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.
The circuit uses a bridge supplied at low voltage to make it easier to implement. The voltage available at TR1 secondary, which reflects the mains voltage multiplied by the transformer ratio (fixed and constant), is rectified by D1, filtered by C1, and stabilized at 12 V by D5. This same voltage is also rectified by D2, but this time is not stabilized and is only slightly filtered by C2 so that the circuit remains responsive. Given the value of R3, R4, and P2, the voltage at the junction of R3 and R4 can be adjusted to 12 V when the mains is at its nominal value. Any increase or decrease in it will then vary the voltage at this point and change the reading of meter M1 accordingly.
There’s no need to use a center-zero meter, thanks to the adjustment available via P1 and P2. All you have to do is decide that when the needle is at the center of its travel, this corresponds to 115 V. In this way, you’ll have a margin in both directions for indicating any increase or reduction. The circuit diagram gives you a choice between two types of widely-available meter, but by modifying R2, and possibly R3 and R4, it can be adapted to use practically any reasonably sensitive .
It’s not difficult to adjust this circuit, but you do need to have access to a variable transformer (variac). As these aren’t particularly common, contact your local technical college, for example, where you should be able to borrow one just long enough for your adjustments. Remember, most variants are auto-transformers, i.e. they do not afford electrical isolation. To adjust, set P1 to mid travel and adjust the variac to 115 V. Now adjust P2 so that the meter reads 0. Then turn the variac up to 130 V and set P1 so the meter reads full scale. Do watch out, though, as the simplicity of the circuit means there is some interaction between the two adjustments, so you’ll need to work by successive approximation to achieve the best compromise — but it’ll still only take you a few minutes.
All that is left for you to do is to graduate the meter scale from, say, 100 to 130 V, and you’ll have a magnificent expanded-scale voltmeter that will let you follow the slightest variation in the AC power line voltage. www.elektor.com/081181