US3087109A

US3087109A - Control circuits

Info

Publication number: US3087109A
Application number: US852016A
Authority: US
Inventors: Bowers Melville Dorey
Original assignee: McGraw Edison Co
Current assignee: McGraw Edison Co
Priority date: 1959-11-10
Filing date: 1959-11-10
Publication date: 1963-04-23
Anticipated expiration: 1980-04-23

Description

April 1963 M. D. BOWERS 3,087,109

CONTROL CIRCUITS Filed Nov. 10, 1959 IN V EN TOR. M cZz/zi Z 5: D. Boa/ens 2 /144! flttorngr 3,687,109 CONTROL CIRCUITS Melville Dorey Bowers, Lake Valhalla, Montville, N.J., assignor to McGraw-Edison Company, Milwaukee, Win, a corporation of Delaware Filed Nov. 10, 1959, Ser. No. 852,016 6 Claims. (Cl. 323-6) This invention relates to control circuits and, more particularly, to a control circuit utilizing a Zener diode as a reference of voltage source.

Zener diodes are commonly utilized in control circuits as a reference voltage source for comparison with a variable input signal so that an output signal or eflect can be obtained which is a function of input signal variations. Zener diodes have, however, a positive temperature coeflicient so that changes in ambient temperature modify the output signal or etfect whereby it no longer reflects true variations in the input signal.

It is an object of this invention to provide a Zener diode with means for compensating variations in its Zener voltage with changes in ambient temperature.

It is a more specific object of the invention to utilize in a control circuit, a positive temperature coefficient resistor for temperature compensationg a Zener diode.

it is a still more specific object of the invention to utilize in a control circuit having a Zener diode, a positive temperature coeflicient resistor and a second resistor having a substantially zero temperature coeflicient to compensate for changes in Zener voltage with temperature.

These and other objects and advantages of the invention will become more obvious from the detailed description of the invention taken in view of the accompanying drawing which shows a control circuit embodying the instant invention.

Referring to the drawing in greater detail, a voltage sensitive bridge 10 is shown having a Zener diode 12 as a reference voltage source. The other legs of the bridge include a pair of

fixed resistors

14 and 15 and an adjustable resistor 16. The input terminals 17 and 18 of bridge 10 are respectively connected to junction 19 between

resistors

14 and 15 and junction 20 between the negative terminal of Zener diode 12 and resistor 16 respectively. A positive temperature coeflicient resistor 22 is connected at one end to junction 23 between resistor 15 and the positive terminal of Zener diode 12 while a second resistor 24, having a substantially zero temperature coeflicient within the normal temperature range of the device, is connected between the other end of resistor 22 and junction 20. A load 28 is connected to output terminals 26 and 27 between

resistors

14 and 16 and resistors 22 and 24 respectively,

In operation, resistor 16 is so adjusted that When a predetermined desired voltage appears across input terminals 17 and 18, the circuit is balanced whereby the current flowing to load 28 is Zero, and the currents flowing in resistors 22 and 2 4 are equal. Under such conditions the voltage drop across resistor 14 is equal to the sum of the voltage drops across resistors 15 and 22. In addition, the voltage drops across

resistors

16 and 24 are also equal. It can be seen too that the sum of the voltage drops across resistors 22 and 24 is equal to the Zener voltage of diode 12.

Should the input voltage rise above the predetermined desired level, the current through resistors 1-4 and 16 will increase resulting in an increase in voltage at terminal 26 with respect to terminal 20. Increased current will also flow through resistor 15 and Zener diode 12, which, assuming perfect operation thereof will cause no change in the voltage between terminals 20 and 23. Thus the current through resistors 22 and 24 and the voltage distribution across them would not change if it were not for the load 28 connecting points 26 and 27. Since the voltage at terminal 26 has increased, a current will flow in load 28 from that terminal to terminal 27 through

resistors

24 and 16. This load current flow will tend to increase the voltage drop across resistor 24 and decrease the drop across resistors 16 and 22. The load current thus tends to reduce the voltage difference between terminals 26 and 27 which would appear if the load resistance were infinite.

On the other hand, should the voltage between input terminals 17 and 18 fall below the predetermined desired value, a reduced current would flow through

resistors

14 and 16 resulting in a decrease in voltage at terminal 26. At the same time, current flow through Zener diode 12 and resistor 15 is reduced but the voltage drop across diode 12 is unchanged tending to maintain the voltage drop across resistors 22 and 24. Now, the voltage drop across resistor 24 is higher than the voltage drop across resistor 16 and current fiow through load 28 is reversed.

It can therefore be seen that changes in input voltage will be reflected as a change in the current flowing through load 28. In order that the current flowing through load 28 reflect changes in input voltage only, it is necessary to maintain the voltage at terminal 27 with respect to terminal 20 invariant.

Since Zener diodes have a positive temperature coeflicient, the voltage drop between terminals 23 and 20 varies with changes in ambient temperature, even though the input voltage between terminals .17 and 18 remains constant. In order to prevent such temperature-produced changes in Zener voltage from being reflected as error signals in load 28, it is necessary to insure that the current in resistor 24 remains unaifected. This is accomplished by utilizing a resistor 22 having a positive temperature coeflicient. By properly matching the temperature coeflicients of Zener diode 12 and resistor 22, any increase or decrease in the voltage across Zener diode 12 can be matched by an increase or decrease in the voltage drop across resistor 22. As a result, the voltage drop across and the current through resistor 24 will remain constant.

For example, assume that the bridge circuit 10 is balanced and that a temperature rise increases the Zener voltage of diode 12 and hence, the voltage drop between junctions 20 and 23. Since the current in resistor 24, as well as its resistance, must remain constant to satisfy the conditions of compensation, the entire increase in the voltage drop between terminals 20 and 23 appears across resistor 22. However, because the current in resistor 22 is equal to the current in resistor 24, under balanced conditions, it therefore also remains constant. Accordingly the increased voltage drop across resistor 22 results entirely from an increase in its resistance. It can be seen, therefore, that resistor 22 must have a positive temperature coefficient which bears a particular relation to the temperature coeificient of Zener diode 12.

Those skilled in the art will appreciate that if the resistance of load 28 is very large with respect to resistor 24, the current through the latter will be equal to the Zener voltage of diode 12 divided by the sum of the resistances of resistors 22 and 24, and the voltage across resistor 24 will be equal to this current times its resistance R. Because the Zener diode and resistor 22 are temperature sensitive, the expression for the voltage drop across resistor 24 at a temperature T is:

R =the resistance of resistor 24; R =the resistance of resistor 22 at T v.0 V =the Zener voltage of diode 12 at T oc tlle temperature coeflicient of Zener diode 12; and or=the temperature coeificient of resistor 22.

Since there is no change in the resistance or current through resistor 24, the voltage drop across it will not change with temperature so that the above expression can be differentiated with respect to temperature and set equal to zero, from which the following expression can be obtained:

This expression indicates that the necessary temperature coeffieient of resistor 22 is a positive value and can be accurately determined from the resistances of resistors 22 and 24 and the temperature coefiicient of Zener diode 2'12.

While the invention is discussed with respect to a voltage sensitive bridge, it will be understood that it has application in any circuit in which a Zener diode is utilized as a voltage reference source.

I claim:

1. A circuit for deriving a reference voltage independent of temperature variations and including a pair of input terminals connected to a voltage source, a Zener diode and a first resistance means series connected across said input terminals, a second resistance means connected at one end to one terminal of said Zener diode, a third resistance means connected between the other end of said second resistance means and the other terminal of said Zener diode, said second resistance means having a positive temperature coefficient, said third resistanec means having a zero temperature co-efficient, whereby a reference voltage will be derived across said third resistance means that is independent of temperature variations.

2. A circuit for deriving a reference voltage independent of temperature variations and including a pair of input terminals connected to a voltage source, a Zener diode and a first resistance means series connected across said input terminals, a second resistance means connected at one end to the positive terminal of said Zener diode, a third resistance means connected between the other end of said second resistance means and the negative terminal of said Zener diode, said second resistance means having a positive temperature coetficient, said third resistance means having a zero temperature coefficient, whereby a reference voltage will be derived across said third resistance means that is independent of temperature variations.

3. A circuit for deriving a reference voltage independent of temperature variations and including a pair of input terminals connected to a voltage source, a Zener diode and a first resistance means series connected across said input terminals, a second resistance means connected at one end to one terminal of said Zener diode, a third resistance means connected between the other end of said second resistance means and the other terminal of said Zener diode, said third resistance means having a zero temperature coefficient, said second resistance means having a positive temperature coefficient given by the expression where R =the resistance of said second resistance means at a reference temperature;

R =the resistance of said third resistance means; and a =the temperature coefficient of said Zener diode.

whereby a reference voltage will be derived across said third resistance means that is independent of temperature variations.

4. A bridge type control circuit having a pair of inputterminals and a pair of output terminals, a first resistance means connected between one of said input terminals and one of said output terminals, a second resistance means connected between the one of said output terminals and the other of said input terminals, a third resistance means connected between the other of said input terminals and the other of said output terminals, and a fourth and a fifth resistance means series connected to each other, said fourth resistance means also being connected to the one of said input terminals and said fifth resistance means also being connected to the other of said output terminals, said third resistance means having a substantially zero temperature coefiicient, said fifth resistance means having a positive temperature coefficient, and a Zener diode connected between said other input terminal and the junction between said fourth and fifth resistance means.

5. A circuit for deriving a reference voltage independent of temperature variations and including a pair of input terminals connected to a voltage source, a diode and a first resistance means series connected across said input terminals, a second resistance means connected at one end to one terminal of said diode, a third resistance means connected between the other end of said second resistance means and the other terminal of said diode, said diode being characterized by a substantially constant breakdown potential and a positive temperature coeffiient, said second resistance means having a positive temperature coefficient, said third resistance means having a zero temperature coefiicient, whereby a reference voltage will be derived across said third resistance means that is independent of temperature variations.

6. A bridge type control circuit having a first leg con sisting of a first resistance means, a second leg consisting of a second resistance means and connected to said first leg, a third leg consisting of a third resistance means and connected to said second leg, and a fourth leg consisting of a fourth and a fifth resistance means series connected to each other, said fourth resistance means also being connected to said first resistance means and said fifth resistance means also being connected to said third resistance means, said third resistance means having a substantially zero temperature coefficient, said fifth resistance means having a positive temperature coeflicient, and a Zener diode connected between the junction of said second and third legs and the junction between said fourth and fifth resistance means.

References Cited in the file of this patent UNITED STATES PATENTS Crowell Aug. 18, 1942 Woodworth Dec. 9, 1958 OTHER REFERENCES

Claims

1. A CIRCUIT FOR DERIVING A REFERENCE VOLTAGE INDEPENDENT OF TEMPERATURE VARIATIONS AND INCLUDING A PAIR OF INPUT TERMINALS CONNECTED TO A VOLTAGE SOURCE, A ZENER DIODE AND A FIRST RESISTANCE MEANS SERIES CONNECTED ACROSS SAID INPUT TERMINALS, A SECOND RESISTANCE MEANS CONNECTED AT ONE END TO ONE TERMINAL OF SAID ZENER DIODE, A THIRD RESISTANCE MEANS CONNECTED BETWEEN THE OTHER END OF SAID SECOND RESISTANCE MEANS AND THE OTHER TERMINAL OF SAID ZENER DIODE, SAID SECOND RESISTANCE MEANS HAVING A POSITIVE TEMPERATURE COEFFICIENT, SAID THIRD RESISTANCE MEANS HAVING A ZERO TEMPERATURE CO-EFFICIENT, WHEREBY A REFERENCE VOLTAGE WILL BE DERIVED ACROSS SAID THIRD RESISTANCE MEANS THAT IS INDEPENDENT OF TEMPERATURE VARIATIONS.