FIELD OF THE INVENTION
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The present invention relate3 to control elements used in electronic circuits. The
present invention in particular relates to a control circuit with a high voltage
sense element. More particularly, the invention relates to a system for sensing
high voltage mains AC to provide safety barrier isolation between hazardous
mains utility grid and an user accessible safe extra low voltage.
BACKGROUND OF THE INVENTION
! , For AC mains sense circuit, a rectifier circuit and a zero cross sense circuit is
provided to detect the polarity of half cycle or a sine-wave signal lifted to
reference Vcc/2. At the sarrie time, some kind of safety isolation barrier, as AC
mains involves with hazardous voltage, for example, a measurement control
circuit is provided. Several methods of AC mains detection are known in the art
for example, high voltage resistor divider network followed by rectification
circuits or, 50/60Hz line step-down line sensing transformer followed by
rectification circuit, or, rectifier stage directly at mains input after that DC signal
transmitted to control circuitry through an optocoupler.
One of the prior art teaches sensing or measurement of mains AC through high
voltage resistor divider network followed by a rectifier stage. It can be clearly
seen from Circuit1 (fig.1) that the safety barrier is provided by high voltage
resistor i.e. R207, R208, R209 and R210. As the safety barrier provided by the
resistors is not falling in galvanic isolation category, the users/operators are not
safe while working on a micro-controller circuitry generally defined as user
accessible part. Second disadvantage of this prior art AC voltage mains sensing/
measurement circuit is that its rectification part is formed of U201AI D201, D202,
R201, R202, R203 and R204. This rectifier stage is called a precision rectifier. It
employs an operational amplifier to compensate the forward voltage drop of the
semiconductor diodes. In this method, at least one add on diode is used to
compensate the effect of forward voltage drop of the rectification diodes. This
prior art has many limitations for resistor network is required to double the one
half cycle. Thus, any tolerance of said resistor when present shall add on one
half cycle and subtract on another half cycle. So the final rectified output signal
becomes doubly unbalanced w.r.t. resistor tolerance. Thirdly, where two diodes
are used, the difference in forward voltage drop which is quite common, one
being for rectification another for compensating the forward voltage drop such
that the difference reflected on the rectified output signal. It is quite difficult to
control the same shape of two half cycles of the signal at rectified output until
the resistors being used in this circuit have zero tolerance and both the diodes
have identical forward voltage drop, which is practically impossible. Another
complication included is that one terminal of AC input needs to be connected
with the control circuitry ground. So a requirement of dual supply for the
operational amplifier becomes inevitable.
Another prior art process for high voltage AC sensing is shown in figure-2, where
the sensing is done through a line sensing transformer followed by a centre tap
rectifier stage. It employs a line sensing transformer made by iron core operated
at 50/60Hz frequency. Disadvantage of this method is the physical size of the
transformer, which is too bulky including the input voltage surge capability.
Typically, weight of 230VAC sensing transformer is 2009 which substantially
reduces the packaging density of the control circuit. In such a case, the line
sensing transformer comprises more than 9O0/0 density of the control circuit of an
user accessible part. Also it has limited voltage surge handling capacity. In a
230VAC utility grid 1500V, the voltage spike or surge can be persisting. But a line
sensing transformer designed for 230VAC/50Hz operation can sustain only 20%
above the rated nominal voltage. Beyond this voltage, the spike / surge causes
the transformer's core to become saturated and the primary winding of said
sensing transformer comme'nces behaving like DC resistance of copper winding.
The impedance of copper wire is too low for that range of AC voltage, which is
practically a short circuit condition. Hence, the reliability factor against voltage
spikes or surge of said prior art line sensing transformer is very poor.
The second stage of this AC voltage sensing scheme makes use of a
conventional rectifier circuit with a centre tap winding or full wave rectifier on the
secondary side winding of said line sensing transformer. Semiconductor diodes
are used to rectify the sine-wave waveform. These diodes have an imherent
property of forward voltage drops which interalia affects the accuracy of the
rectified output signal. In stage of a substantially zero-cross instantaneous
voltage measurement the diode's forward voltage is worst as said forward
voltage of the diode is independent of the input AC signal. So near to a zero
cross AC input signal, the diode is not forward-biased until the AC input signal
reaches to its forward breakdown threshold voltage. Typically, forward voltage
for the diodes is 0.7V. So, upto 0.7V input signals, the diode fails to conduct,
causing the output signal to exhibit zero voltage. This condition worsen when the
full wave rectification process is implemented, the two diodes in series sustain
the input AC voltage signal, leading to diode the diode's forward voltage drop, a
typical voltage drop gets increased upto 2 x 0.7V. Accordingly, the output signals
become inaccurate even affer the input signal crosses the forward breakdown
threshold voltage of the diodes, due to subtraction of the output signal. by the
magnitude of the diode's forward voltage drop. Thus, the output signal exhibits a
I sinusoidal waveform.
To minimize the diode forward voltage drop effect, several circuits are known to
I *
be used. However, when a sensing and measurement process operates under
I
I microcontroller or control circuitry at 3.3V or 5V VCC, the impact of the diode
forward voltage drop is significant. Even O.7V drop introduces ~ 2 0 %er ror for
I 3.3V operated control circuitry. To minimize this error a high voltage AC
1 . rectification process is known, in which the AC signal is rectified at higher AC
input signals like 15V to 30VAC, and transmitted via a resistor divider network, to
allow voltage drop at a level compatible with 3.3V or 5.OV control circuitry. In
this way, the error introduced by the forward voltage by rectification diodes can
be minimized. If the input AC signal level is as high as 30VAC, the error of 0.7V
forward voltage is just 2.3%.
However, this type of known arrangement, has few limitations, like input AC
signal is not allowed to increase beyond 42V peak because of safety reasons.
Also, this arrangement/process can be applied only when the original signal is at
a higher voltage AC. When AC signal itself'is at lower amplitude, the error can't
be minimized.
The third example of prior art process of AC mains sensing in an optocoupler
based circuit is shown in figure 3. In this circuit the mains AC is directly fed to
the rectifier followed by an optocoupler to carry the rectified signal to a control
circuit. Optocoupler can have a good safety barrier between the hazardous AC
main and an user accessible control circuit. However, this process has many
disadvantages. Firstly the rectifier diode sustain all the high voltage surge spikes
available on the AC mains. Also forward voltage drop of these rectifier diodes
generates a non-linearity in the output sense signal.
The biggest disadvantage of using an optocoupler for the particular sensing
applications under discussion, is that the optocoupler has variations in the
current transfer ratio (CTR). So the output signal can't have a fixed ratio
between AC mains and the rectified output signal. To overcome this problem,
some variable resistor is required to compensate this CTR variation at first time
use. The problem of CrR being recurring, an one time adjustment with a variable
resistor is not a complete solution. CTR of the optocoupler further varies
corresponding to temperature and ageing of the optocoupler, which interalia
requires a quite complicated circuit for temperature compensation. Moreover
there is no method available that can compensate the ageing effect of the
optocoupler current transformer ratio.
Publication No. CN202049187 describes a voltage sensor which has a small zero
drift and a rapid response speed. The voltage sensor is provided with a
resistor R1, a second resistor R2, a third resistor R3, which are in serial
connection and are connected with a primary voltage end, one end of the R2 is
connected with a positive input end of an operational amplifier IC1, the other
end of the second resistor R2 is connected with a negative input end of the
second resistor ICl, an output end of the operational amplifier, IC1 is connected
to input end of an isolation amplifier IC2, an output end of the IC2 is connected
to a voltage current switching circuit IC3, and an output end of the IC3 is a
current detecting end M.
US Patent No. 4,527,118 provides a testing device for indicating an electric
voltage, and its polarity for continuity testing, which includes two handles that
are connected by a cable, provided with test prods, each handle containing a
high-resistance resistor connected in series with the test prods, and one of these
handles contains optical indicating elements for displaying the ranges staggered
in the voltage ranges (for instance 6, 12, 24, 50, 110, 220, 380 and 660 V), an
oscillator which can also drive an acoustic signal generator if provided, and an
isolating amplifier which amplifies the input current that is limited by the two
series resistors, switches the battery provided in the testing device into the
circuit and thereby brings about the staggered indication of the voltage to be
tested, optionally brings about the ranges of the polarity, and switches on the
acoustic signal generator.
US Patent No U.S. 4,361,808 describes methods and systems for measuring
dielectric constants of earth formations adjacent to a bore hole. Earth formations
having high brine saturation exhibits a low electrical resistivity while formations
having high oil saturation exhibit a high electrical resistivity. An alternating
current is passed through a portion of the formation and a reference resistor
disposed in series with the portion. Said portion consists of a reference resistor,
a first differential isolation amplifier, and a second differential isolation amplifier
for measuring the voltage across the reference resistor.
Avago Technologies provides an automotive isolation amplifier with R2CouplerTM
isolation for voltage and current sensing in a process of monitoring electronic
motor drives and battery system. In a typical implementation, motor currents
flow through an external resistor and the resulting analog voltage drop is sensed
by the isolation amplifier ACPL-782T. The voltage sensing optical isolation
amplifiers by Avago Technologies are based on sigma-delta modulation that is
specifically optimized for voltage sensing. This relatively new family of opticallyisolated
voltage sensors has a high 1 GR input impedance, which simplifies the
resistor network design by allowing higher-value resistors to be used to scale the
voltage. .
LEM International SA provides AV 100 type voltage transducers which can
measure any kind of signal, DC, AC, pulsed and complex. The voltage to
measure (VP) is directly applied on the primary connections through an internal
resistor network and the associated components allowing the signal to feed an
isolation amplifier.
However, the techniques for sensing and measuring mains AC voltage mentioned
in prior arts suffer from one or more drawbacks such as lack of precision and
stability in measuring the voltage.
Therefore, the present invention provides a system for voltage sensing
comprising resistors network plus isolation amplifier for safely and accurately
measuring the high voltage mains AC.
OBJECTS OF THE INVENTION
Accordingly, it is an object of the present invention to propose a system for
sensing high voltage mains AC to provide safety barrier isolation between
hazardous mains utility grid and an user accessible safe extra low voltage.
Another object of the present invention is to propose a system for sensing high
voltage mains AC to provide safety barrier isolation between hazardous mains
utility grid and an user accessible safe extra low voltage, that has minimum
number of components.
Yet another object of the present invention is to propose a system for sensing
high voltage mains AC to provide safety barrier isolation between hazardous
mains utility grid and an user accessible safe extra low voltage, which can sense
and measure voltage with high precision and accuracy.
Still another object of the present invention is to propose a system for sensing
high voltage mains AC to provide safety barrier isolation between hazardous
mains utility grid and an user accessible safe extra low voltage, which has
excellent offset and gain accuracy and stability over time and temperature.
A further object of the present invention is to propose a system for sensing high
voltage mains AC to provide safety barrier isolation between hazardous mains
utility grid and an user accessible safe extra low voltage, which has lower: profile
component with better linearity.
A still further object of the present invention is to propose a system for sensing
high voltage mains AC to provide safety barrier isolation between hazardous
mains utility grid and an user accessible safe extra low voltage, which is easy to
produce and is cost effective.
Yet further object of the present invention is to propose a system for sensing
high voltage mains AC to provide safety barrier isolation between hazardous
mains utility grid and an user accessible safe extra low voltage, which is enabled
to filter high voltage stress using series connected resistor network string, the
number of series connected resistors being dependent depending on maximum
AC input voltage.
Another object of the present invention is to propose a system for sensing high
voltage mains AC to provide safety barrier isolation between hazardous mains
utility grid and an user accelssible safe extra low voltage, which is capable to
provide galvanic isolation between primary AC circuits and an user accessible
! control circuit by using isolation amplifier and auxiliary control power supply.
I
I I SUMMARY OF THE INVENTION
Accordingly, there is provided a system for high voltage Mains AC sensing that
includes of a full-wave rectifier circuit connected and extended between mains
utility grid and a low voltage controller circuitry and extending with the provision
of an isolation amplifier. The present invention provides a safety reinforced
insulation barrier between the primary circuits that is the mains utility grid with
zero effect of forward voltage drop of rectification diodes,'voltage surge or spike
capacity beyond 3000V, including an effective packaging density for the control
circuit. Variation in shape and amplitude of the output signal is less than 1%
without any trim pot adjustment. The system shows very low ~0.1%v ariation in
relation to the working temperature range.
In a preferred embodiment of the present invention, the mains utility
sine-wave measurement to low voltage operated micro-controller includes a high
voltage resistor network for detecting hazardous sine-wave signal followed by
isolation amplifier to provide a safety barrier between the hazardous mains utility
grid and a user accessible safe extra low voltage.
In another embodiment of the present invention, the sine wave signal
from the utility is propagated to the second side of the isolation amplifier which
has a fixed gain.
In yet another embodiment of the present invention, the isolation
amplifier has a fixed gain for providing the safety barrier isolation between the
primary circuits i.e. mains utility grid and the user accessible safe extra low
voltage control circuit. So mains AC signal is transferred to the control circuit
with a fixed ratio without changing its shape.
In still another embodiment of the present invention, the output of the
isolation amplifier has two terminal AC signals that are fed to a back to-back
connected differential amplifier.
In another embodiment of the present invention, during a positive half
cycle, an AC non-inverting pin of the upper of the two differential amplifier
becomes positive and an inverting pin of the lower of the two differential
amplifier becomes positive. So the upper differential amplifier sourcing the
output current based on its input signal wherein the lower differential amplifier
sinks the output current based on its input signal. Therefore, the effective signal
I
at the input terminal resistor becomes half of the differential amplifier input with
1 a multiplier factor of differential amplifier gain.
In yet another embodiment of the present invention, during a negative
half cycle, the AC inverting pin of the upper differential amplifier becomes
positive and the non-inverting pin of the lower differential amplifier becomes
positive.
T
In still arlother embodiment of the present invention, the resistors may
be connected in series, parallel or in series-parallel combination.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
It is to be noted, however, that the appended drawings illustrate only typical
embodiments of this invention and are therefore not to be considered for limiting
of its scope, for the invention may admit to other equally effective embodiments.
Fig. 1-A illustrates a prior art circuit diagram for a system for AC voltage sensing
or measurement through high voltage resistor divider network followed by some
precision rectifier stage using operational amplifier;
Fig. 1-0 illustrates another prior art circuit diagram of a system for AC voltage
sensing or measurement through high voltage resistor divider network followed
by some rectifier stage using diode;
Fig. 2-A illustrates a still another prior art circuit diagram of a system for AC
voltage sensing through a line sensing transformer followed by centre tap
rectifier stage;
Fig. 2-8 illustrates a prior art circuit diagram of a system for AC voltage sensing
through a line sensing transformer followed by full bridge rectifier stage;
I 1 . Fig. 3 illustrates a prior art optocoupler based AC mains sensing circuit;
Fig. 4-A illustrates a circuit diagram of a system for sensing AC voltage according
to the present invention using isolation amplifier with high voltage resistor string I
l and reactification stage ;
Fig. 4-8 illustrates a circuit diagram of the system of Figure 4A for producing
isolated power supply to drive primary side control circuit of isolation amplifier,
provided with galvanic isolation between hazardous primary circuits and an user
accessible control circuit.
Fig. 5 illustrates a circuit diagram of a system for sensing AC voltage according
to the present invention using isolation amplifier with high voltage resistor string
and output siganl lifted with referance VccJ2;
Accordingly, the present invention provides configurations of a full-wave rectifier
starting from mains utility grid to a controller circuitry for safety reinforced
insulation between primary circuits i.e. mains utility grid with zero effect of
forward voltage drop of the rectification diodes, voltage surge or spike capacity
beyond 3000V, and packaging density to a control circuit. Referring to figs. 4 and
5, a full-wave rectifier operated micro-controller 100 of the present invention
includes a high voltage resistor network 101 in which a plurality of resistors is
connected in series, parallel or in series-parallel combination for detecting
hazardous sine-wave signal. An isolation amplifier 102 provides a safety barrier
between the hazardous mains utility grid and an user accessible safe extra low
voltage i.e. battery operated micro-controller based control circuit. So, sine-wave
coming from the utility grid is fed to said high voltage resistor network 101, and
allowed to be transmitted via a first side of the isolation amplifier 102 such that
the sine wave signal without changing its original shape is propagated to the
second side of the isolation amplifier 102 which has a fixed gain. In this way, the
sine-wave signal at the output of the isolation amplifier 102 retains its original
shape as coming through the mains utility grid. The instantaneous voltage signal
of the sine-wave transmitted through the safety barrier is thereof fixed
divider/multiplication to its original source.
The output of the isolation amplifier 102 has two terminal AC signals which are
feed to back to-back connected differential amplifier. During a positive half cycle
of the input AC signal, first output pin no. 7 of the upper differential amplifier
"Ul01" becomes positive w.r.t. its second output pin no. 6 with a fixed gain at
said upper of the differential amplifier 'UlOl". This AC signal is fed to a back to
back connected differential amplifier made of "U102A and "U102B"
simultaneously.
During a positive half cycle of AC, the non-inverting pin of U102A becomes
positive and inverting pin of U102B becomes positive. So upper one of the
differential amplifier i.e. made of "U102A" sources output current based on its
input signal where lower one of the differential amplifier i.e. made of "U102B"
sinks output current based on its input signal. So the effective signal at the input
terminal resistor 'Rlll" becomes half of the differential amplifier input with a
multiplier factor of the differential amplifier gain. But the signal remains on
I positive side of control circuit as OPamp "U102A" and "U102B" has power supply
rail of OV to 5V.
During a negative half cycle of AC, the inverting pin of U102A becomes positive
and non-inverting pin of U102B becomes positive. So upper one differential
amplifier i.e. made of "UlO2A" sinks output current based on its input signal
wherein the lower one of the differential amplifier i.e. made of "U102B" sources
output current based on its input signal. The effective signal at the input terminal
resistor " R l l l " becomes half of differential amplifier input with a multiplier factor
of differential amplifier gain as supply rail of differential OPamp are OV to 5V.
Waveforms are shown in fig. 6 and 7.
The present invention provides rectified signal to low-voltage control circuit
without using any semiconductor diodes. Hence, the fomvard voltage drop error
of the rectification diode is nullified. The isolation amplifier has a fixed gain
mentioned on its datasheet, so it provides safety barrier between the primary
circuits i.e. mains utility grid and the user accessible safe extra low voltage
control circuit. So mains AC signal is transferred to the control circuit with a fixed
ratio without changing its shape.
Numerous modifications and adaptations of the system of the present invention
will be apparent to those skilled in the art, and thus it is intended by the
appended claims to cover all such modifications and adaptations which fall within
the true spirit and scope of this invention.

WE CLAIM
1. A system for sensing high voltage mains AC to provide a safety barrier
isolation between hazardous mains utility grid and an user accessible low
voltage device, the system comprising:
- a high voltage resistor network operably connected to the AC mains
grid generating sine waves including hazardous signals, the
network eliminating the forward voltage drop at the rectification
stage; and
- an user accessible low voltage operated micro-controller based
device,
characterized by comprising an isolation amplifier operably interposed
between the high voltage resistor network and the user accessible
device to supply to the device the sine wave signal in the Original
shape of the signal initially outputted from the AC main grid so as to
provide a safety barrier between the AC main grid and the low-voltage
user accessible device.
2. The system for high voltage AC mains sensing and measurement as
claimed in claim 1, wherein the resistors in the network are connected in
series or parallel or in series-parallel combination.
3. The. system for high voltage AC mains sensing and measurement as
claimed in claim 1, wherein a ground pin of the isolational amplifier is
connected to a neutral pin of AC input through the series connected high
voltage resistor network to provide a supplementary isolation between the
isolational amplifier and the AC mains utility grid.
4. The system for high voltage AC mains sensing and measurement as
claimed in claim 1, wherein the isolation amplifier has a fixed gain for
providing safety barrier isolation between the mains utility grid and user
accessible low voltage device.
5. The system for high voltage AC mains sensing and measurement as
claimed in claim 1, wherein the mains AC signal is transferred to a control
circuit of the device with a fixed ratio without change in its shape.
6. The system for high voltage AC mains sensing and measurement as
claimed in claim 1, wherein a variation of the shape and amplitude of the
sine'wave signals are'less than 1% without any trim pot adjustment.
7. The system for high voltage AC mains sensing and measurement as
claimed in claim 1, wherein the output of the isolation amplifier has two
terminal AC signals that are fed to a back to a back connected differential
amplifier.
8. The system for high voltage AC mains sensing and measurement as
claimed in claim 1, wherein the combination and interconnection of the
high voltage resistor network provides balanced dropped down signal to
the isolation amplifier.
9. A method of sensing high voltage mains AC to provide safety barrier
isolation between a hazardous mains utility grid and an user accessible
low-voltage device, the utility grid is connected to a high voltage resistor
network having at least a plurality of inter-connected resistors to eliminate
the effect of forward voltage drop at rectification stage, and an isolation
amplifier interposed between the user accessible low-voltage device and
the hazardous mains utility grid, the method comprising the steps of:-
- feeding simultaneously the terminal AC signals of the isolation
amplifier to a lack to lack connected differential amplifier
comprising one each lower and upper part;
- outputting current during a positive half-cycle of the sine wave
signal by the upper differential amplifier based on the input signal
wherein the lower differential amplifier sinks the output current
causing the effective signal at the input terminal to become half of
the differential amplifier input with a multiplier factor, but still
remaining on the positive side;
- outputting current during a negative half-cycle of the AC signal
based on input signal by the lower differential amplifier, wherein
the upper differential amplifier sinks the output current causing the
effective signal at the input terminal to become half of the
differential amplifier input with a multiplier factor of differential
amplifier gain.