US2743413A - Voltage regulator - Google Patents

Description

April 24, 1956 E. L. JOHNSON, JR 2,743,413

VOLTAGE REGULATOR Filed April 4, 1952 AMGIEN'T TEMPERATURE. COMPENSATING 'fiHarzwsToa P AD CQMPENSATING BALLAST HERM 5T Qeam'm E CONSTANT VOLTAGE Pic). 1 THEQMISTOQ BALLAST 2 my :zzss'mucz VOLTAGE REGULATION 3 ZONE P INVENTOR Emil L. Jolznsolglr'.

ATTORNEY United States Patent VOLTAGE REGULATOR Emil L. Johnson, In, Baltimore, Md., assignor' to Bendix Aviation Corporation, Baltimore; Md-., a corporation of Delaware Application April 4, 1952, Serial No. 280,581 4 Claims; (Cl. 323-69) This invention relates generally to voltage regulating apparatus and more particularly to a voltagev regulator embodying a special resistance device having; a. negative temperature coefiicient ofv resistance and which, with associated. circuitry, provides simple and efi'ective regulation of a source of varying voltage.

Such special resistance devices are commonly called thermistors and normally take the form of a sintered combination of ceramic materials and various metallic oxides, the latter group of materials being generally known as semi-conductors. Such units can easily be provided with metallic electrodes, to which tinned copper leads can be conveniently soldered. Hence, there is available an element adapted for. ready insertionin conventional circuitry, and possessing special electrical characteristics which can be availed: of forpurposesof voltage regulation.

It has been found that upon an increase in current flow through a thermistor, a current value is reached wherein the thermistor begins to self-heat, i. e., the internal resistance heat-loss causes an elevation inthe internal temperature of the thermistor, thus causingthe resistance of the thermistors to decrease in value, per mitting more current to flow and'hence further self-heating of the thermistor. A voltage-current curve for this region of self-heating will show a constant voltage drop across the thermistor through a limited range in current values, thus suggesting the possibility of usinga ther-' mister for purposes of voltage regulation. If acurrent limiting resistor is placed in series with a thermistor across a. source of varying voltage, output leads connected across the thermistor will derive therefrom a substantially constant voltage. Under' conditions of constant ambient temperature, such a system would give excellent results. However, normal field use of voltage regulating apparatus imposes variable: ambient temperatures upon the system and since the resistance of the regulating thermistor is also dependent upon ambient temperature, some means must be provided'to compensate for such side-effects. It hasbeen found that a second thermistor, locatedin the output-circuit, can be utilized for such compensation. This second thermistor will have to operate below the self-heating current range so-that it is responsive to'temperature onlyand thus respond properly as a compensator,

Accordingly, it is an object ofthe invention toprovide an improved voltage regulatorembodying simple resistance elements and capable of accurate performance over a wide range in ambient temperatures.

The invention will be best understood, and its advantageous and novel features readily ascertained, upon reference to the detailed description set forth below, when taken in conjunction with the drawings, in which:

Figure 1 shows the improved voltage regulator in its most elementary form, and

Figure 2 shows the current-voltage characteristic curve of the regulating thermistor embodied in Figure l, and

Figure 3 shows a series of current-voltage characteris 2,743,413 Patented Apr. 24, 1956 tic curves of the regulating thermistor of Figure 1 when same is subjected to variations in ambient temperature, and

Figure 4 shows a temperature measuring, circuit sinzodying the voltage regulator of Figure l and having means for compensating for the effects of ambient tern"- perature changes.

Referring to Figure 1, there isshown a thermistor connectedin series. with a ballast resistance, there being an input voltage impressed across the combined circuit. The voltage-current characteristic of the thermistor is generally shown in Figure 2' and inspection of thisTcha-rac' teristic curve readily shows asubstantially constant volt age drop for a limited range of current flow. In this range is the voltage regulation zone, and for the particular thermistor used, it is necessary to have suificient ballast resistance to limit the current flow to the aforementioned range during an excursion of input voltage throughout the permissible deviation.

it was found that excellent results could be attained with the thermistor in the form of a thin rod having a diameter of .018 inch and a length of one and a half inches. Such a thermistor would have a resistance at plusv 30 degrees C. on theorder of 35,000'olnns and its temperature coeflicient of resistance per degree C. at 20 degrees C. is minus 2.7%. The voltage regulationv zone was found to be within the current range of 20' 50 mil liarnperes. Thus, a ballast resistance on the order of 2,000 ohms was found tobe necessary to limit the'current within the proper range for voltagercgulation.

In practice, with the circuit values as set forth above, the thermistor provided a regulated voltage across its terminals of 40 voltsplus or minus 1 volt under a range in impressed voltage. of to 125 volts. For a range in impressed voltage of to volts, the regulated output voltage was found to be 40 volts plus or minusvonehalf volt, and for a total change of 5 volts in supply, the regulator would be. accurate toone-quarter volt.

Now referring to Figure. 3, there is shown-a series of voltage-current characteristic curves for the thermistor under conditions of varying ambient temperature. The current range for voltage regulation decreases with increasing temperature, as does the' voltage drop itself. Obviously, with such a condition existing, some means must be found to compensatev for this temperature effect if the regulator is to perform satisfactorily in a measuring circuit. Furthermore, the ballast resistance must beselected to limit the current to its minimum range at maxi mum temperatures-and for a range inambient temperature of minus 30degrees F. to plus 1 10 degrees F., a value of 2,000 ohms as aforesaid wasadequate.

With the above in mind, and referring to Figure 4, therewill now be described a temperature. measuring circuit embodyinga thermistor for voltage. regulation. A source of alternating current 7 is connected. across the series. combination of thermistor 5 and ballast resistance 6. Connected across the thermistor 5 is the primary winding 8 of a step-down transformer having-its secondary. Winding 9 connected across the potentiometer 11. Between the winding 9 and potentiometer l lthere is interposed a conventional rectifier 12'for converting the alternating current source to a direct current suitable for use in the measuring circuit.

Connected across potentiometer 11 is a Wheatstone bridge measuring circuit comprising potentiometer arm 13, fixed arm 14, and condition sensing arm 15. The element 15 is a thermistor for measuring temperature and may be located remotely from the remainder of the circuit. The indicating diagonal of the bridge network comprises the microammeter 16 and a compensating thermistor 17.

Both the condition sensing thermistor 15 and compensating thermistor 17 are operated well below their self-heating current range so that they respond solely to changes in ambient temperature. Thus, if the ambient temperature decreases, the regulated voltage supplied to thebridge circuit by thermistor'S will increase. However, the compensating thermistor 17 will undergo an increase in resistance, resulting in a constant voltage drop across the microammeter for a given setting of the bridge. Should the ambient temperature increase, the action set forth above would reverse, resulting in the same voltage drop across the microammeter.

Obviously, if the regulator is to function properly, the thermistors 17 and 5 should be subjected to the same, or substantially the same ambient temperatures, or temperatures which are proportionally related to one another. When the entire regulator is located in a single, compact housing, the ambient temperatures would ordinarily be substantially the same for both 'thermistors. However, should the measuring circuit be located at a point remote from the input circuit, then some means may be necessary to regulate the temperatures so that they are proportionally related to one another. In either case, the thermal inertia of the thermistor 17 (which varies in relation to the diameter of a thermistor at a given length of the latter) should be properly correlated with that of the thermistor 5.

Thus, by the use of two thermistors and a ballast resistor, a fully temperature compensated source of regulated voltage is available for use in measuring circuits. The thermistor elements are inexpensive, are unaffected by ambient humidity or pressure, and have an indefinite life, all of which are quite beneficial for the intended use.

What is claimed and desired to be secured by United States Letters Patent is:

1. In a condition-responsive electrical network, a measuring or analogous circuit, a transformer having a primary winding adapted to receive load current from a source of supply and a secondary winding connected to said measuring circuit, means connected between said primary winding and said source for compensating for limited changes in load current comprising a resistor having a negative temperature coefiicient of resistance and a voltage-drop-versus current characteristic such that a subtially constant voltage drop exists thereacross throughout a given range of current flow and a current limiting resistor coacting with said first-named resistor for maintaining the current flow through the latter within said range, and'a temperature-compensating resistor connected in said measuring circuit functioning to automatically compensate for changes in transformer output voltage due to the efiects of ambient temperature on said first-named resistor, said temperature compensating resistor also having a negative temperature coefiicient of resistance and being operated below its self-heating current range so as to respond solely to changes in ambient temperature.

2. In a condition-responsive electrical network, a measuring or analogous circuit, a transformer having a primary winding receiving load currentfrom a source of supply and a secondary winding connected to said measuring circuit, means connected between said primary winding and said source for compensating for limited changes in load current comprising a thermistor having a voltagedrop-versus current characteristic such that a substantially constant voltage drop exists thereacross throughout a given range of current flow and a current-limiting resistor coacting with said thermistor for maintaining the current flow through the latter within said range, and a temperature-compensating thermistor connected in said measuring circuit functioning to automatically compensate for changes in transformer output voltage due to the effects of ambient temperature on said first-named thermistor, said temperature-compensating thermistor being operated below its self-heating current range so as to respond solely to changes in ambient temperature.

3. In a condition-responsive electrical network, a source of supply of load current, means for compensating for limited changes in load current over a given range of current flow comprising a thermistor connected across the supply line and having a voltage-drop-versus-current characteristic such that a substantially constant voltage drop exists across the thermistor throughout said range and a current-limiting resistor coacting with said thermistor for maintaining current flow through the latter within said range, and a temperature-compensating thermistor connected into said network functioning to automatically compensate for changes in the voltage drop across said first-named thermistor due to the effects of ambient temperature on said latter thermistor, said temperaturecompensating thermistor being operated below its selfheating current range so as to respond solely to changes in ambient temperature and said temperature-compem' sating thermistor being subjected to ambient temperatures which are substantially the same or are proportionally related to the ambient temperatures to which said firstnamed thermistor is subjected.

4. In a condition-responsive electrical network, in combination, a Wheatstone bridge circuit having a condition-responsive resistor in one leg thereof, a source of electric potential connected to the input circuit of said bridge, a thermistor connected across the supply lines of said source and having a voltage-drop-versus-current characteristic such that a substantially constant voltage drop exists across said thermistor throughout a given range of current flow through the latter and means for maintaining the value of current flow through said thermistor within said range, and meansfor automatically compensating for the'effects of ambient temperature changes on said thermistor including a second thermistor connected in the bridge circuit, said temperature-compensating thermistor being operated below its self-heating current range so as to respond solely to changes in ambient temperature and said compensating thermistor being subjected to ambient temperatures which are substantially the same or are proportionally related to the ambient temperatures to which said first-named thermistor is subjected.

References Cited in the file of this patent UNITED STATES PATENTS

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