Modification of AC voltage/current/power meters for variacs

Originally published on 2025-12-08, 3 min read, ~600 words.

Warning

Working with high voltage can be lethal—disconnect power before starting and verify all wiring is correct and properly isolated before powering up.

I needed to measure AC voltage and current generated by a variable transformer (variac). A great source for combined AC voltage/current/power meters is AliExpress. Prices start as low as $10. While accuracy is not great, it is sufficient for this use case.

The main issue is that these meters draw their supply power from the voltage they measure. Their integrated switched-mode power supplies usually require at least ~80 V to operate. Consequently, they turn off when the variac’s output voltage drops below this threshold.

However, it is quite easy to modify them so that the two voltages—measurement voltage and supply voltage—are provided separately. The neutral terminal remains common for both of these voltages, but this is not a problem when working with a variac.

I performed this modification for two types of meters: PZEM-020 and SPM003.

Peacefair PZEM-020

This single-phase AC meter measures voltage, current, power, consumed energy, power factor, and frequency. It utilizes a shunt resistor for measuring current. The shunt resistor is placed between the two neutral terminals: input and output.

The manual and the specification taken from AliExpress claim the following:

Parameter	Specification
Working voltage	80 ~ 260 VAC
Test voltage	80 ~ 260 VAC
Rated power	10 A/2200 W
Operating frequency	45-65 Hz

To modify the meter, cut the trace marked by a blue arrow and solder a cable to the resistor R15, highlighted by a red circle in the photo below. The cable should be connected to the 230/120 V supply voltage upstream of the variac.

Look at the photo below. The original phase terminals are now for the measurement voltage; I marked them with wiper in the photo. They are both bridged internally. They should be connected to the wiper of the variac. The other two are the neutral terminals. The one with the AC in mark should go to the variac’s neutral point, the other—to the load.

Sinotimer/Sinometer SPM003

This single-phase AC meter is marketed under brands like Sinometer, Sinotimer. It measures voltage, current, power, consumed energy, power factor, and frequency. It utilizes a current transformer for measuring current.

The manual and the specification copied from AliExpress are as follows:

Parameter	Specification
Accuracy	1% ±2 digits
Range AC voltage	AC 80.0–300.0 V/300.0–450.0 V (depending on version).
Range AC current	AC 0–99.99 A, minimum resolution 0.01 A
Power frequency	45.0–65.0 Hz
Power factor	0.00-1.00 PF
Active power	0–45000 W (0.1 W resolution for 0–9999.9 W)
Electric energy	0–999999 kWh 0.01 kWh resolution for 0–999.99 kWh 0.1 kWh resolution for 1000.0–9999.9 kWh 1 kWh resolution for 10000–999999 kWh
Speed	2 times per second
Size	79x43x48 mm
Installation Size	76x39 mm

To modify the meter for separate power and measurement voltages, you must first cut one trace on the PCB, marked with a blue arrow in the photo below:

SPM003 meter circuit board with blue arrow pointing to the trace that needs to be cut.

Then, solder a supply voltage cable to the large capacitor at the location marked by a red circle in the photo below:

Red circle indicates the place the supply voltage should be connected.

SPM003 modified. L—power supply voltage, W—variac's wiper, measurement voltage, N—neutral.

The measurement voltage remains at its original terminal. It is now possible to connect the meter to the variac in the following manner:

How to connect a modified SPM003 meter to a variable transformer.

I put the modified meter in my testing station for mains-powered devices.