IoT Clamp Meter

Maplins sell a cheap clamp meter that fits neatly in the palm of your hand. However a much more useful device would be a network connected meter that could read both current and waveform information, therefore being a ‘smart meter’ that is part of the Internet of Things. One application would be to record a time trace of current for a given domestic appliance, for example a dishwasher through its operating cycle, aiding diagnostics for repair, especially if a previous trace exists of the machine working correctly.

Hardware Design

The block diagram of the adapted system is as follows:

Block diagram

This is produced by removing the existing circuit board from the clamp meter (only retaining the clamp sensor itself and the case) and filling the remainder of the case with a micro-controller and an Ethernet board. There were very few steps in the physical circuit, with most functions now in software. The physical circuit used an Op Amp to amplify the initial signal; the micro-controller (an Atmel AT Mega 328P) recording the voltage with one of its integral ADC lines; and an Ethernet interface (based on an ENC28J60) connecting the machine to the outside world. It no longer has a local display, although an added LED returns some status information. The In-system Programmer pins are available via a socket, allowing easy firmware changes. The link to the Ethernet is also via the same SPI interface used by the programmer.

These are some before and after pictures:

Before and After.png

Safety point. I believe the pick-up coil is a current transformer and this does mean some counter intuitive points for those used to more usual voltage supplies. For a voltage transformer, one expects to increase a resistance to decrease current and so limit things to safe values. A current transformer will instead endeavour to drive a constant current, and hence connecting it across a higher resistance (and an air gap can qualify as a high resistance) will generate more heat, not less (see the Safety section of the Wikipedia article)

In particular, using a current transformer from an older, non-digital meter seems likely to have a lower turn ratio and hence more dangerous secondary. You should be familiar with these effects before implementing something.

The initial pick up involved passing the transformed current through a resistor and using an Op Amp to sense the voltage drop across that resistor. The Op Amp was chosen to be a single-, rather than split- power device (in this case a LM358N) to allow the shared use of a 5V DC supply to power it and the micro-controller. The bias and gain on the Op Amp were set (via resistors) to achieve a clean waveform with nearly full scale deflection (i.e. not quite clipping at either end) for an input of around 80 Amps to the sensor. This is a credible figure to cover the range over which the main supply to a house might vary. Using a clamp meter on an individual appliance is not normally possible, because the current enters and returns in a single lead carrying both Live and Neutral. This is actually an opportunity however, because by implementing a short extension lead whose Live and Neutral are passed in multiple loops in opposite directions through a short loop of conduit which the clamp meter can encircle, one effectively makes an amplifier.

Adapter.png

Three wires in each direction creates a x6 amplifier, which means that a full scale deflection of 80A corresponds to about the 13A of a standard switched appliance, allowing more accurate measurements of individual appliances. Further this design means the meter need not itself physically switch scales, which is advantageous in avoiding an air gap for the current transformer (see the Safety point above).

The microcontroller needs very little ancillary hardware; mainly just capacitors, to stabilise the power supply and reset pins, and the status LED. The ENC28J60 provides a 25MHz clock, so the microcontroller does not need its own crystal circuit, but does divide that clock by two to operate within specification at 12.5MHz. Most of the microcontroller pins are unused. Given that there are several unused ADC pins, an obvious enhancement would be to provide mains-synchronised low-voltage AC power to the device, which it could rectify for its own power, but sense via a spare ADC channel to derive phase information to compare with the current traces.

Since the ENC28J60 was a 3.3V device, a voltage level shift was required between it and the micro-controller. In hindsight it would have been better to design it to use 3.3V throughout.

A particular problem with the specific ENC28J60 module I used was that it appeared to be mis-wired on the Ethernet side. Some Googling (now a dead link) suggested this was not an unknown problem, and in my case it was solved by producing a mis-wired Ethernet cable to compensate. Specifically I swapped wire 1 with 2 and wire 3 with 6 (a normal cross-over cable switches wires 1 with 3 and 2 with 6). Although modern self-sensing switches will correct such things, the direct connection to a PC (see below) may require it to be right : but other ENC28J60 modules may be correct anyway.

Software

The microcontroller is programmed in C. I had previously produced a full stack of TCP/IP/Ethernet code (on which I will blog later) for the AT Mega supporting various protocols. However only ARP, ICMP, DHCP and UDP are needed in this application. Although I had written a driver for a legacy ISA card for the data link layer (based on ideas from David Clausen), an ISA card was far too physically large for this device and I had to use the ENC28J60 for which I used Guido Socher’s driver (from Tuxgraphics). I would note that Tuxgraphics and David Clausen’s work were the primary inspiration for this device.

The adapted clamp meter uses a locally administered MAC address (as the ENC28J60 card does not come with a global one assigned) although any MAC address could be set in the software. It will seek a DHCP server over the Ethernet link and accept whatever IPv4 address it is given. If it does not find a DHCP sever, it can be setup to also use a defined, static, IP address on the LAN or to assign itself a known IP address in the APIPA range (169.254.x.y), which is the same behaviour as, for example, a Windows machine that cannot find a DHCP server. On this basis it can be used directly connected to a PC or laptop without a LAN, or can be used across a LAN supported by DHCP. To assist setup, it will respond to a ICMP PING (and, less human-friendly, an ARP packet) once it has an IP address.

The work specific to the meter application is relatively trivial. A bespoke UDP packet (i.e. with appropriate magic number) directed at the device will cause it to respond with a packet containing a set of samples from the ADC spanning one 50Hz cycle and a summed current value. Only one return packet is required as there are around 200, 16-bit samples. For memory related reasons the code limits packets to 536 byte payloads which, when combined with a 20 byte UDP header and a 20 byte IP header corresponds to the 576 minimum datagram size required for IPv4 compliance (RFC 791) without fragmentation. UDP packets can be transmitted in quick succession (<1s spacing) to obtain a real-time trace of current consumption.

Although it would likely be well within the capability of the AT Mega, any further processing can take place at the receiver end where far more compute power is likely to be available. For future enhancement, the packet specifies which ADC pin (from the choice of 0 to 5) the microcontroller should read, enabling perhaps voltage-phase information to be returned.

Since the clamp meter uses a 12.5MHz clock and the AT Mega can achieve its maximum 10 bit ADC resolution only at sampling rates below 200kHz, the sampling rate needs to be reduced and the AT Mega prescaler is used at its maximum setting (which allows the full voltage resolution) as well as taking groups of three samples but only recording one from each group. On this basis, 188 samples were found to correspond to a full 50Hz cycle.

Security

In principle, using items from the Internet of Things on your home network is the IT equivalent of allowing people into your house to rummage unsupervised. However the benefit of producing your own IoT devices is that you have tight control of their behaviour (the device will silently drop all packets that are not ARP; PING; expected DHCP responses; other UDP packets without the magic number or with malformed checksums etc). The code is also relatively short and easy to check. Importantly, and unlike many other devices such as home routers, the device cannot be re-programmed through the Ethernet interface. Instead it is flashed through the out-of-band SPI system (although the ENC28J60 is attached to SPI, it does not control the necessary reset line to initiate programming).

The microcontroller with relatively limited computing power will have no significant resistance to a DoS attack, and anyone who knows the magic number can determine your power consumption trace, but these are hardly material threats behind a firewall and possibly not even so if the device is directly connected to the Internet.

Results

Monitoring the main feed to the house shows both waveforms and current draw very clearly The main graph shows current draw over time, and the insets show the current flow over the AC cycles.

Trace 1

Initially there is low current background (probably mainly CFL lights), with a heavily distorted current spectrum. Although the cycle is clear, it is patently not sinusoidal. Adding significantly larger and mainly resistive loads of heating elements (kettle, dishwasher and (tungsten filament) security light) draws notably more current (enough that their individual switch on times can be seen) and this now approximates a sinusoid.

A second set of results show the pattern of the dishwasher heating element over time and the sinusoidal addition of a kettle. But at one point the microwave is used alone (other than background) and has a very distinctive current draw of its own.

Trace 2

Overall this should be a useful diagnostic tool.

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