Here’s a rather special voltmeter that will let you measure the AC grid voltage and also see very accurately how it fluctuates around its nominal value. The voltmeter has a measuring range of around 35 V that you can center around the nominal voltage from the grid.
Enhanced Voltage Monitoring Circuit: Leveraging Low Voltage Bridge
This circuit capitalizes on a low-voltage bridge configuration for ease of implementation. The voltage derived from the TR1 secondary, determined by the transformer ratio and directly proportional to the mains voltage, undergoes rectification via D1. It is further filtered by C1 and stabilized at 12 V through D5. Simultaneously, the same voltage is rectified by D2 but remains unregulated and is only mildly filtered by C2 to maintain circuit responsiveness. Through careful selection of R3, R4, and P2 values, the voltage at the junction of R3 and R4 can be fine-tuned to 12 V when the mains voltage is at its nominal level. Any deviations from this nominal value will consequently alter the voltage at this junction, thereby affecting the reading on meter M1.
Meter Calibration Simplified: Center-Zero Meter Unnecessary
One of the merits of this design is its adaptability to various meter types without necessitating a center-zero meter. By setting a reference point where the needle rests at the center of its range, which corresponds to 115 V, there is ample room on both sides to indicate voltage increases or reductions. The circuit diagram provides flexibility by accommodating two widely available meter types. With potential modifications to R2, and potentially R3 and R4, it can be tailored to work with practically any reasonably sensitive meter.
Adjusting the Circuit: Utilizing a Variable Transformer (Variac)
Although adjusting this circuit is not overly complex, access to a variable transformer, or variac, is essential. As variacs may not be readily available, consider contacting local technical institutions, such as technical colleges, where you may be able to borrow one temporarily for your adjustments. It’s important to note that most variants are auto-transformers, lacking electrical isolation. To calibrate the circuit, set P1 to mid-travel and adjust the variac to 115 V. Subsequently, fine-tune P2 to align the meter reading with zero. Gradually increase the variac to 130 V and adjust P1 to ensure the meter reaches its full-scale reading. It’s worth noting that due to the circuit’s simplicity, there may be some interaction between these two adjustments, requiring iterative adjustments to strike the best compromise. Fortunately, this process can be completed within a matter of minutes.
Completing the Setup: Scaling the Meter for Precision
With the adjustments in place, the final step involves scaling the meter appropriately, typically ranging from 100 to 130 V. This calibration yields an expanded-scale voltmeter that excels at tracking even the subtlest fluctuations in AC power line voltage. www.elektor.com/081181