Voltage Divider Offset Circuit for AC Measurement
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Solution Overview
Problem
Household appliances face operational impairments due to fluctuations in mains voltage and frequency, leading to overvoltage situations and unstable power consumption.
Innovation Solution
A measuring device that uses voltage dividers to scale AC input voltage, providing a scaled and isolated voltage signal for reliable measurement, and an offset unit to generate a stable measurement signal for processing by a microprocessor, enabling efficient voltage monitoring and control within a Safety Extra Low Voltage (SELV) environment.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If voltage dividers with high resistance values are used to scale the input voltage, then galvanic isolation and SELV requirements are met, but the measurement precision may be affected due to the high impedance
Solution Approach 1:
A buffer amplifier is introduced as an intermediary between the voltage divider and the measurement circuit. This buffer has high input impedance that does not load the voltage divider, and low output impedance that drives the measurement circuit effectively. The buffer transfers the scaled voltage signal while maintaining the galvanic isolation provided by the high-value resistors, thus resolving the contradiction between isolation reliability and measurement precision.
2Measurement precision
If the input voltage is directly measured without scaling, then measurement precision is maintained, but safety requirements (SELV) cannot be met due to high voltage levels
Solution Approach 1:
The measurement system is segmented into two distinct domains: a high-voltage side with the voltage divider for scaling, and a low-voltage SELV side with the buffer and measurement circuit. The voltage divider segments the high voltage into a safe scaled version, while the buffer acts as a boundary between the isolated high-voltage domain and the safe low-voltage domain, maintaining both precision and safety compliance.
3Reliability
If offset voltage is added to the scaled voltage, then the measurement signal can be read reliably by the microprocessor, but the circuit complexity increases
Solution Approach 1:
The offset voltage generation and addition functions are merged into a single operational amplifier circuit. The same op-amp that provides buffering also generates the offset voltage through its feedback network and adds it to the scaled input signal. This combining of multiple functions into a single component minimizes the increase in circuit complexity while achieving reliable measurement signals.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
Ensures stable and reliable operation of household appliances by accurately monitoring and adapting to fluctuations in supply voltage, ensuring safe and efficient power management.
Implementation Method 1
The measuring device comprises a first voltage divider configured to couple a first pole, e.g., the neutral conductor or a phase conductor, of the input voltage to a first pole of at least one measuring resistor. The measuring device further comprises a second voltage divider which is designed to couple a second pole, e.g. the phase conductor or the neutral conductor in a complementary manner to the first voltage divider, of the input voltage to a second pole of the measuring resistor.
Implementation Method 2
The first and/or second voltage divider can be designed to effect galvanic isolation of the input voltage from the scaled voltage. Alternatively or additionally, the first and/or second voltage divider can be designed to effect SELV isolation between the input voltage and the scaled voltage. The galvanic isolation and/or SELV isolation can be achieved in particular by the series-connected resistors of the voltage divider. The resistance values can be sufficiently high (e.g. 500 kOhm or more) and the resulting current sufficiently low to effect galvanic isolation and/or SELV isolation.
Data Source
Figure 1~3
Figure 2
AI summary
A measuring device (110) for measuring an AC input voltage (103) is described. The measuring device (110) comprises a first voltage divider (211) configured to couple a first pole of the input voltage (103) to a first pole of at least one measuring resistor (215), such that a scaled voltage (202) is applied to the measuring resistor (215) as a scaled version of the input voltage (103). Furthermore, the measuring device (110) comprises an offset unit (213) configured to shift the scaled voltage (202) by means of an offset DC voltage (203) to provide an offset voltage (204) that does not exhibit a polarity reversal.