GB1570889A - Electrical storage circuits - Google Patents

Description

(54) IMI'ROVEMENTS IN AND RELATING TO ELECTRICAL STORAGE CIRCUITS (71) We, ASSOCIATED ENGINEER ING LIMITED, a British Company. of Ince House, 6() Kenilworth Road. Leamington Spa. Warwickshire, CV32 6JZ. do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed. to be particularly described in and by the following statement: The invention relates to electrical storage circuits for storing electrical signals and is particularly. though not exclusively. applicable to such storage circuits for use in electrically controlled speed responsive systems such as speed responsive systems for engines and/or motor vehicles for controlling the vehicle at a predetermined speed or limiting it to a predetermined speed.

According to the invention. there is provided a storage circuit for storing an electrical signal, comprising a capacitor connected to be charged by the electrical signal through an electronic switch capable of assuming a high impedance state to minimise subsequent discharge of the capacitor, and a field effect transistor connected to have its conduction controlled by the charge on the capacitor, the output circuit of the field effect transistor being connected as part of a potential divider circuit which includes means for controlling the current therein to be substantially constant at a predetermined value and across part of which is derived an output signal of the storage circuit in dependence on the charge on the capacitor. the said predetermined value of current being such as to minimise the temperature dependence of the conductivitv of the field effect transistor. and in which the gain of the field effect transistor is such that the relationship between the output signal and the charge on the capacitor varies with the charge on the capacitor.

An electrical storage circuit for storing electrical signals and embodying the invention will now be described, by way of example, with reference to the accompanying drawing which shows a circuit diagram of the storage circuit.

The storage circuit shown is particularly, though not exclusively, suitable for use in a system for automatically controlling a motor vehicle to run at a predetermined speed which is set by the driver. Such a system is described in more detail in the Patent Application from which this Application is divided. Briefly, in such a system, once the vehicle speed has been selected and set by the driver, the system operates automatically to hold the vehicle at that speed. In the system, a transducer is provided which produces an electrical signal dependent on vehicle speed, and the system includes circuitry for monitoring the variations in this signal so as to control the vehicle speed accordingly. In order to bring the system into operation, the driver can accelerate (or brake) the vehicle to the required speed and then operate a control which causes the value of the speed-dependent signal at that time to be stored in an electrical storage circuit and subsequently used as a reference value against which any changes in vehicle speed can be detected - and thereby controlled.

The accompanying drawing shows the circuit diagram of one form which such an electrical storage circuit can take.

As shown, the storage circuit has an input line 36 which receives a variable voltage Vs whose value at a particular instant (as selected in this example by the motor vehicle driver) is to be stored.

The storage circuit is connected between a positive voltage line (+V) and a zero volt line (0V) supply line, and comprises a capacitor 114 which is connected to be charged by the voltage V5 through a resistor 116 and a field effect transistor 118. Two offset-adjusting resistors 115 and 117 are provided which connect respective ends of the resistor 116 to the zero volt line. The gate of the field effect transistor 118 is connected to the zero volt line through resistors 120 and 122, and transistor 118 can be momentarily rendered conductive, when it is desired to store a value of the voltage VK, by applying a control voltage to a control line 124. When this is done, capacitor 114 becomes charged to the level of the voltage V5, as modified by the values of resistors 116 and 117.

The stored voltage on capacitor 114 controls the conduction of a second field effect transistor 126 whose conduction path is connected between the positive and zero volt lines through resistors 128, 130 and 132, and through the emitter-collector path of a control transistor 134. Transistor 134 has its base-emitter path connected across a zener diode 136 which is fed from the positive volt line through a resistor 138. The effect is, therefore, that the transistor 134 maintains the current level through the field effect transistor 126 substantially constant. The constant current is selected to be at such a value that transistor 126 is held at a point on its operating characteristic where the effect on its conductivity of temperature change (over a normal ambient range) is minimised.

The voltage Vm. representing the particular stored value of Vs, is taken off by a line 140 and will of course depend on the charge stored on the capacitor 114.

It may be advantageous in certain circumstances for Vm to differ from the stored value of Vs and for the difference to vary with the actual stored value of Vs. This may be achieved by using the inherent change in gain across the source-follower connected field effect transistor 126. The effect is trimmed by the resistors 116 and 117.

Resistor 115 sets the starting d.c. level of the storage circuit. The function of resistors 128 and 130 is to offset the gate-source voltage of the field transistor 126.

The storage circuit described is advantageous in that the current level through the field effect transistor 126 is deliberately selected to be at such a value that this transistor is held at a point on its operating characteristic where the effect on its conductivity of temperature change (over a normal ambient range) is minimised.

Reference is made to our co-pending Patent Application No. 10134/76 (Serial No.

1570887) from which this Application is divided and to our co-pending Patent Application No. 38259/77 (Serial No. 1570888) which is also divided from Application No.

10134/76. (Serial No. 1570887).

WHAT WE CLAIM IS: 1. A storage circuit for storing an electrical signal, comprising a capacitor connected to be charged by the electrical signal through an electronic switch capable of assuming a high impedance state to minimise subsequent discharge of the capacitor, and a field effect transistor connected to have its conduction controlled by the charge on the capacitor, the output circuit of the field effect transistor being connected as part of a potential divider circuit which includes means for controlling the current therein to be substantially constant at a predetermined value and across part of which is derived an output signal of the storage circuit in dependence on the charge on the capacitor, the said predetermined value of current being such as to minimise the temperature dependence of the conductivity of the field effect transistor, and in which the gain of the field effect transistor is such that the relationship between the output signal and the charge on the capacitor varies with the charge on the capacitor.

2. A storage circuit according to claim 1, in which the electrical switch is a further field effect transistor.

3. An electrical storage circuit according to claim 1 and substantially as described with reference to the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (3)

1. **WARNING** start of CLMS field may overlap end of DESC **.

   provided which connect respective ends of the resistor 116 to the zero volt line. The gate of the field effect transistor 118 is connected to the zero volt line through resistors 120 and 122, and transistor 118 can be momentarily rendered conductive, when it is desired to store a value of the voltage VK, by applying a control voltage to a control line 124. When this is done, capacitor 114 becomes charged to the level of the voltage V5, as modified by the values of resistors 116 and 117.

   The stored voltage on capacitor 114 controls the conduction of a second field effect transistor 126 whose conduction path is connected between the positive and zero volt lines through resistors 128, 130 and 132, and through the emitter-collector path of a control transistor 134. Transistor 134 has its base-emitter path connected across a zener diode 136 which is fed from the positive volt line through a resistor 138. The effect is, therefore, that the transistor 134 maintains the current level through the field effect transistor 126 substantially constant. The constant current is selected to be at such a value that transistor 126 is held at a point on its operating characteristic where the effect on its conductivity of temperature change (over a normal ambient range) is minimised.

   The voltage Vm. representing the particular stored value of Vs, is taken off by a line 140 and will of course depend on the charge stored on the capacitor 114.

   It may be advantageous in certain circumstances for Vm to differ from the stored value of Vs and for the difference to vary with the actual stored value of Vs. This may be achieved by using the inherent change in gain across the source-follower connected field effect transistor 126. The effect is trimmed by the resistors 116 and 117.

   Resistor 115 sets the starting d.c. level of the storage circuit. The function of resistors 128 and 130 is to offset the gate-source voltage of the field transistor 126.

   The storage circuit described is advantageous in that the current level through the field effect transistor 126 is deliberately selected to be at such a value that this transistor is held at a point on its operating characteristic where the effect on its conductivity of temperature change (over a normal ambient range) is minimised.

   Reference is made to our co-pending Patent Application No. 10134/76 (Serial No.

   1570887) from which this Application is divided and to our co-pending Patent Application No. 38259/77 (Serial No. 1570888) which is also divided from Application No.

   10134/76. (Serial No. 1570887).

   WHAT WE CLAIM IS: 1. A storage circuit for storing an electrical signal, comprising a capacitor connected to be charged by the electrical signal through an electronic switch capable of assuming a high impedance state to minimise subsequent discharge of the capacitor, and a field effect transistor connected to have its conduction controlled by the charge on the capacitor, the output circuit of the field effect transistor being connected as part of a potential divider circuit which includes means for controlling the current therein to be substantially constant at a predetermined value and across part of which is derived an output signal of the storage circuit in dependence on the charge on the capacitor, the said predetermined value of current being such as to minimise the temperature dependence of the conductivity of the field effect transistor, and in which the gain of the field effect transistor is such that the relationship between the output signal and the charge on the capacitor varies with the charge on the capacitor.
2. 2. A storage circuit according to claim 1, in which the electrical switch is a further field effect transistor.
3. 3. An electrical storage circuit according to claim 1 and substantially as described with reference to the accompanying drawings.