HK1170080A - Electrical isolators - Google Patents

Description

Electrical isolator

Technical Field

The present invention relates to a low power electrical isolator for providing isolation of a low voltage circuit from a high voltage circuit while providing coupling of an analog signal from the high voltage circuit to the low voltage circuit.

Background

Mains voltage powered consumer products, such as multimedia home network nodes, require electrical isolation between the mains circuit and the low voltage circuit for safety reasons. Despite the presence of electrical isolation, it is often desirable to communicate signals across an electrical isolation barrier between the supply voltage circuit and the low voltage circuit. Determination of a location (e.g., a zero crossing point on a supply voltage signal from a low voltage side) is one example of such a need involving the communication of a signal from a high voltage side to a low voltage side. The determination of the position on the supply voltage signal may find application, for example, in providing synchronization with a supply voltage cycle. Synchronization with the supply voltage cycle may be used to provide synchronous communication in and between various networked products, such as multimedia home network nodes. The determination of the location on the supply voltage signal may also find application in detecting a change in the phase of the supply to determine whether the mains supply is likely to fail. For example, if there is more than a 2% deviation in the phase of the mains supply on average over twenty-four hours, the likelihood of a power failure increases. Precautions can then be taken, such as connecting an uninterruptible power supply.

Fig. 1 shows an electrical isolator circuit 10 for communicating signals from a supply voltage circuit to a low voltage circuit while maintaining isolation between the supply voltage circuit and the low voltage circuit. The electrical isolator 10 includes an opto-isolator 12 having an infrared Light Emitting Diode (LED)14 and a phototransistor 16. A resistor 18 in series with the LED14 may limit the current through the LED 14. A load resistor 20 is connected in series with the phototransistor 16 between the phototransistor and the positive line. The capacitor, capacitor 22, represents the parasitic capacitance of the circuit connected to the output 24 of the phototransistor. In use, a high voltage AC signal is applied to the electrical isolator circuit via output 26, thereby operating LED 14. Light from the LED is received by the phototransistor, which generates a current that flows through a load resistor to provide a corresponding voltage at output 24. A representative high voltage AC signal 28 is shown in fig. 2, along with a corresponding output voltage 30 from a zero-crossing detector having as its input the voltage signal from output 24.

A disadvantage of the electrical isolator circuit in fig. 1 is its high power consumption problem at the low and high voltage side (which is particularly pronounced). The time accuracy of the output 24 depends on the switching speed of the isolator circuit 10, the switching speed of the isolator circuit 10 being determined by the RC time constants of the resistor 20 and the capacitor 22. The maximum value of the resistor 20 is determined by the given load and the required accuracy. The maximum value of the resistor 20 and the required voltage amplitude in turn determine the required minimum phototransistor current Ic. Based on the Current Transfer Ratio (CTR) of the opto-isolator, and noting the phototransistor current Ic, the LED forward current If can be determined. For high voltage signals, most of the voltage is dropped across resistor 18. The expected power dissipation of a typical opto-isolator is 0.5 watts, while the power dissipation of resistor 20 and resistor 18 is also 0.5 watts. If the forward current is reduced to a significant degree that reduces power consumption, not only will the current on the phototransistor be reduced, but the current transfer ratio of the opto-isolator will also be reduced. The combination of these effects results in a more significant reduction in the output swing, necessitating an increase in resistor 20, which in turn reduces the switching speed of the isolator circuit by a corresponding amount. The galvanic isolator circuit in fig. 1 therefore gives an unacceptable trade-off between power consumption and switching speed.

The present invention was conceived in light of the above-mentioned problems of known electrical isolator circuits.

It is therefore an object of the present invention to provide an improved electrical isolator circuit for isolating a low voltage circuit from a high voltage circuit and at the same time providing coupling of signals from the high voltage circuit to the low voltage circuit.

It is a further object of the present invention to provide an improved electrical isolator circuit including an opto-isolator for isolating a low voltage circuit from a high voltage circuit while providing coupling of signals from the high voltage circuit to the low voltage circuit.

Disclosure of Invention

According to a first aspect of the present invention, there is provided an electrical isolator circuit comprising:

an input stage comprising a transmitter of an opto-isolator, the input stage for receiving a high voltage AC signal; and

an output stage operable at a low voltage, the output stage comprising: a receiver of the opto-isolator; an active circuit in series with the receiver between the receiver and a power rail of the output stage; and a first resistor connected to the output of the active circuit and a predetermined voltage applied at the input of the active circuit, the output stage being configurable to produce a varying output voltage across the first resistor in response to input of a high voltage AC signal to the input stage.

In use, application of a predetermined voltage to the input of the active circuit may keep the voltage across the receiver substantially constant. The output stage may be configured to maintain the voltage across the receiver substantially constant over the predetermined voltage. The influence of the electrodes formed by the load resistance and the load capacitance is thus substantially eliminated, allowing the load resistance to increase without the time constant increasing to an unacceptable extent, taking into account the switching requirements. Thus, the input resistance can be increased to reduce the forward current I flowing through the LED_fThereby reducing power consumption. With respect to the effect on the varying output voltage, the output stage may be configured to level shift the varying output voltage by a voltage corresponding to the predetermined voltage.

More specifically, the active circuit and the receiver are in a high side configuration (high side configuration). Thus, the active circuit may be connected in series between the receiver and the positive power rail of the output stage.

Alternatively or additionally, the active circuit has first and second inputs, the first input forming a series electrical connection of the receiver and the active circuit, the second input receiving the predetermined voltage.

Alternatively or additionally, the impedance of the active circuit is lower than the impedance of the receiver. More specifically, the impedance of the receiver is at least two times higher than the impedance of the active circuit. More specifically, the impedance of the receiver is at least five times, e.g., ten times, higher than the impedance of the active circuit.

In a first configuration, the active circuit includes an active load transistor. More specifically, the receiver is a phototransistor, and the active load transistor and first resistor are in a cascode (or common gate) configuration in which the first resistor operates as a resistive load.

In a second configuration, the active circuit includes a differential amplifier having a first input electrically connected to an output of the receiver (e.g., a collector or emitter, where the receiver is a photodiode) and a second input connected to a predetermined voltage, a first resistor connected between the output of the differential amplifier and the first input. The differential amplifier is included in an operational amplifier. More specifically, the first input of the differential amplifier is an inverting input.

Alternatively or additionally, the active circuit is configured to operate in saturation. The different output voltages are thus in the form of digital signals or in the form of generally square waves.

More specifically, the electrical isolator circuit further comprises a bi-stable circuit, such as a latch, and an output provided from the active circuit to an input of the bi-stable circuit. Thus, the bi-stable circuit can be used to provide a digital signal that, in effect, can provide zero-crossing information.

Alternatively or additionally, the output stage may be configured such that the varying output voltage is an analogue output voltage and such that the electrical isolator circuit further comprises a signal determination circuit electrically connected to the output of the active circuit and operable to determine a position on the analogue output voltage. For example, the active circuit includes an operational amplifier that may be configured not to operate in saturation to provide an analog output voltage, i.e., a voltage that is neither in digital form nor in the form of a general square wave.

More specifically, the signal determination circuit may be configured to operate as a voltage level crossing detector.

More specifically, the signal determination circuit is configured to compare the analog output voltage to a reference voltage. The reference voltage may be of a level such that the signal determination circuit is operable to determine a position on the analog output signal corresponding to a zero crossing point. The reference voltage may correspond to a predetermined voltage applied to an input of the active circuit such that the signal determination circuit may operate as a zero-crossing detector. The electrical isolator circuit may be operable to adjust a reference voltage of the signal determination circuit according to a value of the output voltage. Thus, when the RMS voltage of the high voltage AC signal changes, or during calibration, the reference voltage may change to compensate for changes in electrical device parameters.

Alternatively or additionally, the first resistor comprises a variable resistor and the electrical isolator circuit comprises a gain feedback circuit for varying a resistance value of the variable resistor in dependence on the varying output voltage. The gain feedback circuit is configured to vary the resistance of the variable resistor in accordance with a value of the varying output voltage, such as a peak value, which may be determined by an analog-to-digital converter or a peak detection circuit. In use, the variable resistor and the gain feedback circuit may be used to adjust for variations in the RMS voltage of the high voltage AC signal applied to the input stage, or during calibration, to compensate for electrical equipment parameter variations. For example, if the high voltage signal is increased from 90VRMS to 240VRMS, a value, e.g., a peak value, of the different output voltage may be determined and the gain feedback circuit may be used to decrease the resistance of the variable resistor. The variable resistor and the gain feedback circuit may provide an alternative to adjust the reference voltage of the signal determination circuit when the RMS voltage of the high voltage AC signal changes.

Alternatively or additionally, the low voltage in the context of the present invention may be a voltage of less than 50VRMS or a DC voltage of less than 120V according to standards defined by the international electrotechnical commission. More specifically, the low voltage signal may be a DC voltage of less than 15 volts, such as a voltage of 12 volts. More specifically, the low voltage signal is a DC voltage of 5 volts or less. Alternatively or additionally, the high voltage AC signal in the context of the present invention may be an AC voltage of 50VEMS or greater according to the standards defined by the international electrotechnical commission. Alternatively or additionally, the input stage may be operable to receive a high voltage AC signal at a frequency of less than 500HZ, for example, at a frequency of 50HZ or 60HZ for domestic power supplies, or at a frequency of 400HZ for foreign power supplies.

In one configuration, the galvanic isolator circuit includes a signal determination circuit including a comparator having a first input receiving the varying output voltage and a second input providing a reference voltage. The reference voltage may, for example, be set at a particular voltage level to determine when the varying output voltage exceeds 1.5 volts in increasing from less than 1.5 volts to greater than 1.5 volts. The electrical isolator circuit is operable to determine a zero crossing point on a high voltage AC signal based on a known time period between a zero crossing point and a point at which the varying output voltage exceeds the reference voltage. The reference voltage is set at a level corresponding to zero voltage and the comparator may function as a zero crossing detector.

In another configuration, the electrical isolator circuit includes a signal determination circuit that includes an analog-to-digital converter and a digital processing circuit (e.g., embedded in a microprocessor) that are operable to convert the varying output voltage to a digital value and compare the digital value to a reference value, respectively. The reference value corresponds to a reference voltage, e.g., 1.5 volts, or zero voltage required for a zero-crossing function.

Alternatively or additionally, the input stage further comprises a second resistor, the transmitter and the second resistor being electrically connected in series. The resistance of the second resistor may be selected to limit the current through the transmitter.

Alternatively or additionally, the emitter comprises a Light Emitting Diode (LED). The LED may be an infrared LED.

Alternatively or additionally, the receiver comprises a photodetector, such as a phototransistor.

Alternatively or additionally, the output stage comprises a current source in parallel with the receiver, the current source for providing a current in the opposite direction to the receiver. The current source may be used to compensate for dark current flowing through the phototransistor. The current source is variable, and the galvanic isolator circuit further comprises a feedback circuit for controlling the variable current source in dependence on a signal received from an output of the active circuit. The feedback circuit may be active during a portion of the cycle of the high voltage AC signal, for example, during a negative cycle of the high voltage AC signal when the phototransistor is off. Thus, the varying current source and the feedback circuit may be used to compensate for variations in dark current flowing through the phototransistor.

Alternatively or additionally, the input stage comprises a diode in parallel with the transmitter. The emitter includes an LED, and the diode and LED are electrically connected anode to cathode. In use, the diode may be used to reduce the likelihood of damage to the transmitter, for example due to the application of a supply voltage across the transmitter.

Alternatively or additionally, the output stage comprises a diode in series with the emitter. The emitter includes an LED, and the diode and LED are electrically connected anode to cathode. In use, the diode may limit the reverse bias voltage on the LED such that the LED emits only a portion of the high voltage AC signal, thereby reducing the power consumption of the output stage.

Alternatively or additionally, the emitter comprises two back-to-back leds. In use, the back-to-back LEDs may provide full-wave emission of the high voltage AC signal.

The electrical isolator circuit may be integrated in the same integrated circuit chip as the circuit comprising the electrical isolator circuit, e.g. a circuit component of a home network node.

According to a second aspect of the present invention there is provided an electrical isolator circuit comprising:

an input stage comprising a transmitter of an isolator, the input stage for receiving a high voltage AC signal; and

an output stage operating at a low voltage, the output stage comprising: a receiver of the isolator; an active circuit in series with the receiver between the receiver and a power rail of the output stage; and a first resistor connected to the output of the active circuit and a predetermined voltage applied at the input of the active circuit, the output stage being configurable to produce a varying output voltage across the first resistor in response to input of a high voltage AC signal to the input stage.

More specifically, the isolator includes an optical isolator.

Other embodiments of the second aspect of the present invention include one or more features of the first aspect of the present invention.

Drawings

Other features and advantages of the present invention will become apparent from the following detailed description, which is given by way of example and with reference to the accompanying drawings.

FIG. 1 is a circuit diagram of a prior art electrical isolator circuit;

FIG. 2 is a graphical representation of input and output voltages of the electrical isolator circuit of FIG. 1;

FIG. 3 is a partial circuit diagram of a first embodiment of an electrical isolator circuit according to the present invention;

FIG. 4 is a partial circuit diagram of a second embodiment of an electrical isolator circuit according to the present invention;

FIG. 5A is a graphical representation of input and output voltages of the electrical isolator circuit of FIG. 4;

FIG. 5B is a graphical representation of the input and output voltages of the electrical isolator circuit of FIG. 4 when the amplifier is operating in saturation;

FIG. 6 is a circuit diagram of a third embodiment of an electrical isolator circuit according to the present invention;

fig. 7 is a circuit diagram of a fourth embodiment of an electrical isolator circuit according to the present invention;

FIG. 8 is a circuit diagram of an electrical isolator circuit for calibration according to the present invention;

fig. 9A, 9B, and 9C are three different alternatives of the input stage of any one of the first to fourth embodiments.

Detailed Description

Fig. 3 shows a first embodiment of an electrical isolator circuit 40 according to the invention. As with the prior circuit of fig. 1, the electrical isolator circuit 40 also includes an opto-isolator 42 having an infrared Light Emitting Diode (LED)44 and a phototransistor 46. An input resistor (input resistance) 48 is connected in series with the LED for limiting the current flowing through the LED. The high voltage AC signal is applied to input 58 between input resistor 48 and the cathode of LED 44. The electrical isolator circuit also includes a field effect transistor 50 (which constitutes the active circuit) in series with a load resistor 52 between the phototransistor 46 and the positive power rail in the high-side common-gate configuration. The electrical connection between the field effect transistor and the load resistor defines an output connection 54, and a reference voltage (which constitutes a predetermined voltage) is applied to the gate 56 of the field effect transistor. Although not shown in fig. 3, electrical isolator circuit 40 also includes a zero crossing detector for detecting a location on the signal on output 54 that corresponds to a zero crossing on the high voltage AC signal applied to input 58 of electrical isolator circuit 40. The zero-crossing detector will be described below with reference to fig. 6.

Fig. 4 shows a second embodiment of an electrical isolator circuit 60 according to the present invention. Like the prior circuit of FIG. 1, electrical isolator circuit 60 also includes an opto-isolator 62 having an infrared Light Emitting Diode (LED)64 and a phototransistor 66. An input resistor 68 is connected in series with the LED for limiting the current flowing through the LED. The high voltage AC signal is applied to an input 69 between an input resistor 68 and the cathode of the LED. The galvanic isolator circuit 60 further includes an operational amplifier 70 (which constitutes an active circuit), the operational amplifier 70 having a non-inverting input 72 (which constitutes a first input) related to a reference voltage (which constitutes a predetermined voltage), and an inverting input 74 (which constitutes a second input), the inverting input 74 being electrically connected to one end of a variable output resistor 76 and to the collector of the phototransistor 66. The other end of the variable output resistor 76 is connected to the output 77 of the operational amplifier. The operational amplifier 70 and the phototransistor 66 are in a high-side configuration.

The first stage of the operational amplifier 70 includes a differential amplifier according to normal design practice. The configuration of the electrical isolator circuit is such that the operational amplifier is in series with the phototransistor between the phototransistor and the positive power rail, e.g., the electrical path is defined from the collector of the phototransistor to the inverting input of the differential amplifier (included in the operational amplifier) until from one leg of the differential amplifier to the positive power rail. The electrical isolator circuit 60 further comprises a variable current source 78 connected in parallel with the phototransistor for providing an inverted source current to the current of the phototransistor, the electrical isolator circuit 60 further comprising a feedback circuit 80 for receiving a voltage from the output 77 of the operational amplifier and controlling the current flowing through the variable current source 78 in dependence on the received voltage. Switch 82 is located at the output of feedback circuit 80. As described above, the variable current source 78, feedback circuit and switch 82 are used to compensate for opto-isolator dark current. Although not shown in the figures, electrical isolator circuit 60 also includes a zero-crossing detector for detecting a location on the signal of output 77 that corresponds to a zero-crossing point on the high-voltage AC signal applied to input 69 of electrical isolator circuit 60. The zero-crossing detector will be described below with reference to fig. 6.

The operation of the electrical isolator circuits 40 and 60 of fig. 3 and 4 will now be described with reference to fig. 5A. A high voltage AC signal, such as a 240VRMS supply voltage signal, is applied to the inputs 58, 69 of the electrical isolator circuits 40, 60. Opto-isolators 42, 62 operate in the manner described with reference to fig. 1, except that a much lower forward current flows through LEDs 44, 64 due to operation of active circuitry 50, 70 on the low voltage side of the electrical isolator circuits 40, 60. More specifically, a forward current of 15 μ A RMS (i.e., a high side current) flows through the electrical isolator circuits 40, 60 in FIGS. 3 and 4, while a forward current of 2mA RMS (i.e., a high side current) flows through the electrical isolator circuit in FIG. 1.

In each of the electrical isolator circuits 40 and 60 of fig. 3 and 4, the load resistor 20 of the existing electrical isolator circuit of fig. 1 is replaced with an active circuit for fixing the voltage across the phototransistor 46, 66 at the reference voltage. In the circuit of fig. 3, the active circuit is a field effect transistor 50, while in the circuit of fig. 4, the active circuit is an operational amplifier 70. The current flowing through the phototransistor will pass through the load resistor of the circuit in fig. 3 in response to the current flowing through the LED52 and variable output resistor 76 of the circuit of figure 4. The clamping of the voltage across the phototransistors 46, 66 may substantially eliminate the influence of the electrodes formed by the load resistor 20 and the load capacitor 22 of fig. 1, thereby allowing the values of the load resistor 52 and the variable output resistor 76 to be increased without increasing the time constant to an unacceptable degree in view of the switching requirements. Thus, the value of the input resistors 48, 68 may be increased to reduce the forward current I flowing through the LED_fThereby reducing power consumption.

The lower graph in fig. 5A represents the supply voltage signal 90 applied on the inputs 58, 69 of the galvanic isolator circuits 40, 60. The higher graph in fig. 5A is the voltage signal on the output 54, 77 of the electrical isolator circuit 40, 60, the half-wave portion 92 representing the variable voltage applied by the opto-isolator 42, 62 across the load resistor 52 or variable output resistor 76 in response to the coupling of the high voltage AC signal, and the voltage offset 94 representing the level shift provided by the reference voltage. Returning to the circuit of fig. 4, compensation for opto-isolator dark current is accomplished by means of a variable current source 78, a feedback circuit, and a switch 82. Switch 82 operates to close during negative periods of the high voltage AC signal applied to the electrical isolator circuit and to open during positive periods of the high voltage AC signal. Thus, the feedback circuit 80 can be used to vary the current of the variable current source 78 only when the opto-isolator is off and dark current is flowing in the phototransistor 66. Varying the current of the variable current source 78 in a "dead" manner compensates for variations in dark current caused by changing temperature, etc.

Fig. 5B shows a graphical representation of the input 96 and output 98 of the electrical isolator circuit of fig. 4 when the amplifier 70 is operating in saturation. As shown in fig. 5B, operation of the amplifier 70 in saturation may cause the amplifier output voltage 98 to present a generally square wave path in response to application of the high voltage AC signal 96 to the input of the electrical isolator circuit. The output voltage 98 of the amplifier is input to a latch or similar such circuit to provide a regular digital signal suitable for digital processing. Thus, when amplifier 70 is operating in saturation, the circuit of fig. 4 can provide a digital output signal that can be processed to obtain zero-crossing information without the use of a comparator or analog-to-digital converter, as will be described below with reference to fig. 6, 7, and 8.

Fig. 6 is a circuit diagram of an electrical isolator circuit 100, the electrical isolator circuit 100 comprising the electrical isolator circuit of fig. 3 or 4, as well as an analog-to-digital converter/comparator 102 and an analog-to-digital converter 104. As described in the previous paragraph, when the amplifier 70 of fig. 4 is not operating in saturation, an analog-to-digital converter/comparator is required. Returning to fig. 6, the reader's attention is directed to the description of the circuits of fig. 3 and 4, and fig. 6 identifies the same components using the reference numerals used in fig. 4. Each of analog-to-digital converter/comparator 102 and analog-to-digital converter 104 is connected to an output from active circuitry 70. Digitizer/comparator 102 is configured to operate as a comparator. Where the digitizer/comparator 102 has the composition of the comparator itself, the output of the active circuit is compared to a reference voltage, the value of which is selected to correspond to the voltage level applied on the high voltage AC signal at the input 69 of the electrical isolator circuit, which is offset from the zero crossing by a known amount of time (i.e., a time offset). Thus, as the high voltage AC signal crosses the voltage level, the corresponding voltage on the output of the electrical isolator circuit crosses the reference voltage, causing the comparator to transition and register (register) the zero crossing of the high voltage AC signal; since the time offset is known, the time location of the zero crossing can be determined. Wherein the digitizer/comparator 102 is in the form of an analog-to-digital converter that samples the voltage on the output of the active circuit 70, converts the sampled value to a digital value, and compares the sampled digital value to a reference voltage in the form of a digital value (e.g., by a microprocessor and associated circuitry (not shown)) to register a zero crossing of the high voltage AC signal. As described above, the registration of zero crossings may, for example, be used to provide synchronization of the low voltage circuit with the mains voltage cycle, thereby in turn providing synchronized communication between and among the low voltage circuits of a plurality of networked products. Alternatively, the registration of zero crossings may be used to detect the changing phase of the power supply to determine whether the power supply may fail. The analog-to-digital converter 104 in the electrical isolator circuit 100 in fig. 6 is used to sample and convert the output of the active circuit 70 to provide a digital form of the output for analysis purposes, e.g., a curve form (curve profiling) for diagnostic purposes. Alternatively, the analog-to-digital converter 104 may be used to adjust the level of the reference voltage used by the digital converter/comparator 102 during the calibration process to account for changes in the parameters of the electronics in the electrical isolator circuit or to account for changes in the RMS voltage level of the high voltage AC signal applied to the input of the electrical isolator circuit. More specifically, for example, the RMS voltage of the high voltage AC signal is increased from 90VRMS to 240VRMS, and the output of the analog-to-digital converter is used to increase the level of the reference voltage used by the digitizer/comparator 102 so that the increased level corresponds to a zero crossing point on the high voltage AC signal applied to the electrical isolator circuit (taking into account the time offset).

Fig. 7 is a circuit diagram of the electrical isolator 110. The electrical isolator 110 includes the electrical isolator circuit of fig. 3 or 4, as well as an analog-to-digital converter/comparator 112 and a gain feedback circuit 114. The reader's attention is directed to the description of the circuits of fig. 3 and 4, and fig. 7 utilizes the reference numerals used in fig. 4 to identify like components. An analog-to-digital converter 112 is connected to the output of the active circuit 70 and an input of a gain feedback circuit 114 is connected to the output of the analog-to-digital converter 112. The output of the gain feedback circuit 114 is connected to the variable output resistor 76 of the active circuit 70 (see fig. 4). In use, the analog-to-digital converter 112 and the gain feedback circuit 114 are used to vary the resistance value of the variable output resistor 76 to change the form of the output signal of the active circuit (with respect to the reference voltage used by the analog-to-digital converter/comparator 112) to provide registration of zero crossings in the high voltage AC signal corresponding to the input to the galvanic isolator. The resistance value of the variable output resistor 76 needs to be changed to account for changes in the parameters of the electronics in the electrical isolator or to account for changes in the RMS voltage level of the high voltage AC signal applied to the input of the electrical isolator circuit. As described in the preceding paragraph, the varying resistance value of the variable output resistor 76 may provide an alternative to adjusting the level of the reference voltage used by the digitizer/comparator.

Calibration to account for variations in component characteristics, and in particular variations in the gain of an opto-isolator (current transfer rate (CTR)), will now be described with reference to fig. 8. In a batch of opto-isolators, the CTR varies from device to device by up to four times the change in value. Fig. 8 is a circuit diagram of an electrical isolator circuit 118, the electrical isolator circuit 118 comprising components of the electrical isolator circuit 100 given in fig. 6. Accordingly, the components of the circuit in fig. 8 are identified by the same reference numerals as the circuit in fig. 6 and the circuits in fig. 3 and 4. The description of the constitution and function of the components in fig. 8 may refer to the above description. To calibrate electrical isolator 118 in fig. 8, a predetermined voltage corresponding to the desired output voltage is applied at input 69. The voltage on the output of amplifier 70 is sampled and converted by analog-to-digital converter 104 to provide a digital output value. The digital output value is then compared to a stored digital value corresponding to the desired output voltage, and based on the comparison, the gain of the amplifier 70 is adjusted so that the voltage at the output of the amplifier 70 is substantially the same as the desired output voltage.

Fig. 9A to 9C show different configurations of the input stage of the electrical isolator circuit described above. The input stage shown in fig. 9A includes an input resistor 122 in series with an optocoupler including two parallel LEDs 124, 126, the LEDs 124 and 126 being connected from cathode to anode. The LEDs 124, 126 provide full wave coupling of the high voltage AC signal applied at the input stage of the phototransistor of the electrical isolator circuit. The input stage 130 of fig. 9B includes an input resistor 132 in series with an optocoupler including a single LED 134. A diode 136 is connected in parallel with the LED134 and is anode-to-cathode configured to prevent electrostatic discharge. Input stage 140 of fig. 9C includes an input resistor 142, with input resistor 142 in series with an optocoupler including a single LED 144. The diode 146 is in series with the LED144 and is anode-to-cathode configured such that the LED144 transmits only a portion of the high voltage AC signal, thereby reducing the power consumption of the input stage 140.

The embodiments discussed herein are illustrative of the invention. Since these embodiments of the present invention have been described with reference to examples, various method and/or specific structural modifications and adaptations of the present method will be apparent to those skilled in the art. All such modifications, adaptations, or variations that rely upon the teachings of the present invention, through which these teachings may advance to those skilled in the art, are deemed to be within the spirit and scope of the present invention. The description and drawings are, accordingly, not to be taken in a limiting sense, and it is understood that the invention is in no way limited to the described embodiments.

Claims (34)

1. An electrical isolator circuit, comprising:

an input stage comprising a transmitter of an opto-isolator; and

an output stage, the output stage comprising: a receiver of the opto-isolator; an active circuit in series with the receiver between the receiver and a power rail of the output stage; a first resistor connected to the output of the active circuit and applying a predetermined voltage to the input of the active circuit, the output stage being configurable to produce a varying output voltage across the first resistor in response to the input of the high voltage AC signal to the input stage.

2. An electrical isolator circuit according to claim 1, wherein the output stage is configurable to maintain the voltage across the receiver substantially constant over the predetermined voltage.

3. An electrical isolator according to claim 1 or claim 2, wherein the output stage is configurable to level shift the varying output voltage to a voltage corresponding to the predetermined voltage.

4. An electrical isolator circuit according to any preceding claim, wherein the active circuit and the receiver are in a high side configuration.

5. An electrical isolator circuit according to any preceding claim, wherein the active circuit has first and second inputs, the first input forming a series electrical connection of the receiver and the active circuit, the second input receiving the predetermined voltage.

6. An electrical isolator circuit according to any preceding claim, wherein the impedance of the active circuit is lower than the impedance of the receiver.

7. The electrical isolator circuit of claim 6, wherein the impedance of the receiver is at least twice the impedance of the active circuit.

8. A galvanic isolator circuit according to any preceding claim, wherein the active circuit comprises an active load transistor.

9. The electrical isolator of claim 8, wherein the receiver is a phototransistor and the active load transistor and first resistor are in a cascode configuration in which the first resistor operates as a resistive load.

10. An electrical isolator circuit according to any one of claims 1 to 7, wherein the active circuit comprises a differential amplifier having a first input electrically connected to the output of the receiver and a second input connected to a predetermined voltage, a first resistor being connected between the output of the differential amplifier and the first input.

11. The electrical isolator circuit according to claim 10, wherein the differential amplifier is included in an operational amplifier.

12. The electrical isolator circuit according to claim 11, wherein the first input of the differential amplifier is an inverting input.

13. An electrical isolator circuit according to any preceding claim, wherein the active circuit is arranged to operate in saturation.

14. The electrical isolator circuit of claim 13, further comprising a bi-stable circuit, and an output provided from the active circuit to an input of the bi-stable circuit.

15. An electrical isolator circuit according to any one of claims 1 to 12, wherein the output stage is configurable such that the different output voltage is an analogue output voltage, and such that the electrical isolator circuit further comprises a signal determining circuit electrically connected to the output of the active circuit and operable to determine a position on the analogue output voltage.

16. The electrical isolator circuit of claim 15, wherein the signal determination circuit is configured to operate as a voltage level zero crossing detector.

17. The electrical isolator circuit according to claim 16, wherein the signal determination circuit is configured to compare the analog output voltage to a reference voltage.

18. An electrical isolator circuit according to any preceding claim, wherein the first resistor comprises a variable resistor and the electrical isolator circuit comprises a gain feedback circuit for varying the resistance of the variable resistor in dependence on the varying output voltage.

19. The electrical isolator circuit of claim 18, wherein the gain feedback circuit is configured to vary the resistance of the variable resistor based on a value of the varying output voltage, the value of the varying output voltage being determined by at least one of an analog-to-digital converter and a peak detection circuit.

20. An electrical isolator circuit according to any preceding claim, wherein the output stage operates at a low voltage comprising at least one of a voltage of less than 50VRMS and a DC voltage of less than 120V.

21. The electrical isolator circuit of claim 20, wherein the low voltage signal is a DC voltage of less than approximately 15 volts.

22. An electrical isolator circuit according to any preceding claim, wherein the input is for receiving a high voltage AC signal comprising an AC voltage of 50VRMS or greater.

23. An electrical isolator circuit according to any preceding claim, wherein the output stage is adapted to receive a high voltage AC signal at a frequency of less than 500 Hz.

24. An electrical isolator circuit according to any preceding claim, comprising a signal determining circuit comprising a comparator, a first input of which receives the varying output voltage, a second input of which provides a reference voltage.

25. An electrical isolator circuit according to claim 24, wherein the electrical isolator circuit is operable to determine a zero crossing point on a high voltage AC signal in dependence on a known time limit between a zero crossing point and the point at which the varying output voltage exceeds the reference voltage.

26. An electrical isolator circuit according to any preceding claim, further comprising a signal determining circuit comprising an analogue to digital converter and a digital processing circuit for converting the different output voltages to digital values and comparing the digital values with a reference value, respectively.

27. An electrical isolator circuit according to any preceding claim, wherein the input stage further comprises a second resistor, the transmitter and second resistor being electrically connected in series.

28. An electrical isolator circuit according to any preceding claim, wherein the emitter comprises a Light Emitting Diode (LED).

29. An electrical isolator circuit according to any preceding claim, wherein the receiver comprises a light detector.

30. A galvanic isolator circuit according to any preceding claim, wherein the output stage comprises a current source in parallel with the receiver for providing current in the opposite direction to the receiver.

31. The electrical isolator circuit of claim 30, wherein the current source is variable, and further comprising a feedback circuit for controlling the variable current source in accordance with a signal received from an output of the active circuit.

32. A galvanic isolator circuit according to any preceding claim, wherein the output stage comprises a diode in parallel with the transmitter.

33. A galvanic isolator circuit according to any preceding claim, wherein the output stage comprises a diode in series with the transmitter.

34. An electrical isolator circuit according to any preceding claim, wherein the emitter comprises two back-to-back LEDs.