INSTRUMENTATION In the expanding field of transistor applications, the advent of silicon p o w e r rectifiers is a landmark T H H E R E m u s t be m a n y old-timers in this research business who recall with nostalgia t h e days when research laboratories were equipped with 110and 220-volt d.c. mains. No one, since T h o m a s E d i s o n ' s early fulminations, has deplored t h e widespread and almost exclusive generation and use of alternating current. T h e fact remains t h a t in t h e preponderance of scientific and technological investigations one is faced with t h e problem of rectification. Our electronic friends are prepared to do this for us in m a n y ways, u p to any power r a t i n g and with t h e m o s t exacting self-regulation. At times t h e chemist m a y be astonished a t t h e ponderous chassis which is presented to him in answer to a request, say, for a d.c. source to furnish 1 ampere at 100 volts. T h e design of electronic power supplies is a highly developed a r t ; indeed, it is a special branch of applied electronics. Nevertheless, t h e d e m a n d s are so varied t h a t continuous inquiry into the problem is essential. There is one trend in i n s t r u m e n t a t i o n which offsets the d e m a n d for improved power supplies. This is t h e expanding field of transistor applications. I n m a n y transistorized circuits, power dem a n d s are several orders of m a g n i t u d e smaller a n d t h e extreme compactness of the circuit elements calls for equally compact supplies—usually miniature batteries. Silicon
Power
Rectifiers
Another t r e n d which simplifies current d e m a n d s for d.c. supplies is t h e continuous development a n d improvem e n t of dry-disk or crystal rectifiers. A notable l a n d m a r k is t h e a d v e n t of silicon power rectifiers. W e would like to describe t h e essential features of this new development. I n a recent article b y W. E . Slusher of t h e T r a n s i t r o n VOLUME
27, NO.
10, O C T O B E R
Electronic Corp., Melrose, Mass., t h e principal characteristics a n d uses of these devices are described (Electrical Manufacturing, J u n e 1955). F u r t h e r information on these devices is available in t h e company's Bulletin T E 1321. T h e new power rectifier owes its distinction primarily to t h e fact t h a t it can m a k e effective use of conduction cooling. T h e silicon disk is m o u n t e d directly to a copper base. T h e copper base, in t u r n , can be fastened directly to a chassis or panel and t h u s can remove t h e h e a t with m i n i m u m t e m p e r a t u r e rise. F r o m this a n d other characteristics, t h e m a i n a d v a n t a g e s of t h e silicon power rectifier can be summarized as follows: 1. Reliable operation from —55° to + 125° C. ambient temperature. 2. Higher inverse voltage ratings, which result in improved rectification efficiency. 3. Silicon rectifiers can bo completely protected from unfavorable atmospheres by hermetic sealing. 4. Negligible forward aging combined with minute inverse leakage assures longterm stability in balanced circuits. 5. Considerable size reduction is possible because of conduction cooling design and reduced power losses. T h e inverse characteristics of all metallic rectifiers are temperature-dependent. Selenium rectifiers increase in leakage a t b o t h high and low temperat u r e s a n d m u s t be derated in inverse voltage when operated a t t e m p e r a t u r e s above 100° C. G e r m a n i u m rectifiers h a v e m u c h lower leakage currents a t room t e m p e r a t u r e s , b u t these increase rapidly with t e m p e r a t u r e . Above 65° C. t h e inverse voltage of g e r m a n i u m m u s t be derated to a large extent, a n d operation above 100° C. is not feasible. T h e inverse leakage in silicon power rectifiers is very low, well below 1 m a . a t 150° C. I t is believed t h a t with further improvement, operation can be ex1955
i f ·-- Hi ,i by Ralph H. Müller
tended to 200° C. N o voltage derating a t high t e m p e r a t u r e s is required. I n t h e case of silicon, t h e m a x i m u m inverse voltage (Zener voltage) actually increases with t e m p e r a t u r e . In a typical application, t h e use of two I N 4 1 2 silicon power rectifiers is shown in a full-wave center-tapped power supply. A small transformer supplies 35 volts r.m.s. either side of center t a p t o t h e two crystal units. A 10-henry choke a n d capacitor are used in t h e filter circuit. T h i s s u p p l y will furnish 25 to 30 volts d.c. a t 4 to 30 amperes. T h e voltage regulation is excellent, with approximately 0.1-volt drop per ampere of o u t p u t current. T h e device is particularly useful when used with magnetic amplifiers, which require rectifiers with high inverse resistance and stable high forward conductance. T h e inverse resistance of the silicon rectifiers below 125° C. is so high t h a t magnetic amplifier gain a n d sensitivity no longer need b e limited b y t h e rectifiers, b u t rather b y t h e magnetic materials a n d other associated circuitry. I n m a n y magnetic amplifier systems balanced rectifiers are required. T h e characteristics of t h e silicon rectifiers are good enough to eliminate t h e need for elaborate t e m p e r a t u r e compensation. Of t h e available silicon rectifiers, r a t ings u p to 500 inverse volts a t 400-ma. o u t p u t and 1 ampere a t 100 inverse volts are offered. Heavier current re55 A
INSTRUMENTATION
Industrial Mass Spectrometry
Here's how it pays off in sulfur recovery
By reporting around-the-clock on the concentration of H 2 S and SO., in sulfur-extraction plant tail gas, CEC's Type 21-610 Mass Spectrometers pay off in two important ways. By means of its continuous, accurate monitoring, the ratio of the two gases in the stream can be controlled within critical limits, which increases the profitable elemental sulfur output by as much as IV2 tons per day in a typical plant. And, important to community relations, air pollutants are reduced to an absolute minimum, for when the ratio is just right, the release of harmful, irritating sulfur compounds to the atmosphere is practically eliminated.
N o w . .. t w o process-monitoring mass spectrometers
mass 80
mass 150
TYPE 21-610 is moderately priced and a truly general-purpose instrument. Although primarily designed for continuous petroleum and petrochemical stream analysis, it is also valuable for productionline leak detection or laboratory analysis. Its applications have ranged from on-the-spot acetyleneplant monitoring to hospital clinical tests on lungs.
TYPE 21-620 has the highest mass range of any instrument in its compact size range. Using the new "Cycloidal Focusing" principle, it goes beyond the 21-610 for accurate readings from mass 2 to mass 150. Medical laboratories, petrochemical plants and general research organizations will all find it ideal for their analytical problems. Send for Bulletin CEC
1824A-X24.
Consolidated Engineering Corporation
E L E C T R O N I C I N S T R U M E N T S F O R M E A S U R E M E N T AND C O N T R O L
300 North Sierra Madre Villa, Pasadena 15, California Sales and Service Offices Located in: Albuquerque, Atlanta. Buffalo, Chicago, Dallas, Detroit, New York, Pasadena, Philadelphia, San Francisco, Seattle, Washington. D. C. For further informatian, circle number 56 A on Readers' Service Can), page G1 A
56 A
quirements can be met by high power rectifiers rated at up to 10 amperes at 125° C. continuous duty. Units handling up to 50 amperes are under development. The above described rating can be doubled when operating at 50° C. or below. It is pointed out that silicon, as a source material, is an abundant element, second only to oxygen, and its availability is limited only by refining facilities. Selenium and germanium, on the other hand, must be considered as critical materials. Silicon rectifiers are competitive in cost with high-temperature selenium rectifiers. They are more expensive than the latter in application at 50° C. and below, but this is partly offset by considerations of size, weight, and rectification efficiency. Motor Generators as D.C. Source One might imagine that the Electrical Age had provided us with all the possible sources suitable for the Air Age, Atomic Age, or Chemical Age, whichever we choose to consider ourselves to grace. New demands and applications, however, make the search unending. Solar cells are charging reserve storage batteries in communication systems and an atomic battery, using 500 millicuries of Sr*>, will deliver 8000 volts at a steady drain of 10~9 ampere. At the risk of proposing the revival of Victorian techniques, we bring up the question of motor generators as sources of d.c. Some very fancy and elegant circuit devices are available for an improvement of the principle. A synchronous motor can drive a generator, and to a first approximation the d.c. output will^depend only upon line-frequency stability. A shunt or compound generator has well known regulatory possibilities by automatic regulation of the field excitation. Frame temperature is a remaining problem, but thermistors used in conjunction with Thyrite or other nonlinear elements could compensate for these factors. Modern motors can be overloaded to an astonishing degree as a consequence of improved magnetic materials, high temperature insulation, and silicone lubricants. Sealed bearings contribute to these factors. With these improvements, considerable economy of space can result. ANALYTICAL
CHEMISTRY
oa
INSTRUMENTATION
complete process instrumentation
F&P improved glass pipe coupling
F & P IMPROVED GLASS PIPE COUPLING ELIMINATES BOLTS AND NUTS Greatly simplifies glass-to-metal and glass-toglass connections in glass pipe installations.
F&P has developed a new coupling which provides maximum flexibility of glass pipe installations. The ease of assembly and disassembly is the unique feature of this new method of achieving a positive pressure seal without the use of tools. Leakage and gasketing problems are eliminated by the perfect alignment and even take-up assured by this improved coupling. Naturally, the time required to change or install glass pipe sections is greatly reduced. Features of the F&P coupling are:
age or spillage, as pressure in the line cannot force the plug from the barrel of the stopcock. This stopcock will with stand greater pressure than the pipe itself. In addition to these unique glass prod ucts, F&P carries a complete stock of various elbows, tees and other standard fittings as well as straight lengths of glass pipe. Special fittings can be supplied to customers specifications. For further information send for our catalog
• Mo bolts, nuts, or flanges • Simplicity of installation and maintenance • Perfect alignment is assured Φ No tools required- can be fitted by hand 2 types available—glass-to-metal and glassto-glass . Sizes fo fit: V,
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Running noise would be a negligible factor, especially in fractional horse power ratings. Vacuum tube regulated generators have been in limited use since World War I. As a matter of fact, some of the flying "box-kites" of that era were equipped with a wind-driven generator mounted on a strut. In one design, a two-bladed propeller drove a generator mounted in a streamlined housing. The tail section of the housing contained a tungsten filament diode. The filament current and anode voltage were supplied by the generator and the current through the diode was used to control field excitation in the generator. Since the anode current increases exponentially with filament temperature, the output of the tube is extremely sensitive to generator speed. This arrangement maintained the generator output voltage at 115 volts with variations of no more than a volt at propeller speeds between 1000 and 15,000 r.p.m. For many years 400-cycle a.c. has been used in aircraft power plants be cause very considerable weight saving can be effected in using transformers, chokes, etc., designed for this higher frequency. For obvious reasons, en gineering laboratories in the aviation industry are wired and supplied for this frequency as well as the conventional 60 cycles. Inasmuch as a great variety of electrical and electronic equipment is available for operation at 400 cycles, more frequent installation of 400-cycle supply would seem to be advisable in research laboratories. Although our discussion has referred primarily to d.c. and rectification problems, the inter polation is not entirely pointless because filtering out residual ripple is also much easier and economical at the higher fre quency. The chemist is an important customer as far as electrical energy is concerned. He need not look on helplessly while electrical engineers ponder the future of power generation. After all, he is largely responsible for the dry cell, the storage battery, and the international standard of e.m.f.—the Weston cell. He may have many direct contributions to make in the future as logical improve ments upon the labors of Volta, Daniell, Faraday, and Edison. ANALYTICAL
CHEMISTRY
