Generating Mains-Borne Carrier with a Versatile Circuit
This circuit serves the purpose of superimposing a 143-kHz carrier wave onto the mains voltage, opening doors to various applications. A prime example of its utility is in the creation of the ‘Mains Remote Switch.’ Beyond the power supply, the circuit comprises a sine-wave oscillator, a buffer stage, and an output transformer that ensures isolation from the mains network. The oscillator, centered around IC1a, adheres to the standard Wienbridge design, with its frequency dictated by components R1, C1, R2, and C2. The synergy between R3, R4, and the amplitude stabilization setup involving T1 results in a gain factor of 3. Within this configuration, FET T1 operates as a controllable resistance, leveraging the interplay of R6, R7, and C3 to impart a degree of linearization to the channel resistance.
Feedback Mechanism for Stable Output Voltage
In this setup, the output voltage undergoes rectification through D1, resulting in a negative voltage concerning the virtual ground. This negative voltage is then smoothed by the combination of C4 and R9 before being applied to the gate of T1 via R7. As the amplitude of the signal increases, the negative gate voltage rises, leading to a corresponding increase in the channel resistance of the FET. Consequently, the gain of IC1a decreases. The FET’s specific characteristics dictate the oscillator’s output voltage. With the BF254A variant utilized in this design, the peak-to-peak value of the output voltage hovers around half of the supply voltage.
However, it’s crucial to note that FET traits are subject to significant device-to-device variations. The opamp choice, AD827, was made due to its speed, ensuring minimal impact on the oscillation conditions. The frequency is deliberately set at 143 kHz, aligning closely with the midpoint of the band spanning from 140 kHz to 148.5 kHz, as per the Cenelec standard 50065-1. Employing E24 values for the frequency-determining components, this frequency range adheres to the general use case, where the maximum allowed voltage stands at 116 dBμV.
Signal Modulation and Compensation
The oscillator output is directed to buffer stage IC1b via header K1. This setup offers the flexibility of modulating or coding the signal externally, with a potential need to bypass C5 depending on the specific external circuit used. To account for oscillator tolerance variations, a potentiometer is placed at the input of IC1b, allowing adjustments to meet standard requirements. In the output stage of the buffer, two power transistors (BD139 and BD140) are configured as complementary emitter followers. The quiescent current through this stage, determined by the voltage drop across D2/D3 and the values of emitter resistors R13 and R14, is kept at a few milliamperes.
The maximum signal excursion is controlled by current sources T2 and T3, as well as the current gain of the output transistors. R12 enhances behavior in the zero-crossing region. To establish isolation from the mains network, an output transformer (Tr1) is employed. However, for safety, it is prudent to consider the entire circuit as connected to the mains potential and adhere to maximum insulation resistance; the bare wire can be substituted with vanished wire.
Power Supply and Practical Considerations
The power supply follows the standard layout: transformer, bridge rectifier, electrolytic capacitor, and a voltage regulator (IC2). Operating from asymmetric supply voltages, the circuit incorporates voltage divider R18/R19 and decoupling capacitors C11/C12 to ensure stability. When fitting the circuit into an enclosure or using it for various applications, it’s vital to bear in mind its asymmetric supply nature. The transformer’s primary winding is driven through C9, and its turns ratio is chosen to achieve the maximum allowable value without significantly exceeding it. To isolate the 230-V mains voltage from the 143-kHz carrier signal, a large capacitor (C10) is needed, necessitating the use of an X2-type capacitor. In the event of fuse F1 blowing, R16 and R17, placed in parallel with C10, discharge the voltage on K2 immediately.
Practically, winding transformer Tr1 is a DIY task, and it’s relatively straightforward. The primary winding consists of 6 turns, while the secondary has 1 turn. Using an EPCOS core with a 16 mm diameter made from N30 material, windings employ 1-mm diameter wire with synthetic insulation, resulting in a total diameter of 2.5 mm. The primary winding is bifurcated into equal halves, accommodating the secondary winding precisely between them. The transformer leads emerge on opposite sides. Additionally, capacitors C11/C12 are essential for referencing IC1 to half of the supply voltage, with the stabilized +12 V available for potential expansion circuits via connector K1.