A New Development in Thermionic Relays HOWARD &I. W A D D L E AND WALTER SAE&IAN
Georgia School of Technology, Atlanta, Ga.
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thousand ohms is hooked across M. If there is danger of grounding the circuit through the control leads, PI or P I , another resistor, R,, may be inserted to avoid this danger. Its value should be about one tenth that of Ra. Connecting the control apparatus as shown in the main diagram causes the relay magnet to open when the control contacts close. The inset shows an alternative connecticln in which the relay magnet closes when the control contacts close. Ordinarily a 2000-ohm relay should operate !vel1 with about 5000 ohms on the grid bias resistor, Rt, but if the relay shows a tendency to stick when it should open, the grid bias may be increased. For each relay used with this circuit the grid bias resistance should be set high enough to permit the free opening of the relay when the control contacts close. Other factors which enter into the proper operation of this apparatus are the adjustment of the spring tension on the relay armature, and the modification of the contact between the armature and the pole pieces of the magnet. By cementing a thin piece of paper or mica over the pole pieces an air gap is introduced in the magnetic circuit and metal-to-metal contact is avoided. A small plate current must flow, even when the control contacts have closed, in order to produce a negative grid bias; and this current, although small in comparison with the plate current when the control contacts are open, nevertheless may produce sufficient magnetism to cause the armature t o stick if there is a closed, metallic, magnetic circuit. However, this latter adjustment is seldom necessary.
HE applications of radio tube relay circuits are numerous in experimental work. Controlling temperature in water and oil baths and regulating the pressure in vacuum distillation are suggested uses. I n general, the relay here described can be used with any apparatus in which i t is necessary to keep the current in the control circuits very low in order to avoid sparking or corrosion of electrical contacts, as in pressure-regulating manometers (1, 2 ) and thermoregulators ( 5 ) . The advantages of this relay circuit are low cost and simplicity of construction, wide range and ease of adjustability for use with different relays, fine sensitivity, and strong plate current. I n many of these respects it is an improvement over other types of relay circuits described in previous articles (3, 4,. Even relays made by rewinding lorn-priced electric bells with sufficient S o . 38 copper enameled wire to make their resistance 1500 ohms or more, worked verywell. The entire apparatus, using a rewound electric bell relay, has been constructed a t a cost of less than $3.00 for material.
The advantages claimed for this circuit are gained by using a more suitable amplifier tube, operating i t a t double the voltage used in the previously mentioned circuits (3, 4), and using a variable resistor in order to provide simple and accurate control of grid bias. The control circuit current is of the order of a few microamperes, and, when properly adjusted, the time lag between the control circuit and the relay circuit is negligible. Several different types of relays have been used with this circuit , including type ABTXl from Struthers and Dunn Company, and others which had been rewound to operate in this circuit, and all worked equally well. This develop ment should answer well the needs of workers who must depend on their own resources to build their control apparatus, or who do not wish to invest in the more expensive types of commercial relays now on the market.
Literature Cited (1) Bachman, G. B., IXD. ENG.CHnr., Anal. Ed., 7, 201 (1935). (2) Ferry, C. W., I b i d . , 10, 647 (1938). (3) Heisig, G . B., and Gernes, D. C., Ib?d., 6, 155-6 (1934). (4) Huntress, E. H., and Hershberg, E. B., I b i d . , 5 , 144,344 (1933). (5) Yee, J. Y . , and Davis, R. 0. E., I b i d . , 8, 477 (19:36).
This apparatus operates directly from 110-volt alternating current lines. The wiring diagram shows the manner in which the connections are made. TIis a 25115 rectifier tube used as a voltage doubler. CIand Ct are two 4-mfd. electrolytic condensers. If the relay armature shows a tendency to hum, a 1-mfd. condenser, C3, may be used, but this is seldom necessary since continuous rectified current flows through the relay. T z is a 6C5 amplifier tube whose maximum plate current can be raised t o about 15 milliamperes. R1 is a 40-watt Mazda lamp used as a filament resistor for the t w o tubes. Rz is a 10,000-ohm, IO-watt, variable resistor, and through it the grid bias voltage may be regulated. RBis a 2-megohm resistor used to prevent excessive currents from flowing through the control circuit. PI and Pt indicate the contact points in the control apparatus. A relay from one to several 225
