Electronic Timer IRA C. BECHTOLD' National Bureau of Standards, Washington, D. C.
R = value of shunt resistance in ohms C = capacity in farads = Xapierian logarithmic base
This paper describes a resistaiice-capacilance circuit which, when used in conjunction with a thyratron-type tube, provides a time-delay relay instantly adjustable and capable of operating mechanical devices automatically at predetermined time intervals.
Factor e given in Equation 1 as the potential to which the condenser falls in time t, is also the critical grid potential below which the thyratron will conduct a plate current. To determine its value in the absence of experimental data it is necessary to refer t o the characteristics chart of the 885 (or 884) tube. For any given values of E antl e the time interval may be made almost any chosen value, since a large number of combinations of R and C may be obtained from parts which are commercially available. Furthermore, the time interval may he controlled b y utilizing variable components for either R or C or both. Figure 1 shows the diagrain of a circuit which will deliver :in electrical pulse a t definite time intervals, 1% ith instant control of the interval by the aid of a calibrated dial.
I
S THE control of laboratory apparatus and in certain types (Jf industrial control problems, it is frequently desirable t o operate mechanical devices or electrical circuits in a cyclical manner with predetermined time intervals. Usually it is required that control devices be of such a nature that they will operate automatically antl without attention over long periods of t'iiiie. An obvious means of accomplishing this end is t o employ :I motor-driven rotary switch which develops the required tiniing pulses. Such devices are not usually capable of instant variation over wide ranges of time intervals and frequently require changing gearr or other speed-reduction elements. Rotating contacts are not always reliable for continuow operat,ion, especially when relatively large currents are being interrupted. Thermally operated devices are also available hut they usually present, t'he same disadvantages as ha\-e heell attributed to rotary apparat,us. A less commonly used method of timing is one \\-hich relies upon the time characteristics of :t i.esistance-c:tpacitance network for control of the time interval. Such a network, v41e1iused in conjunction with a tube of the gas-filled or thyratron type, will provide a time-delay relay which is capable of controlling :tn electromechanical relay having a currentcurrying capacity of considerable magnitude.
The apparatus described here is essentially the same as that described by Goldberg ( 3 ) for a specific use in photography. Similar circuits with other adaptations have been described by Gilson ( 2 ) , Smiley (5), and Mucher (4). Of course, the fundament:il resistance-capacitance netviork is well known and is thoroughly treated in textbooks on the subject as well as in summaries in handbooks and similar publications ( 1 ) . The principal advantage of t'he present apparatus is that it provides a repeated time pulse automatically, whereas that used by Goldberg requires resetting after each operation.
The diagram indicates the use of type 885 gas triode with a 2.5-volt filament supply. The type 884 gas triode which requires 11 6.3-volt filament supply may be substituted for type 885 if so desired. The parts used are all available through the usual radio trade channels. In certain instances manufacturers will supply equivalent parts whirh mag he Substituted. Care should be taken to secure apparatus of good design and construction. This is partirularlg true of the R-C network components where cvndensers of low leakage value and good stability to atmospheric changes are required if accurate and stable calibration is desired. Hence, rare shonld be taken t o see that the desired rapacities are
8 8 5 SOCKET
\Then a condenser is allowed to discharge through a resistance shunted across the condenser the time interval required for the condenser voltage t o reach a given level is expressed by
010
FIGURE 1. WIRISCDIAC:HAM 3 Western Electric telephone type condenwrq (pappr), i c t u a l measured values 2 23 2.18 1 10 = 5.51 total pfd. C ,. 8-ufd. 250 WV electrolvtic. Aerovox PBS-8 1-ufd. 450 WV paper cs . I-megohm potentiometer, Centralnb No,6 tape, Ri. Ri. 50,000-ohm 1-watt carbon Rzl Ra. 10,000-ohm 1-watt carbons T. 2.5-volt, 2.5-ampere filament transformer, Stancor 1'61.11 Re. Double-pole double-throw relay, 2000-ohm coil, Guardian 885. RC.l T y p e 885 gas-filled triode F. 2.0-ampere Littelfuse SI. Single-pole single-throw switch S?, Single-pole single-throti rwitch. rionlockinu, iin iuiinual control ('I.
wtirrc
1
E
=
e
=
voltage to which condenser is initially charged voltage to which condenser falls in time t
I'rment address, T h e t ' l u o i Corporation, T,td., 1,os
Aiiei~lt... Calif.
+
429
+
430
INDUSTRIAL AND ENGINEERING CHEMISTRY
obtained. Frequently, "rated" or "replacement" values are given for paper condensers which are much greater than the actual effective capacities. Potentiometer RI should also be of good construction, so that it will repeat its setting and will not be subject to sudden and unpredictable resistance variations. The relay, Re, should be carefully selected, since many of the cheaper varieties are not reliable and may fail to make contact at times because of poor mechanical construction.
The operation of the circuit is simple and may be described by folloning through a complete cycle. Let us start at that point in the cycle where a plate current has just started t o flow and before the relay has had time to close. The thyratron acts as a half-wave rectifier. Because of the current in the plate circuit the relay coil is energized and there is also a potential drop across the relay coil and R4 as indicated by the signs of Cz. As soon as the relay contacts have closed, this potential drop acts as an electromotive force in the circuit containing condenser Ct, the terminals of which are connected to the grid and cathode of the thyratron. As a consequence the condenser is charged and the potential of the grid becomes decidedly negative with respect to the cathode. This negative potential of the grid has no effect on the plate current during the time the thyratron is ionized, However, it prevents reionization following the deionization which occurs during that half of each current cycle in which the plate is negative. The rectified current, therefore, ceases and relay contacts PI and PZopen, leaving condenser Ct charged. This charge then leaks off through R1 and Ra according to Equation l, and as it does the potential of the grid becomes less negative. After the critical value is reached the tube reionizes as soon as the potential of the plate reaches a peak in the positive direction, thus completing the cycle. The relay is de-energized most of the time. It receives short pulses at time intervals determined by grid circuit elements C1, R1,and RP. If it is desired to have current flowing in the load circuit during the discharge interval instead of a momentary pulse,
Vol. 14, No. 5
the "back contacts" of the relay may be used. The values given for R1,Rn,R4,and C1 provide a time-interval range from 140 to 9 pulses per minute over the range of adjustment of potentiometer Rt. Resistance R2determines the minimum time interval available. Changing Ra will change the time-interval range because of changes produced in the charging voltage, E. If a greater range is desired, a group of resistors may be used with a selector switch as shown by Goldberg (S), or other condensers may be switched into the circuit by similar means. The relay indicated will carry currents up to 5 am eres at 110 volts through its contacts. If the controlled l o a f requires a p t e r current, other relays may be selected or a second relay of igh current-carrying capacity may be operated from points PI. Either alternating or direct current may be used for load operations. I n the particular application shown in Figure 1 the controlled load was actuated by current from a 110-volt direct current source. Condenser Csis used to prevent arcing a t oints PI. This condenser should be selected to have the best vake for the particular reactance and resistance characteristics of the controlled load.
The apparatus described here may be applied to a wide variety of timing operations. For time intervals not available with the components shown in Figure 1 it is only necessary to replace R1, RP,and C1with components having the desired values which may be calculated from Equation 1.
Literature Cited (1) Electronics, Reference Charts, 14,No. 6 (1941). (2) Gilson, W.E., Photo Technique, 2, Part 2, 28 (1940). (3) Goldberg, H., Ibid., 3,Part 1, 56 (1941). (4) Mucher, G., Electronics, 9, No. 4, 38 (1936). ( 5 ) Smiley, G., Ibid., 14,X o . 1, 29 (1941). PAPERNo. 41 of the Portland Cement Association Fellowship a t the National Bureau of Standards.
Design for Rectifying Column E. 0. RAMLER AND J. H. SIMONS School of Chemistry-and Physics, The Pennsylvania State College, State College, Penna.
FP10"
rectification of boiling in the range -60" C., there is frequent need for a smalllow-temperature column which can be readily moved about the laboratory or put away when not in use. Such a column has been described for materials boiling around -50" C. or below, but vhile simple to operate, it is not easy for the average amateur glass blower t o construct. The low-temperature column here described is a simple modification of that described by Simons ( f ) , and has been used successfully in this laboratory with liquids boiling at -30" and -5" C. I n Figure 1 is a diagram of the column, which is about 45 cm. (18 inches) in height and fits conveniently into an ordinary quart vacuum flask. A is the liquid container with a volume of about 50 cc. Sealed on the bottom is the nipple, B, which is wound with a heating coil of asbestos-covered Nichrome wire, B. & S. S o . 26. The packed section,,C, of the column is 9-mm. tubing and contains glass helices supported by a small glass cross bar. D is the take-off tube in which is sealed a small thermocouple well, contaming a copper-constantan couple. The condenser, E , consists of two concentric tubes of 42and 37-mm. outside diameter, sealed at the top. The 7-mm. tube, F , sealed at the top
of the condenser, is to permit the escape of air as the material is condensed in A , The fractions may be obtained by the use of a three-wav StODcock on D. or of a chain of traDs in series. The column is very simple to operate. It is placed in a quart-size unsilvered Dewar flask, which is surrounded by a radiation shield of aluminum sheet containing slots cut into it for observation of the pot and reflux. Leaving tube F open, the material is condensed in A through the take-off tube with the aid of the cooling mixture in E and in the Dewar flask. The Dewar flask is now removed, emptied, and replaced around the column. With D closed, A is heated if necessary to bring about a reflux, When equilibrium is established, F is closed and D is opened, and the condensate is received in a series of traps, the last one of which is open to the air. If the take-off is too slow, it may be increased by the use of a very slight vacuum obtained from an ordinary laboratory watersuction pump, applied to the last trap through a stopcock. The amount of take-off can be regulated by adjustments of both the heat supplied to the pot and the vacuum on the receiver. The temperature a t the top of the column can be determined with a potentiometer.
Literature Cited FIGURE1
(1) Simons, IND.ENQ. CHEM.,ANAL.ED.,10, 648 (1938).
