Window Discriminator or Voltage Range Sensor

The invention relates to a window discriminator circuit which with a changing direct voltage supply having a magnitude within a certain voltage range emits a defined output signal.

In many applications it is necessary to enforce a defined starting condition on certain logic or storage circuits when applying the operating voltage. A light dimmer is mentioned as an example of this which draws its supply voltage from the mains. When there is a failure in the mains it is necessary to ensure that the light dimmer remains disconnected when the mains voltage comes back. This means that while the mains voltage comes back operating instructions stored in a logic circuit for the light dimmer must be cancelled out.

The object underlying the invention is therefore to provide a window discriminator circuit which generates a defined output signal, within a certain voltage range, as the supply voltage is being restored. The output signal may be used, for example, to cancel information contained in a logic circuit or to re-establish a defined starting condition.

According to the invention from one aspect there is provided a d.c. voltage range sensor having an input for a d.c. voltage to be sensed and an output for providing a signal when a voltage at the sensor input falls within a predetermined range. The sensor comprises a differential switch having first and second inputs, and an output coupled to the sensor output; bias means coupled with the first input adapted to provide a bias voltage which is substantially independent of the voltage at the sensor input; and circuit means coupled between the sensor input and said second input to the differential switch adapted to provide at said second input a voltage related to the sensor input voltage which voltage effects switching of the differential switch to one state when the d.c. input voltage reaches a predetermined lower level and to the other state when the voltage reaches a level determined by said bias voltage.

According to another aspect of the invention, there is provided a window discriminator circuit which with a changing direct voltage supply within a certain voltage range emits a defined output signal, characterized in that a circuit is connected to the supply voltage source which produces a checking voltage depending on the supply voltage; that a Schmitt trigger is connected to this circuit, the Schmitt trigger being connected in defined manner when a certain lower checking voltage value is achieved; that a further circuit connected to the Schmitt trigger is provided at which a reference voltage substantially independent of the supply voltage is produced, its value determining the upper checking voltage value which when reached switched the Schmitt trigger.

In order that the invention and its various other preferred features may be understood more easily, an embodiment thereof will now be described, by way of example only, with reference to the drawing.

The drawing is a circuit diagram of a discriminator circuit constructed in accordance with the invention.

In a preferred embodiment of the window discriminator, a circuit is connected to a supply voltage source for producing a checking voltage dependent on the supply voltage. A Schmitt trigger is connected to this circuit, the trigger being connected in a defined manner when a certain lower checking voltage value is achieved. A further circuit is provided which is connected to the Schmitt trigger for producing a reference voltage which is substantially independent of the supply voltage, the value of the reference voltage determining the upper checking voltage value. The Schmitt trigger is switched when this checking voltage value is reached.

The supply voltage source may comprise a capacitor which is charged from the mains via a rectifier circuit.

A multi-collector transistor, which operates as a current image circuit and emits a supply current to the different circuit parts in accordance with the size ratios of the different collectors, is preferably provided for current supply of the voltage-producing circuits and of the Schmitt trigger. In an essentially conventional manner, a comparator may comprise two transistors connected together at the emitter electrodes whereby these emitter electrodes are connected to a collector of the multi-collector transistor. The changing checking voltage is supplied to the base electrode of one transistor, while the fixed reference voltage is present at the base electrode of the other transistor.

In the FIGURE a preferred circuit construction is shown which employs bipolar technology. This circuit is particularly suited for use in monolithically integrated circuits.

The entire circuit is connected between the two poles of a direct voltage supply and this means that a capacitor C supplying or smoothing the operating voltage is connected in parallel with the entire circuit. An operating voltage U.sub.B is impressed across the capacitor C and increases from a voltage O to an upper voltage value when the mains are connected, the upper voltage value being predetermined by a further circuit not shown in the FIGURE.

The emitter electrode of a multi-collector transistor T.sub.1 is connected to one pole of the supply voltage and has three collectors in total in the embodiment shown. One collector electrode which is also connected to the base electrode of the multicollector transistor T.sub.1 is twice as large as the two other collectors. This collector electrode is connected to a voltage divider which supplies a checking voltage that is dependent on the supply voltage. This voltage divider comprises series-connected resistors R.sub.1, R.sub.2 and R.sub.3 in the embodiment shown. The resistor R.sub.3 is connected to the other pole of the supply voltage.

A diode chain made up of diodes D.sub.1 to D.sub.4 connected in the flow direction is connected to one of the other two collectors of the multi-collector transistor T.sub.1. A Schmitt trigger circuit comprising the transistors T.sub.2, T.sub.3 and T.sub.4 is connected with the base electrode of transistor T.sub.2 connected to the diode chain D.sub.1 -D.sub.4 and the emitter electrodes of the two transistors T.sub.2 and T.sub.3 are connected together and to the remaining collector of the multi-collector transistor T.sub.1. The diodes D.sub.1 to D.sub.4 are connected in series with each other and the diode chain is connected in parallel with the base collector path of the transistor T.sub.2 so that the voltage drop across the diodes is the input voltage for the transistor T.sub.2. The base electrode of the transistor T.sub.3 is connected to the junction between resistors R.sub.1 and R.sub.2. This transistor T.sub.3 has two collector electrodes preferably of the same size one of which is connected to the base electrode of a switching transistor T.sub.4 and to a resistor R.sub.4. The other collector electrode of transistor T.sub.3 is connected to the base electrode of a switching transistor T.sub.5 and to a resistor R.sub.5. These resistors R.sub.4 and R.sub.5 bridge the base emitter paths of the related switching transistors T.sub.4 T.sub.5, respectively. The emitter collector path of the transistor T.sub.4 bridges the resistor R.sub.3 of the voltage divider so that this resistor is short-circuited when transistor T.sub.4 is fully conductive. An output signal is derived at the collector electrode of the transistor T.sub.5. The emitter electrodes of the transistors T.sub.4 and T.sub.5 as well as the collector electrode of the transistor T.sub.2 are connected to one pole of the supply voltage source.

The transistors T.sub.4 and T.sub.5 are complementary to the transistors T.sub.2 and T.sub.3 of the Schmitt trigger and to the multi-collector transistor T.sub.1. Thus the transistors T.sub.4 and T.sub.5 are npn-transistors having, for example, their emitter electrodes connected to the grounded negative pole. The remaining transistors T.sub.1, T.sub.2 and T.sub.3 are pnp-transistors wherein the emitter electrode of the transistor T.sub.1 is connected to the positive pole of the supply voltage source.

The circuit shown and described in detail functions as follows: when the mains voltage is first connected or restored the capacitor C slowly charges. With a very small voltage the operating condition of the circuit is not definable since the current flowing through the multi-collector transistor T.sub.1 is not sufficient to drive the output transistor T.sub.5 into its conducting state wherein current would flow between its collector and emitter electrodes. With a minimum value of U.sub.B i.e.:-

the transistor T.sub.5 becomes conductive. This minimum value is:

R.sub.3 must be taken into consideration here since the transistor T.sub.4 is blocked and thus the current I.sub.1 also flows through the resistor R.sub.3. The transistor T.sub.1 operates as a current image circuit whereby the current through the voltage divider is twice as large as the current through the Schmitt trigger or through the diode chain because of the selected size ratios of the collectors. Thus:

is true.

The current flowing through the Schmitt trigger is distributed evenly through the collectors of the transistor T.sub.3 in one operating condition. This means that (I.sub.2 /2) flows through each collector of T.sub.3. The minimum current at which the transistors T.sub.4 and T.sub.5 become conductive is: ##EQU1##

Since I.sub.3 is just as large then, in accordance with formula 3: ##EQU2## is true, if the resistors R.sub.4 and R.sub.5 are of the same size. Also, if it is stipulated that the base emitter voltages required to switch the transistors T.sub.4 and T.sub.5 on are of equal size then, for the current I.sub.1, the value: ##EQU3## is true.

If this value is introduced into Formula 2 then for the voltage U.sub.1 at which the transistor T.sub.5 becomes conductive, the following value is obtained: ##EQU4##

If the voltage U reaches this value U.sub.1 then a "0" signal is produced at the output of the transistor T.sub.5 and a logic or storage circuit connected to transistor T.sub.5 will respond in accordance with the "0" signal for example.

If the voltage across the capacitor C is greater than U.sub.1, then the current flowing through the transistor T.sub.3 is sufficient to drive the two transistors T.sub.4 and T.sub.5 into conduction. R.sub.3 is short-circuited therefore by the collector-emitter path of transistor T.sub.4 and this means that the current I.sub.1 , which is determined by the voltage divider, is increased since its magnitude is not effectively determined only by the resistors R.sub.1 and R.sub.2 . This condition continues until the base potential at the transistor T.sub.3 reaches the value of the base potential across the transistor T.sub.2. This is the case at the voltage

Thus by neglecting the saturation voltage for the transistor T.sub.4 the following is true: ##EQU5##

Where U.sub.F.sbsb.1 to U.sub.F.sbsb.4 means the forward voltage drops across diodes D.sub.1 to D.sub.4 of the diode chain respectively. If U.sub.B reaches the value U.sub.2 the transistor T.sub.3 is blocked and the transistor T.sub.2 is conductive. If T.sub.3 is blocked then transistors T.sub.4 and T.sub.5 are also blocked and the output signal of T.sub.5 passed to the logic circuit becomes a "1" so that new instructions are again impressed by further signals supplied to the logic circuit from other circuit components. The base potential at the control electrode of T.sub.3 is not determined by the resistors R.sub.1, R.sub.2 and R.sub.3. Connection and disconnecton of the resistor R.sub.3 causes hysteresis of the circuit shown since the transistor T.sub.5 is only driven into conduction again with a voltage:

which is smaller than the voltage U.sub.2 . This occurs if the voltage across the capacitor again decreases. Because of the effectiveness of all three resistors of the voltage divider the formula: ##EQU6## is true for U.sub.3.

In this case the forward voltage drops U.sub.F of the four diodes D.sub.1 to D.sub.4 are of equal size. The hysteresis of the circuit is determined by the voltage difference between the values U.sub.2 and U.sub.3.

The circuit in accordance with the invention may be used in a digital light dimmer for example in order to set a defined starting condition when there is a failure in the mains and when the voltage comes back. The capacitor C is charged to a maximum voltage of 8 V for example. With a voltage U.sub.1 of approximately 2 V the transistor T.sub.5 is driven into conduction. With a voltage of approximately 5 V the transistor T.sub.5 is blocked again. A preferred position is enforced on a logic circuit connected to the output of transistor T.sub.5 by the starting command which is provided in the voltage range between 2 and 5 V by the transistor T.sub.5. Alternatively, the storage content of a command circuit can be cancelled, for example. Only when a voltage of approximately 8 V is reached is a pulse supplied to the logic circuit by a further circuit not described here, and the light dimmer circuit can then be shifted into any desired operating condition again according to this pulse by means of external commands.

It will be appreciated that instead of the Schmitt trigger other types of differential switch may be employed.

It will be understood that the above description of the present invention is susceptible to various modification changes and adaptations.