US2889470A - Inverter - Google Patents

Description

Jung 2, 1 959 fw. GRAY ET AL 2,889,470

INVERTER FiledJune 25, 1958 POTENTIAL LEFT RIC HT LEFT RIGHT LEFT RIGHT lfo f T 7 T T T Y JNVENTORS i JOHN w. GRAY .Z 9 BY 1 HENRY HUNTER m ATTORNEY.

ates

2,889,470 Patented June 2, 1959 2,sss,47e

INVERTER John W. Gray and Henry L. Hunter 3rd, Pleasantvilie, N.Y., assignors to General Precision Laboratory In corporated, a corporation of New York Application June 25, 1958, Serial No. 744,437

Claims. (til. 307-408) This invention relates to devices for inverting electrical signals and more specifically to a device which emits a signal similar, in current or voltage waveform, to an impressed signal but having at each instant the opposite polarity or sense.

An obvious form of inverter which has been much used consists of an electronic amplifier having an odd number of stages. Amplification may be controlled or eliminated by feedback with retention of the inversion property. When a varying potential signal is applied to such an inverter the signal is reproduced at the amplifier output with some degree of fidelity but with the waveform inverted, so that, for example, each positive peak of the input is reproduced as a negative peak.

The present invention accomplishes the same result but with a small fraction of the number of components of an electronic or transistor amplifier. It accomplishes this function Without amplification, with some degradation of waveform and with increases in output impedance and in time constant. These characteristics, however, may be relatively unimportant and the device compares favorably with older devices in some uses. The present invention additionally may be employed to isolate the output from the input, either with or without inversion of the waveform.

The present device consists of a vibrator or chopper and two capacitors, together with resistors, if desired, to protect the vibrator contacts from capacitor discharge current damage.

The purpose of the invention is to provide an improved electrical inverter and isolator.

A further understanding of the invention may be secured from the detailed description and drawings, in which:

Fig. 1 is the schematic circuit of an embodiment of the invention.

Figs. 2 and 3 illustrate graphs depicting the operation of the circuit of Fig. 1.

Fig. 4 is the schematic circuit of another embodiment of the invention.

Referring now to Fig. l, a two-arm vibrator or chopper comprises a solenoid 11 and two contact arms 12 and 13. These arms are operated in concert by the solenoid. The solenoid 11 is connected at terminals 14 to a source of electrical power having a selected frequency such as 60 or 400 c.p.s. This frequency may be made as high as 1000 c.p.s. with presently-available choppers. The contact arms 12 and 13 make contact in concert on one side with fixed contacts 16 and 17 respectively, and on the other side with contacts 18 and iii. The only provision regarding the accuracy of this contacting action is that one arm must leave its contact on one side before the other arm makes contact on the other side. The two contact arms 12 and 13 are electrically coupled by a capacitor 21 in series with a resistor 22 of low resistance R. This resistor has the function of limitin capacitor charge and discharge currents enough to prevent injury to the chopper contacts. The fixed contact 16 is connected to one input terminal 23 and the fixed contact 17 is connected to the other input terminal 24. The fixed contact 19 is connected to one output terminal 26 and the fixed contact 18 is connected to the other output terminal 27. In general the input terminals 23 and 24 can be completely independent of the output terminals 26 and 27. However, if the inverter input and output have a common terminal this is symbolized by the dashed line 28 and the potential reference datum thereof can be conventionally shown and termed as ground potential. A capacitor 29 is connected between the output terminals 26 and 27. This capacitor preferably has a capacitance several times that of capacitor 21. The output load is assumed, for the purpose of description, to be purely resistive. It is symbolized by the resistor 31 connected between the output terminals 26 and 27.

The input signal E, is applied between input terminals 23 and 24. Alternatively, when input and output terminals are independent, two separate input terminal potentials e, and e at terminals 23 and 24 respectively may be referred to one datum potential and the output ter minal potentials 6 and a, may be referred to another datum potential different from and entirely independent of the input datum potential. Thus, instead of a single input potential E two independently varying potentials e and e are applied to the two input terminals 23 and 24 and the effective input potential at any instant will be the difference of e, and c or E =e -e Similarly the output potential, E may be measured between terminals 26 and 27 and is the difference of each measured to the datum potential. When the circuit is used as an inverter the output sign is the opposite of the input sign, so that 2 and e, are the datum reference potentials and and E0: -E

The input signal potential, E is limited in its permissible rate of change to that imposed by the time constant of the circuit. This time constant equals the resistance R, of the equivalent circuit multiplied by the capacitance, C of capacitor 29. The input signal frequency is limited to a value much less than the frequency of chopper operation. An equally important limitation on the input signal rate of change, however, is that it shall always be less than the transient rate of charge of the capacitor 29 in combination with the action of the chopper. This action and the nature of the charging transient will be described later. If the rate of change of the input signal is greater than prescribed by these limitations, the output waveform will not be a faithful reproduction of the input waveform.

In describing the operation of this circuit, however, an input signal having an abrupt change of voltage, a voltage step, is selected in order to make clear how the output voltage varies with time to constitute a transient charging voltage. This input voltage step is negative and is depicted in Fig. 2 by graph 8. In this graph the impressed voltage, E is zero from time t to time 2 The several cycles of operation of the chopper arms indicated at A do not, during this time, affect the potentials of the capacitors 2i and 29, both terminals of each remaining at zero and the output potential, E remaining at zero.

At time t, the terminal 23 abruptly becomes negative by potential V relative to terminal 24, so that and these potentials are maintained thereafter. This change is depicted by the step 32. At time t when the arms 12 and 13 next touch the input fixed contacts 16 and 17, capacitor 21 commences charging toward V. The potential change across the capacitor 21 is depicted in graph C, Fig. 2, by curve 33. This curve shows the potential of the left or input capacitor terminal relative to that of the right or output terminal. Charging is not instantaneous but exponential, with a time constant RC in which R is the resistance of resistor 22 and C is the capacitance of capacitor 21. The graph C is drawn to show operation when the time constant RC is materially less than the period of one-half cycle, but the circuit operates effectively even when this time constant is greater than the half period.

The contact arms now move to the right and at time t make contact with fixed output contacts 18 and 19. Capacitor 21 is thus connected for transfer of its charge to the capacitor 29. With the connection 28 absent, polarities can be chosen so that the output terminal 26 and the associated side of capacitor 29 can be connected to either side of capacitor 21. The connection is chosen, in this example, so that the potential sense of the terminal 26 relative to terminal 27 will be opposite to the sense of input potential 23 relative to terminal 24. The device thus acts as an inverter of the potential polarity.

The charge in capacitor 21 now distributes itself between capacitors 21 and 29 in inverse ratio to their ca pacitances C and C the change being exponential as indicated by curve 34-. Capacitor 21 is discharged to a potential less than zero by the amount This is also the negative potential to which the 2 terminal 27 of capacitor 29 is charged relative to its other terminal 26. If the former terminal 27 be taken as the output datum terminal, the latter terminal 26 is raised to a positive potential by that amount. This is indicated in graph D by the rise 36. When, at time 13 the arms again move toward their input contacts the output terminals and capacitor 29 are isolated and C retains its potential.

At time 1 the arms 12 and 13 make input contact again, and again charge capacitor 21 to V as indicated at curve 37, graph C. At time t the capacitors are again joined but now capacitor 29, already being partly charged, takes less charge and its increment of potential change is less than before. It is equal to the reduction of potential 38 of capacitor 21 and is indicated at rise 39.

Thus each increment of potential added to capacitor 29 is less than the last and the sum of all increments, increasing exponentially, approaches V as the limit under infinite resistance load conditions. This is depicted in Fig. 3 which is merely the graph D of Fig. 2 redrawn to a much smaller scale.

If the rate of change of the input signal be less than the maximum slope of this exponential rise of Fig. 3 the output E will reproduce the input E, with reasonable faithfulness, assuming no load. In a relatively few cycles of the chopper the output potential will be substantially that of the inverted input.

In general, the resistance, R of the load resistor 31 is less than infinite. The combination of the chopper and capacitor 21 then behaves as if it were a resistor in an equivalent circuit having the value E after its transient has died out, is no longer substantially equal to E,, but is Rae.

A second embodiment has the advantage of requiring only a single-arm chopper. This form of the invention is depicted in Fig. 4, in which the input signal E, is applied through a low-resistance resistor 41 to a small capacitor 42. The other side of capacitor 42 is connected through a resistor 43 to a capacitor 44 the other side of which is grounded. An output voltage E is taken from across capacitor 44 and applied to a load 46. A one-arm 4 chopper or vibrator comprises a solenoid 47 operated on an alternating supply connected to terminals 48 and a contact arm 49 with fixed contacts 51 and 52 connected to capacitor 42.

Resistor 43 has a relatively high resistance R and the resistance R of the load 46 to which E is applied is preferably at least as large as that of resistor 43. The time constant R C in which R is the resistance of resistor 43 and C is the capacitance of capacitor 42, is preferably not less than twice the chopper period. The time constant R C in which C is the capacitance of capacitor 44, is substantially the output time constant of the circuit, and is preferably large.

In the operation of this circuit, when arm 49 dwells on contact 51 it grounds the left side of capacitor 42. Assuming equal dwells on contacts 51 and 52 and assuming zero travel time, the left side of capacitor 42 is at the potential E for one-half of the time and at zero for the remainder of the time. The right side of this capacitor is then, with infinite impedance loads, at E for onetalf of the time and at zero for the remainder, so that the average potential is /z E,. The long time constant of resistor 43 and capacitor 44 integrates or smooths the potential E which thus exponentially rises toward the asymptote value /2 E.

When a less than infinite resistance load is applied, by reasoning similar to that for the circuit of Fig. 1, the steady state output potential is shown to be What is claimed is:

1. An inverter comprising, an input circuit and an output circuit having a common terminal, a charging capacitor, means periodically applying the potential of said input circuit to a first terminal of said capacitor and alternately connecting said first terminal to the common terminal of said input and output circuit, a storage capacitor connected in shunt to said output circuit, and means applying the potential existing at a second terminal of said charging capacitor to that terminal of said storage capacitor which is remote from the common terminal of said input and output circuit during the periods in which the first terminal of said charging capacitor is connected to said common terminal whereby the potential of said input circuit is transferred to said output circuit in the opposite sense.

2. An inverter comprising, an input circuit and an output circuit having a common terminal, a charging capacitor, a vibrating contactor periodically applying the potential existing at said input circuit to a first terminal of said capacitor during a first time interval and connecting said first terminal to the common terminal of said input and output circuit during a second time interval, a storage capacitor connected in shunt to said output circuit, and means including said vibrating contactor for applying the potential existing at a second terminal of said charging capacitor to the terminal of said storage capacitor which is remote from the common terminal of said input and output circuit during said second time interval.

3. An inverter as set forth in claim 2 in which the capacity of said storage capacitor is several times that of said charging capacitor and the charging time constant of said charging capacitor is a fraction of said first time interval.

4. An inverter comprising, an input circuit and a storage capacitor each having a pair of terminals with one terminal connected in common, a charging capacitor, means alternately connecting one terminal of said charging capacitor to the alternate ones of said pair of input terminals and simultaneously and alternately connecting the other terminal of said charging capacitor to the alternate ones of said pair of storage capacitor terminals,

and an output circuit connected in shunt to said storage capacitor.

5. An inverter comprising, an input signal circuit having first and second terminals, a capacitor having first and second terminals, a storage capacitor having first and second terminals constituting circuit output terminals, said input signal circuit second terminal and storage capacitor second terminal being conductively connected, first means periodically connecting said capacitor first terminal to said first and second input circuit terminals in alternation at a selected frequency, and second means periodically connecting said capacitor second terminal to said second input circuit terminal and to said storage capacitor first terminal in alternation at said selected frequency, the capacitor first terminal being connected to the input first terminal during the interval that the capacitor second terminal is connected to the input second terminal.

6. An inverter comprising, a capacitor, a grounded input circuit, a double-arm double-throw vibrator, one arm thereof alternately connecting one terminal of said capacitor to the ungrounded and grounded terminals of said input circuits, a grounded storage capacitor, the other arm of said vibrator alternately connecting the other terminal of said capacitor to ground and to said storage capacitor, whereby the storage capacitor is incrementally charged toward an asymptote potential equal to said input circuit potential but of opposite sense.

7. An inverter comprising, a capacitor having first and second terminals, an input circuit having first and second terminals, a storage capacitor having first and second terminals constituting output terminals, said input second terminal and said storage capacitor second terminal being connected together, a vibrator having two contact arms making contacts in synchronism and phase, one of said arms connecting said input first terminal to said capacitor first terminal while the other of said arms connects said capacitor second terminal to said input second terminal, and said one arm connecting said capacitor first terminal to said input second terminal while the other said arm connects said capacitor second terminal to said storage 6 capacitor first terminal, whereby said output first terminal is charged toward the potential of said input first terminal but of opposite sense.

8. An inverter comprising, a grounded input circuit, a grounded storage capacitor, a charging capacitor having one terminal connected to the ungrounded terminal of said input circuit and the other terminal connected through a resistor to the ungrounded terminal of said storage capacitor, switching means alternately grounding the terminals of said charging capacitor, the time constant of said storage capacitor and resistor being longer than the period of said switching means, and an output circuit connected in shunt to said storage capacitor.

9. An inverter comprising, a capacitor, a grounded input circuit connected to one terminal thereof, a grounded storage capacitor, a resistor connecting said storage capacitor to the other terminal of said capacitor, and single-arm double-throw vibrator alternately grounding the terminals of said capacitor whereby the storage capacitor is incrementally charged toward an asymptote potential representative of the potential of said input circuit but opposite in sense.

10. An inverter comprising, a capacitor having first and second terminals, an input circuit having first and second terminals, said first input terminal being resistively connected to said capacitor first terminal, a storage capacitor having first and second terminals, said storage capacitor second terminal being connected to said input circuit second terminal, said storage capacitor terminals constituting circuit output terminals, a resistor connecting said storage capacitor first terminal and said capacitor second terminal, a vibrator having an arm connected to said input circuit second terminal and having two fixed contacts, and a connection from each said fixed contact to a respective terminal of said capacitor whereby said storage capacitor first terminal is charged incrementally to a potential representative of said input circuit first terminal potential but opposite in sense.

No references cited.

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