An Electronic 1000-Cycle Sine Wave Oscillator MORRIS J . HELDMAN University of Southern California, Los Angeles, California
T
HIS paper describes a source of alternating current convenient for measuring the electrical conductivity of electrolytes. Microphone hummers and ordinary inexpensive electronic oscillators often have poor wave forms, especially when they are fed into low-resistance circuits, i. e., those which consume any appreciable amount of power. The instrument to be described produces a pure sine wave in any load from a few ohms up, without the use of bridging transformers. It can be constructed from ordinary and currently available radio parts a t a cost of materials not exceeding $10; an ordinary midget radio can be converted into such an oscillator for a few dollars a t most. It is completely free of mechanical noise and operates directly from 110 volts a. c. or d. c., in contrast to a microphone hummer. No adjustments are necessary once the instrument is finished; the frequency and amplitude of the pure sine wave form are stable indefinitely. Finally, repairs are simple and infrequent. For the reasons above, i t can be seen that the oscillator is adaptable to student and routine work, as well as many research applications. In measuring the conductivity of electrolytes i t is essential that the source of alternating current fulfill a t least two minimum requirements. (1) The frequency should be rapid enough to avoid polarization. One thousand cycles has generally been found satisfactory. (2) The wave form should be such that the current passing in one direction is equal to that in the opposite direction. Otherwise the residual current acts effectively as a direct one. One microphone hummer in use in the University's laboratory showed such a d.-c. residual. It has been shown that the conductivity of electrolytes between platinized electrodes depends slightly but appreciably upon frequency.' Thus a wave form meeting the criteria above but containing harmonics of the fundamental would tend to diffuse the minimum in the phones or other detecting devices. Even for student work, then, a sine wave is highly desirable. The device to be described meets the specifications above. It produces a sine wave of approximately 1000 cycles, with no noticeable distortion in the wave form when viewed on a 5-inch cathode ray oscilloscope. The wave form remains unchanged a t any external load from a few ohms up, with an output of about 0.5 volt a t a load of 10 ohms, rising to about 3 volts when loaded with 100 ohms or more. CONSTRUCTION
CI -20 Cz -20 Ca -20
mfd.-200-volt electrolytic condenser mfd.-200-volt electrolytic condenser mfd.-200-volt electrolytic condenser C4 4 0 mfd.-25-volt electrolytic condenser Cs 4 . 0 5 mfd.-200-volt tubular paper condenser Cs 4 . 0 5 mfd.-200-volt tubular paper condenser CY -0.05 mfd.-200-volt tubular DaDer condenser Cs -50 mmfd.-mica condenser Cg -50 mmfd -mica condenser 'Cm - 4 3 0 mmfd.-trimmer condenser R, -170 ohms; line cord, ballast tube, or 25-watt wire-wound resistor RS -30 o h m s 4 5-watt carbon resistor R8 -200 ohms-l-watt carboh resistor R4 -0 1 megohm4.5-watt carbon resistor R6 -800 ohm-l-watt carbon resistor Ra -0.1 megohm-0.6watt carbon resistor
' ~ & 3 megohms0.5-watt carbon reristot *RII-3 megohms-0.5-watt carbon resistor rl; -midget output transformer, pentode tube to voice coil Chl-mideet filter choke. 10 henries. 50 ma. * R L ~ L > - w a t t . 120-"olt tun&& , - mazda l a m.~.s candelabra base Sw,single-pole, single-throw switch ' VI -I2SJ7GT/G. Vg-12SK7GT/G. Vs-35L6GTIG. V435Z5GT/G. *See k t .
various disadvantages, among which are frequency drift, harmonic distortion, etc. The oscillator herein described operates on a somewhat diierent principle, namely, that of a resistance-capacitance Wien bridge. In recent years such bridge circuits have been used widely in commercial wide-frequency-range os~illators.~ These instruments are unnecessarily large, complex, and expensive for most routine conductivity work. For the present purpose neither a range of frequencies nor critical load-matching is necessary, as has been shown. Operation a t 1000 cycles makes i t possible to simplify the power supply, omit frequency tuning, decrease the size of certain components, and omit
Most audio oscillators of the past have been of either the Hartley or beat-frequency type. These have Several models are manufactured by Hewlett Packard ComTAYLORAND ACREE,I.Am. Chem. Soc., 38,2415 (1916). pany. Palo Alto, California. 553
shielding to such an extent that the entire oscillator is easily built on a 5 X 8-inch chassis. No shield box is necessary.
f
r I
In the schematic diagram, Figure 1, only the parts marked with an asterisk are critical as to value or adjustment. The other resistors, capacitors, etc., can he of any standard make and tolerance; higher voltagerating capacitors, larger wattage resistors, or a larger transformer or choke can he substituted if physical limitations permit. The lamps RL1 and RL2 should he of the type specified. For most efficient operation the values of Rlo and RI1 should he matched as closely as possible. Sufficient matching is assured by purchasing nominally priced resistors of 5 per cent tolerance. Because stray capacitances are unavoidable, Clois used to balance the two arms of the Wien bridge
s RC, the circuit. Since the resonant frequency is '/% oscillator can he constructed to produce other frequencies by using resistors and/or capacitors of different values. However, other aspects of the design limit the usefulness of the circuit to the middle audio range. 400 to 5000 cycles. Figure 2 is a photograph of one of these instruments in use in the University's physical chemistry laboratory. Although the placement of parts is not critical the one shown is satisfactory. Plate and grid leads should be kept as direct and short as possible, hut no other special precautions are necessary in the wiring. I t is convenient to mount all tube sockets, the transformer, choke, resistance lamp sockets, switch, etc., before wiring is begun. Thereafter a convenient order is power supply, filaments, grid and plate circuits, and finallv. ,. cathode circuits. A bottom cover is desirable. hut not essential. Any . type .. of mountina- or cabinet is satisfactory. After the instrument is assembled, the wave form can be checked and final adjustments made. A cathode ray oscilloscope is employed, and the output of the oscillator is fed directly into the vertical amplifier of the oscilloscope, in parallel with various resistive loads. Clo should be adjusted for maximum output. Although the value of 2500 ohms for RQworked well in the instrument described, it is advisable to try different values, finally using the largest resistance which still gives a pure sine wave with the lowest load that will be encountered in the oscillator's use. No further adjustments should he necessary. The instrument shown in the photograph has been in use by the writer and students for over six months without adjustment or repair. Another such oscillator has been built from an old a.-c., d.-c. midget radio by removing all coils and rewiring as shown in Figure 1. The additional cost was negligible, and the machine is operating satisfactorily.
