A Pulse Generator for Circuit TestingH. P. Manning and V. J. Young Citation: Review of Scientific Instruments 13, 234 (1942); doi: 10.1063/1.1770021 View online: http://dx.doi.org/10.1063/1.1770021 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/13/5?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Logarithmic pulse generator for long-term creep and relaxation testing Rev. Sci. Instrum. 76, 056102 (2005); 10.1063/1.1897667 A new pulse compression circuit for low impedance pulse power generation Rev. Sci. Instrum. 66, 5640 (1995); 10.1063/1.1146034 Digital Transistor Circuit for Generation of Timing Pulses for Pulsed NMR Rev. Sci. Instrum. 33, 96 (1962); 10.1063/1.1717674 Spectrum Generator for Testing of PulseHeight Analyzers Rev. Sci. Instrum. 30, 805 (1959); 10.1063/1.1716759 Generator of Nanosecond Light Pulses for Phototube Testing Rev. Sci. Instrum. 30, 31 (1959); 10.1063/1.1716351

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234 LABORATORY AND SHOP NOTES

. TABLE I. Performance tests and approximate circuit constants.

Lamp 20 watt 40 watt 100 watt

Input volts 115 230 230 Lamp volts (rated) 62 108 72 Lamp current (rated) 0.35 amp. 0.41 1.45 Output volts 63 120 95 R,+R, (ohms) 140 250 80 R, (ohms) 16 39 12 Maximum load current* 0.4 amp. 1.0 2.5 Stabilization ratio ( '" lie;/ lieo)

for ±5 percent lie; 100 100 50 Minimum recommended tube

current (amp.) 0.20 0.30 1.20 Maximum recommended tube

current (amp.) 0.50 0.50 1.70

* Load current for which the lamp goes out with a S-percent decrease in input voltage.

constant temperature, are listed·in Table I. These measure­ments were made with a time interval of twenty seconds between an abrupt changing of the input voltage and the observation of the output voltage. The stabilization ratio, defined as the ratio of a 5-percent change in input voltage to the resulting percent change in output voltage, is roughly independent of load current. A resistive load was used in making these measurements.

With a capacitive or an inductive load, the stabilization ratio is very nearly the same as with a resistive load.

For the time intervals of! to 20 seconds, the stabilization ratio is less in each case than that given in Table I, being perhaps as small as 20. For most applications the input voltage changes gradually rather than abruptly, and with such input fluctuations the stabilization ratio is always high. For time intervals of one hour the output voltage drifts by as much as one percent. Of course, with two stabilizer units in series, e.g., the 100-watt lamp supplying the 40-watt lamp, the over-all stabilization ratio should be much greater than the ratios of the single units. (Experi­mentally it has been found that the over-all stabilization ratio is considerably less than the product of the ratios of the single units, however.)

We have used the stabilizer satisfactorily to supply, through a transformer, the power to heat a filament of an ionization gauge, and of a magnetron tube, in an advanced teaching laboratory. With the ionization gauge, a ten­percent change in the input voltage was accompanied by a t-percent change in the grid current of the ionization gauge. Use of the stabilizer is also quite satisfactory in applications requiring an appreciable change in the load current if the resistances are readjusted each time such a current change is made. An ammeter in series with the lamp is a convenient indicator of proper adjustment, a 5-percent change in lamp voltage being accompanied by about a 25-percent change in the lamp current.

A relay switch is provided, as indicated in Fig. 1, to prevent overload output voltage when the lamp is not in discharge. R. is chosen to provide constant lamp current whether the relay is open or closed.

Needless to say, the stabilizer also provides, as a by­product, very satisfactory illumination.

, C. Morton. J. Sci. Inst. 14. 161 (1937). has used several neon type tu bes in voltage stabilization.

A Pulse Generator for Circuit Testing H. P. MANNING AND V. J. YOUNG

Department of Physics. New York University. New York. New York March 9. 1942

T HIS device is useful for checking the many circuits commonly used with Geiger counters and ionization

chambers sensitive to single particles. It puts out a pulse which may be conveniently varied in duration, voltage, and frequency so as to simulate actual operating conditions in the scaler, coincident circuit, or other unit under test.

Cl and Rl make possible a continuous frequency range from 0.8 to 30,000 pulses per sec. C. allows a choice of pulse breadths varying from 40 microseconds to 3 milli­seconds. R. varies the pulse voltage from zero to 150 volts. The reversing switch changes the polarity of the pulse without changing its shape or disturbing the ground con­nections. All five controls are essentially independent of each other.

FIG. 1. Circuit diagram.

Ri. l~megohm potentiometer; R2, SOOO-9hm potentiometer; R 3 ,

500.000 ohms; R •• 15.000 ohms; Rs, 1000 ohms; R,. 50.000 ohms; R7. 10.000 ohms; R •• 100.000 ohms; Ro. 1 megohm; RIO. 100.000 ohms; RH. 100.000 ohms; R12. 500.000 ohms; R13. 25.000 ohms; R". 50.000 ohms; R15. 600.000 ohms; R16, 12.500 ohms; R17. 1 megohm.

C,. 9'position switch with capacity of 2 I'f; 1 I'f; 0.4 I'f; 0.15 "f; 0.065 I'f; 0.03 "f; 0.01 I'f; 0.003 "f; 0.0007 1'1.

C,. 5-position switch-{).OI I'f; 1450 I'l'f; 500 I'l'f; 250 I'l'f; <1001'"f. C" 0.001 I'f; C. =C,. 0.5 I'f; C. =0.05 "I.

A Vacuum Tube Intensity Meter ROBERT B. TAFT

103 Rutledge Avenue, Charleston. South Carolina January 16. 1942

A N amplifier is shown here which uses an ordinary radio tube, is rugged and reasonably stable, but has a

high enough sensitivity to produce deflections on a milli­ammeter when activated by ionization currents from a thimble chamber placed in an x-'ray beam.

The type 1 T4 tube is used but it is absolutely essential that this be selected and properly treated to give the required sensitivity. A test circuit is set up with voltages and bias resistor as shown in Fig. 1, but with a 0-10 ma meter in the plate circuit and without the ionization chamber and switches. At first, the tube draws about 4 ma;

. however, some tubes will fall to about 1.4 ma in 2! hours and others will continue to draw the same current. Only those tubes which show the current drop are suitable,

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NEW INSTRUMENTS 235

FIG. 1. \Viring diagram.

though all may check identically on the commercial tube checker both before and after treatment.

The circuit is grounded to the shield at the positive side of the B battery which eliminates the nuisance of having the ionization chamber at a different potential from the cabinet. The A battery is used to buck the normal plate current through the meter so that it may be returned to zero, thus enabling the meter to indicate only the increased current caused by ionization. The switching system allows a 0-100 microammeter to be used for small ionization currents, or to be shunted to read 0-1 ma for larger cur­rents. For the measurement of such x-ray intensities as are used clinically, one may use a 0-1 ma meter.

Tube, socket, and grid resistor are placed in a small metal box fastened to the ionization chamber stem. (See Fig. 2.) This box is filled with melted paraffin which prevents air ionization as well as surface leakage. The box is connected by 30 feet of three-wire shielded microphone cable to the steel cabinet which houses the batteries, switches, and meter. Batteries recommended are: one Burgess XX45 or Eveready 467; and one Burgess 4F or Eveready 742. Two "Penlite" cells are used for the bias.

Only a few minutes are needed for the circuit to stabilize, after which time there is practically no drift even on the microampere scale.

FIG. 2. Complete instrument. Metal box attached to ionization chamber contains vacuum tube, socket. and grid resistor. Cable con­nects this to cabinet containing batteries, meter, and switches.

\Vhen used with a lucite thimble chamber, volume 1.6 cc, and aluminum wire central electrode, the following readings are obtained from x-r~diation generated by 140 kv: 86.4 r/minute, meter reads 0.93 ma; 1.5 r/minute, meter reads 8 microamperes.

While this device wiIl not displace the x-ray measuring instruments now in use, it appears that an instrument which can measure the intensity of an x-ray beam with an extremely small time lag should be useful to the radiologist as well as the physicist.

New Instruments Section Editor: Wm. F. Roeser.

National Bureau of Standards, Washington, D. C.

These descriptions are based on information supplied by the manu­jacturer and in some cases from ipdependent sources. THE REVIEW assumes _no_responsibility for their correctness.

Duo-Seal Pumps The Welch Duo-Seal pump consists essentially of a tight

metal case in which is mounted the pump unit-a sta­tionary cylinder, or stator, with a cylindrical hole in which another smaller cylinder (the rotor), mounted eccentrically with reference to the stator but concentrically with reference to its own periphery, is free to turn by means of a shaft connected to the driving mechanism. The contact point of rotor and stator is a cylindrical seat milled into the stator with the same radius as the rotor, and located between the inlet and the outlet of the pump. This is called the seal. The rotor is provided with two cast-iron vanes mounted diametrically opposite each other, with a stiff spring between, which presses them against the inner wall of the stator and makes them move in and out of their slots as the rotor revolves, thus sweeping the crescent­

shaped air space twice each revolution. As the stator is of harder material than the vanes, the latter are the ones which wear, although the oil film reduces this wear to a very insignificant minimum.

The method of operation is simple. As a vane leaves the seal moving in a clockwise direction, it begins to sweep the air-filled, crescent-shaped space and forces the air out through the outlet, the latter being provided with a ball check valve. A patent feature, called the duo-seal, provides a by-pass at the seal to carry off the last increment of gas which may escape the exhaust port. This operation is repeated twice each revolution, or at least 600 times per minute, and with each repetition the pressure in the vessel connected to the intake is reduced and, therefore, a higher and higher vacuum formed.

Evidently this operating combination of stator, rotor, and vanes constitutes the vital part of the pump. One of these units is used in the single-stage pumps while two connected in series are mounted on the same drive shaft in the two-stage pumps.

The casing is designed with fins to provide air-cooling. This means there is no oil jacket, as such, the oil reservoir being located on top. Therefore, the design and the use of

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