The other day, whilst walking through my local Jaycar Electronics, I picked up an inline mains power meter for $16.95 (it was on special, down from $21.95). At that price, I was curious, how does someone manufacture and sell for some sort of profit a device that can measure up to 2400W of power. For the low price, how is the current measured. Does the device use a current transformer (CT)? Or a resistive shunt which would be much cheaper? Is there some sort of custom IC performing the energy measurement?
Also, is there any scope to hack the device, similar to the Tweet-a-Watt?
Getting right into it, 2x tri-point screws were removed from the rear. A flat blade screwdriver is useful for prising the case apart. I found it easier to prise the unit apart from the bottom.
Once the case is apart, the internals are quite compact. Immediately, a resistive shunt used to measure current can be seen on the bottom right of the green PCB. The two heavy gauge wires are the active connection. When in service, current from the rear male prong connects to the obscured red wire, passes through the resistive shunt and then through the other red wire to the front female contacts.
I don’t know a great deal about the requirements or standards relating to electronic circuits interfaced to mains rated voltages. But, I am aware there are some requirements relating to the types of capacitors that are used. It appears that the circuit is using an X2 type capacitor – used to filter out differential mode noise and is connected between active and neutral. Check out this link, it has some good overview information regarding mains rated caps.
On the rear of the main PCB, things are more interesting.
On the right hand side is a Cirrus Logic CS5460 “single-phase, bi-directional power/energy IC”. Reviewing the datasheet, the device is a dedicated chip for performing voltage, current and energy consumption calculations. Current measurement is performed using a resistive shunt. It appears that the designers of the power meter have closely followed Cirrus Logic’s recommended circuit design.
To the right hand side, the SOIC-8 device is a “HK24C02”, I couldn’t find much detail on the chip, apart from it being a 256 x 8bit EEPROM. This would be used for the auto-boot function described in the CS5460 datasheet and it would contain calibration and configuration settings.
There is quite a bit of information in the CS5460 datasheet and it uses a “3wire digital serial interface” to read and write to the CS5460’s registers. A bonus is that the serial interface pins are clearly labelled on the PCB adjacent to the ribbon cable on the top right. There is some scope for customisation here.
The operation of the LCD and adjustment of the power meter’s settings are not performed by the CS5460. Behind the LCD is an epoxy encapsualted IC with no details. This device would be interfacing to the CS5460 via the serial interface.
An interesting feature of the CS5460 is that for “low cost” applications, two pins can provide a pulse output for energy consumed or exported to the grid. When configured, the EDIR and EOUT pins can be configured to pulse when a unit of energy is consumed or exported. The settings and operation of these pins do require configuration so the EDIR and EOUT pins may not function “straight out of the box”.
Anyway, a resistive shunt is used for the low cost current measurement and a custom IC handles all the power measurement calculations. There may be some potential for re-purposing the hardware in the power meter, either using the serial interface or the EDIR and EOUT pins.