped­ ance to compensate for voltage drops in lines between the power supply and the consumer. The instruments are of the upright-meter style housing for space conservation and are equipped with both voit and milliampere meters.

A

Regulated Power Supply The new Millivac R P series of regu­ lated power supplies approaches the goal of perfect regulation in unique man­ ner. These regulators are dynamically compensated. Their output voltage can be made to rise with increase in load to balance out the rising voltage drops in leads between power supply and con­ sumer. The amount of compensation is adjustable and can be matched to spe­ cific requirements. Even overcompen­ sation is available, causing a small rise of Β plus with increasing load, thus pro­ viding a negative rather than positive internal impedance. These new supplies provide a degree of protection against motorboating of amplifiers never achieved before. Three- and five-stage high-gain, nonmotorboating amplifiers have been fed from these supplies successfully without any stage decoupling whatsoever. The Millivac-type R P supplies are not only self-stabilizing d.c. regulators; they are also equipped with two effec­ tive compound circuits which compen­ sate or even over compensate both dy­ namic load changes and line voltage variations. A slot-driven control in the middle of the panel marked "Load Com­ pensation, Over-Under" enables the user to set this control in such manner that the regulated supply voltage either slightly increases with increasing load or VOLUME

2 6, N O .

3, M A R C H

slightly decreases, or, if adjusted accord­ ingly, remains completely constant. The same control is also marked "In­ ternal Impedance, Positive-Negative." If the supply is adjusted to overcompensate external loads, this means that the output impedance is negative. Such setting provides a most powerful weapon against amplifier-motorboating, as regenerative coupling between the out­ put stage of the amplifier and earlier stages through the power supply is thus eliminated. High-gain threeand even five-stage amplifiers can, there­ fore, be operated from these supplies without decoupling if the compensating control is carefully adjusted to an effec­ tive true zero or slightly negative im­ pedance. The regulated reference voltage in these supplies is taken from conven-

F i g u r e 1.

1954

Dynamically Compensated Power Supply

Müller

tional OA2 or OB2 regulator tubes. I t is a well-known fact that all regulator tubes have inherent small regulating errors resulting from supply voltage changes. Millivac R P supplies have a special internal line voltage compensating control which feeds a small unregulated, opposing signal into the d.c. amplifier of the supply, thus creating a corrective compound effect which neutralizes the slight imperfections of the gas regulator reference tube. An unusual degree of line stability is thereby obtained. This internal control is factory-adjusted and need not be touched under normal conditions. Four models are available with nominal ratings ranging from 250 volts, 0 to 50 ma., up to 400 volts, 0 to 100 ma. Each of these supplies may also be used at higher voltages and reduced current or at lower voltages and full current. Thus the Model RP-41 has a nominal rating of 0 to 50 ma. output at 250 volts but may be adjusted to provide as low as 50 volts 0 to 50 ma., or as high as 400 volts at an output of 0 to 1 ma. All regulators have a static stability of 0.5% at rated voltage over the full load range with line voltage variations between 10% above and 10% below rated value of 117 volts, 60 cycles per second. Their dynamic stability can be adjusted to better than 0.001% with a 10-cycles-per-second load fluctuating between 100 and 50% of maximum rated load at all voltages listed. All supplies have heater power outputs rated 6.3 volts, 2 amperes, and 60 cycles per second. Ripple in all supplies is well below 10 mv. Linear Motion Transducer A new linear motion transducer produces accurate electrical output corre_ 37 A

INSTRUMENTATION,

Look who uses

GOLDFISCH fat extractors!

• Wisconsin Alumni Research Founda­ tion (above,) • Wilson and Company • The Borden Company • American Can Company • State of California Feed Control • Schenley Distillers • General Mills • Joseph E. Seagram and Sons • U. S. Department of Agriculture • Cornell University Experiment Station Using the Goldfisch solvent process fat extractor, you, too, get results in 2 to 4 hours that used to take all day. Due to automatic release and seal of the condensation chamber, vapor­ ized solvent quickly condenses, thus speeding extractions. The modern Goldfisch reclaims a high percentage of solvent . . . operates safely in open rooms . . . soon pays its way in time and money saved.

FREE ILLUSTRATED BOOKLET Write t o d a y for prices and

free

illustrated

booklet with full descrip­ tions and installation pic­ tures of f a t , fiber and protein apparatus.

Ask

for booklet T-54.

Laboratory Construction Company 1115 Holmes Street

38 A

Kansas City, Missouri

continued

sponding to motion or position in ranges of ±0.5, ± 1 , or ± 2 inches of travel. This linear motion transducer converts linear motion or position into propor­ tional electrical signal for remote indi­ cation or control. It is sensitive to very small movements; in a typical instal­ lation, it was used to measure a solenoid valve stroke within 0.0003 inch over a range of 0.03 inch with a time interval between energization and operation being measured within 1 millisecond. The transducer is based on the mag­ netic reluctance principle, wherein the moving slide of the transducer controls the inductive difference between two coils. The instrument can be used in a.c. carrier systems at frequencies rang­ ing from 60 to 10,000 cycles per second. When operated in a bridge circuit, full scale output is ± 4 0 mv. per volt with maximum excitation of 7 volts. The unit is small, consisting of a machined aluminum case lVe inches square, from which the moving slide projects. An alternative model is hermetically sealed for corrosion resistance. This instru­ ment is a product of North American Instruments, Inc., 2420 North Lake Ave., Altadena, Calif. Electronic Proximity

Switches

The electronic measurement of change in capacitance has been used for many years as an effective transducer. The arrangement is often referred to as a proximity switch and has been success­ fully applied to many industrial and laboratory problems. Some appli­ cations for which the proximity switch is recommended are: (1) a temperature control, by detection of expansion or contraction, by the movement of a pointer or an indicating instrument, or of a column of mercury; (2) a level con­ trol for liquids or solids; (3) an interface detector, when the instrument may be arranged to differentiate between liquids of different electrical conductivity; (4) a sorting device or counter; and (5) a fire or burglar alarm. A recent study has been concerned with the effective humidity variations on the operation of electronic proximity switches [Long, R. A. K., J. Sci. Instr., 30, 422 ( 1953) ]. This investigation has shown that the control point of a prox­ imity switch is shifted by changing rela­ tive humidity of the air over and beyond that which is to be expected from a change in permittivity of the air. The conclusions drawn by this author are that there is no evidence to support the continued assumption that at radiofrequencies and low potential difference, adsorption of water by the insulation between plates has any marked effect. Stainless steel plates and rhodiumflashed plates have given similar curves

of capacitance against humidity, dis­ tinct in form from those given by the readily oxidized aluminum plates. The form of the curve indicates adsorption by the oxide surface, in addition to the theoretical increase in the permittivity of the air. Greatest sensitivity—-i.e., smallest re­ lay differential—is obtained with the smallest practicable value of capacitance at the electrodes, but, in these circum­ stances, change of humidity has its greatest effect on advance of level. For continuous accuracy, the electrode must, therefore, be shielded against drafts and temperature changes; under rigorous conditions of high temperature and high or low humidity, it should be totally enclosed. Thermistors Thermistors are being applied to numerous measuring and control prob­ lems of interest to the analytical chemist. A temperature controller using thermis­ tors has been described which will main­ tain an 8-gallon tank of water constant to within 0.01° C. [Noltingk, Β. Ε., and Snelling, Μ. Α., J. Sci. Instr., 30, 349 (1953)]. Some of the distinctive features of this thermostatic control system are the rugged mounting for the thermistor unit of low heat capacity and permitting rapid heat interchange. The thermistor is connected in an a.c. bridge circuit. The output of this bridge is amplified and fed to a phase-sensitive detector containing a grid control recti­ fier. The output is used to control heat input to the bath. The arrangement is such that change in gain of the amplifier introduces no error of the control point but only a change in sensitivity. Some further observations on the performance of thermistors used as hygrometers have been made by G. W. Smith [/. Sci. Instr., 30, 414 (1953)]. The author has found that the thermistor hygrometer described by Pennman and Long, when used under tropical condi­ tions, is not reliable unless, used in the shade. Several proposals have been made for improving the performance in high light intensities, such as reducing the surface area of the thermistor caps while still retaining about the same value for their thermal capacity, or re­ ducing the influence of the wet and dry bulbs on each other by altering their relative positions. The hygrometer, if it could be made reasonably accurate, has several ad­ vantages over more conventional types. It is portable and light, and its sensitive head has only small dimensions. I t can be placed in positions not normally accessible to other hygrometers, as in cracks in rocks, holes in the ground, and amid branches of trees. ANALYTICAL

CHEMISTRY