Modifying AC Power Meters to Read Down to Zero Volts From Their Spec of > 50 to 80 VAC

These PZEM AC power meters sold under various brands are pretty amazing little devices and are great for use in variable AC power supplies based on varactors / variable transformers but do not read down to zero volts. This simple modification converts these meters to read from 0-280VAC instead of from their minimum spec of ~ 60-280VAC.

The modification involves working with AC voltage which can kill you. Only do the modification if you are comfortable working with AC power. You will need the PZEM power meter and wire, solder, etc.

These are available from Amazon, Ebay, etc. under various brands. A good search term is PZEM-020 v2.

I have linked a couple below that were current when I wrote this instructable with prices from ~US$15-25.

"AC Current Voltage Amperage Power Energy Panel Meter LCD Digital Display Ammeter Voltmeter Multimeter with Split Core Current Transformer CT AC 80-260V 100A Multi-Function Power Monitor"

"Peacefair Pzem-020 Ac Multifunction Digital Meter Power Energy Voltage Current Tester 10a"

I traced the circuit diagram of the power supply in these PZEM meters. The circuit is quite sophisticated but the required modification becomes clear. The same AC input is shared between the AC voltage measurement ('volt sense' voltage divider R12 and R12) and the input to the AC to DC converter ('R15') used to convert the 60-70 to 280VAC to 3.3VDC. So by breaking that shared link would separate the measurement function from the DC supply function.

If we break the circuit board trace before the 'R15' fuse and then solder a wire to the R15 input we have broken the connection between the 'volt sense' and the AC-DC power supply. The volt sense will still read the input voltage (all the way down to 0 volts) and the power supply is still fed the minimum voltage it needs (> 70VAC) from the wire we have soldered to that input.

The diagram summarizes this modification and how it can be used to read the 0 volts from a variable transformer. For a different way to achieve the 'read to 0VAC' please see my instructable on an variable AC power supply. In that modification I fed 5 V just before the 3.3V Power Supply IC shown in the circuit diagram.