US2970227A

US2970227A - Voltage transfer switch

Info

Publication number: US2970227A
Application number: US655982A
Authority: US
Inventors: William F Horton; Jr Samuel H Auld
Original assignee: Lear Inc
Current assignee: Lear Inc
Priority date: 1957-04-30
Filing date: 1957-04-30
Publication date: 1961-01-31
Anticipated expiration: 1978-01-31

Description

Jan. 31,1961 w. F. HORTON ET AL 2,970,227

VOLTAGE TRANSFER SWITCH Filed April 50, 1957 I .Z'Z z-a-J /a 24 I /4 A j Awa 7 4040i Ill/11w; Alfluzo, Jet, MAL/1M A #0,? MM,

@6 1 %jMW United Sfttes Patent VOLTAGE TRANSFER SWITCH William F. Horton, Pacific Palisades, and Samuel H.

Auld, Jr., Canoga Park, Califl, assignors to Lear, Incorporated Filed Apr. 30, 1957, Ser. N0. 655,982

3 Claims. (Cl. 307-885) This invention relates to voltage control devices, and more particularly to a voltage transfer switch for connection between first and second points in a circuit to control the appearance at the second point of voltages produced at the first point.

In many applications, it is important that voltages developed at one point in a circuit be controllably introduced into another part of the circuit. One example is found in aircraft, Where flight error indicating signals in the output of a command receiver are applied to the input of an autopilot. When the autopilot input is first connected to the output of the receiver, the existing error signal may, for example, indicate that a sizeable turn is necessary to wipe out the error. If this voltage were applied directly to the autopilot, the plane might be turned sharply and create stresses which would cause substantial structural damage. To avoid such consequences, it is necessary that the voltage applied to the autopilot should build up gradually, whereby the plane can be put into the requisite turn smoothly. Presently known arrangements for accomplishing this purpose are characterized by moving parts; potentiometer means, such as a manually operated mechanical dash-pot or automatically operated motor-driven potentiometer, are usually employed. Such parts are of course subject to wear and tear and require frequent attention and maintenance to maintain a safe level of reliability. Furthermore, such equipment unduly taxes space and weight requirements.

It is an object of this invention to provide an improved voltage transfer switch having no moving parts.

It is another object of this invention to provide an improved voltage transfer switch for controlling the appearance at an input circuit of voltage in a preceding output circuit, which employs a minimum number of component parts of simple design, and which is smaller in size and lighter in weight than voltage transfer devices of the prior art.

Still another object of this invention is to provide an improved voltage transfer switch employing semiconductors, wherein parts are not subject to wear and tear, and which requires a minimum of repair and replacement of parts.

The above and other objects and advantages of this invention will become apparent from the following description, taken in conjunction with the accompanying drawing, in which preferred embodiments are illustrated by way of example. The scope of the invention is pointed out in the appended claims. In the drawing,

Fig. 1 is a schematic diagram of an improved voltage transfer switch employing a pair of transistors, in accordance with this invention, and

Fig. 2 is a schematic diagram, similar to Fig. l, of a voltage transfer switch employing a single transistor, further in accordance with this invention.

Briefly, this invention comprises transistor means connected between the output of one stage and the input of a succeeding stage, with a time constant or delay network connected to the base circuit of the transistor means.

A D.-C. voltage is applied to the delay network, which effects an exponential increase in current flow through the transistor means, and hence causes a gradual build-up at the input circuit of the succeeding stage of the voltage existing at the output of the preceding stage.

Referring to Fig. 1, a first pair of terminals 10, 12 represent the output terminals of one stage of an electrical circuit, and a second pair of

terminals

14, 16 represent the input terminals to a succeeding stage. A load 18 connected between

terminals

14, 16 represents the input impedance to the succeeding stage. A pair of

n-p-n junction transistors

20, 22 have their

collector electrodes

24, 26 connected to respective terminals 10, 14; terminals 12, 16 are directly connected. The

emitter electrodes

28, 29 of the

transistors

20, 22 are each connected to a point of reference or ground potential. The base electrodes 30-, 32 are directly connected, and their junction 34 is connected through a series connected resistor 36 and capacitor 38 to ground. Resistor 36 is a conventional current limiting device, to prevent excessive currents being conducted by the transistors which might damage them.

A source of D.-C. voltage, illustrated as a battery 40, has its positive terminal connected to a single-throw switch 42, and a resistor 44 is connected between switch 42 and the junction 46 of resistor 36 and capacitor 38. A grounded resistor 48 is connected to the switch side of resistor 44, and a unidirectional conductive device, shown as a diode 50, shunts resistor 44.

When switch 42 is closed, capacitor 38 charges exponentially, and hence the D.-C. voltage at the

bases

30, 32 increases exponentially. Since the dynamic impedance of each transistor decreases as the D.-C. voltage applied to its base increases, the conductivity of the transistors gradually increases from zero to a maximum. Assuming that a signal voltage exists across terminals 10, 12 when switch 42 is closed, a voltage appearing between

terminals

14, 16 builds up gradually from zero to a maximum corresponding to the magnitude of the signal voltage. The time for this build-up is governed by the time constant of capacitor 38 and

resistors

36 and 44.

When switch 42 is opened, capacitor 38 discharges through diode 50 and resistor 48. To permit rapid discharge, resistor 48 is much smaller than resistor 44; however, resistor 48 must be sufficiently large so as not to overheat while it is connected across source 40. This quick-discharge provision allows the load 18 to be quickly disconnected from the source of signals upon opening switch 42.

The back-to-back connections of the collector and emitter electrodes in Fig. l compensates for the unsymmetrical dynamic impedances of typical single transistors to current flow in opposite directions. It will be appreciated that the alternating voltage appearing between

terminals

14, 16 will resemble the signal voltages presented at terminals 10, 12 only if the impedances to positive and negative current excursions are substantially equal. Two transistors having substantially equal characteristics, and connected back-to-back as shown in Fig. 1, accomplish the desired result.

One voltage transfer switch as above described employs the following components:

Battery 40 28-volts. Resistor 36 20,000 ohms. Resistor 44 20,000 ohms. Capacitor 38 microfarads. Resistor 48 1,000 ohms.

A voltage transfer switch constructed with components having these values allows the build-up of voltage at ter- 0

minals

14, 16 to occupy a period of approximately one second.

Fig. 2 illustrates a voltage transfer switch employing a single transistor 20'. From the foregoing discussion, it will be apparent that this transistor should present the same impedance to current fiow in opposite directions. Transistors of this type are currently being developed, and this invention embraces the use of such in a voltage transfer switch.

Diode 50 in Fig. 2 is shown connected in the reverse direction to that in Fig. 1. This arrangement insures that capacitor 38 will be quickly charged, and gradually discharged.

Although n-p-n junction transistors have been described and illustrated for use in this invention, it will be apparent that pen-p junction transistors could be employed. In such case, it would be necessary only to connect switch 42 to the negative terminal of battery 49. Further, it will be apparent that the emitter-collector connections could be reversed, in which case the ground connection preferably would be made to terminals 12, 16.

What is claimed is:

1. A voltage transfer switch comprising first and second transistors each having emitter, collector and base electrodes, said base electrodes being directly connected, a first resistor and a capacitor connected in series between the junction of said base electrodes and a point of reference potential, a switching device, asecond resistor connected between said switching device and the junction of said first resistor and said capacitor, a resistive connection between the junction of said switching device and said second resistor to said point of reference potential, at unidirectionally conductive device shunting said second resistor, first and second pairs of terminals each having a respective load impedance connected therebetween, said emitter and collector electrodes being connected in series between respective terminals of each pair so that the emitter-collector paths of said transistor are in back-toback relation, and means to provide D.-C. path from said emitter electrodes to the point of reference potential.

2. A voltage transfer switch as defined in claim 1, wherein the unidirectionally conductive device is connected in the forward direction between the junction of said first resistor and capacitor and the junction of said switch and said second resistor.

3. The voltage transfer switch as defined in claim 1, wherein the unidirectionally conductive device is connected in the forward direction between the junction of said switching device and said second resistor and the junction of said first resistor and capacitor.

References Cited in the file of this patent UNITED STATES PATENTS 2,584,990 Dimond Feb. 12, 1952 2,628,330 Williams Feb. 10, 1953 2,703,368 Wrathall Mar. 1, 1955 2,763,832 Shockley Sept. 18, 1956 2,823,322 Trousdale Feb. 11, 1958 2,899,571 Myers Aug. 11, 1959 OTHER REFERENCES Pub. I Junction Transistors Used as Switches, by

Bright, A.I.E.E. Transactions, part I Communications and Electronics, vol. 74 March 1955, pages 119-120.

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