Elektor_Mag_-_January-February_2021

66 January & February 2021 http://www.elektormagazine.com

This project was inspired by the AC/DC Power Meter published
in Elektor Magazine in September 2015 [1]. At first I thought it was
very interesting and I wanted to build one myself, but I found some
limitations and drawbacks in this design, so I decided to develop an
“improved” design. Things I wanted to improve were:

The input circuit: It should be possible to measure current and
voltage independently: the current through the connecting leads
should not influence the voltage reading.
The sampling rate: the low sampling rate makes it only possi-
ble to measure the first eight harmonics of a 50 Hz signal (with
perfect filter). This should be increased to the first 40 harmonics.
The range switching and offset correction must be made
automatically.

This means that the input and amplifier circuit need a complete

redesign. Also, a microcontroller is needed at the hot-side, to make

automatic offset correction and auto-ranging possible. The block

diagram in Figure  1 shows a main board and (up to) three satellite

boards.

The Main board

The schematic (see Figure  2 ) is largely a copy of the EasyPIC V7 board,

with a graphic LCD touch screen as user interface. A square wave

oscillator (circuit around IC1) generates a 12 Vpp square wave with

a frequency of about 150 kHz for powering the satellite boards. The

data communication with the satellite boards will be via I2C, where

the microcontroller on the main board is the master and the satellite

boards are the slaves.

The power supply section of the main board supports either a 12 V

stabilized or a 15...18 V supply without stabilization. The selection can

be made by a solder joint (SJ1). There are two other solder bridges/

jumpers: MAX1 and MAX2. With these, the number of channels can

be set for the master: closing MAX1 only means one, closing MAX2

means two and closing both means three channels (satellite boards).

Some jumper wires (marked with Jx in schematics and on the main

PCB layout) are used to avoid the need for a multilayer PCB.

The Satellite board

On the Satellite boards (schematic in Figure  3 ) I wanted to measure

voltage and current independently. This is not fully realizable, but with

the circuit the low voltage input and the current input can float about

+/- 1 V with respect to each other. This is realized by the diode D3,

D4, D7, D8. Without measures taken, the circuit can be damaged by a

wrong connection: the high potential to the low voltage input and also

the current input at the low voltage side. To prevent this damage, the

circuit with T10...T14 and reed relay K1 is added. A first current limita-

tion is realized by R11: at 750 V input voltage the maximum current is

0.5 A. If the current through R11 is larger than some 10 to 20 mA, there

Multi-Channel

Power Analyzer

Up to 3 Channels, with Graphic and Alphanumeric Display

By Wil Dijkman (The Netherlands)

When making voltage, current and power measurements on AC-powered devices things are

getting more complicated than measuring DC, because waveform and phase-shift between

voltage and current play an important role here. This instrument not only measures and

calculates quantities, but also shows waveforms and spectra of AC-signals on a graphical LCD.

LCD +

Ch 2 mainboard touchscreen

Ch 1

Ch 3

1 2V

I

I

I

I

I

I

V

V

V

V

V

V

Satellite boards

Power

I^2 C

Figure 1: Block diagram of the Power Analyzer.

homelab project