Circuit: AC-DC Converter

A clever 1997 Elektor design shows how accurate RMS-to-DC conversion can be achieved using just two op-amps and a handful of components. This circuit delivers better than 2% accuracy across a wide input range, making it a surprisingly powerful solution for simple AC measurement tasks.

In July 1997, Elektor presented H. Bonekamp’s compact AC-DC converter project that demonstrated how a precise RMS-to-DC conversion can be achieved with minimal circuitry. The design used just two op-amps and a few of discrete components.

The Design

Unlike rectifier circuits that relied on multiple stages for rectification, filtering, and buffering, this design integrated all three functions efficiently. The first op-amp (IC_1a) performed half-wave rectification of the incoming sinusoidal signal, with carefully selected resistor values ensuring unity gain, Elektor explained. The BAT85 Schottky diodes minimized forward voltage drop, improving accuracy at low signal levels.

The second stage, built around IC_1b, processed the rectified signal further by amplifying and averaging it to produce a DC output proportional to the RMS value of the input. By feeding back a scaled version of the signal via R₂, the circuit effectively reconstructed the absolute value of the waveform and applied a calibrated gain factor, Elektor noted. The RC network (R₅–C₁) introduced a defined averaging time constant (approximately 2.22 s), smoothing the output into a stable DC level. Additionally, IC_1b served as a buffer, ensuring low output impedance and consistent performance when driving external loads.

The simple design provided accuracy. Offset errors were minimized through the R₆–R₇–P₁ network, allowing precise calibration with a known reference signal. The circuit accommodated a wide input range (50 mV to 7 V RMS) and operated across frequencies from 10 Hz to 10 kHz, making it suitable for general-purpose measurement applications.

“Setting is simple,” Elektor explained. “Apply a 1 kHz sinusoidal signal at a level of 50 mV r.m.s. to the input of the converter and adjust P₁ until the output is a direct voltage of 50 mV,” Elektor explained.

With an accuracy better than 2% and a current consumption of only about 3 mA, this circuit was an option for engineers seeking a straightforward yet effective AC measurement front end.

The AC-DC Converter Project

The original article, “AC-DC Converter,” appeared in Elektor July/August 1997. Here is the article.

Editor's Note: The article first appeared in a 1997 issue of Elektor. Due to the project’s age, some parts and components might not be available. Still, the project should inspire you to start a new design.

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