The Internet Power Line Adapter - University of Queensland

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Home Automation, Power Lines and the Internet Quenten Alick The basis of the circuit is the measurement of current. Since the adapter is plugged into the main supply, it is reasonable to expect that the supply voltage will be a constant 240V AC. If the current can be measured then by sampling the current and averaging over a time period, then using the result in a simple “power equals voltage times current” equation, a rough measure of the power should be attained. The main concern with this circuit is choosing a suitable series resister that is able to dissipate the power that may run through it.. For this, a 0.1Ohm 7-Watt power resister can be placed in series with the load i.e. the appliance. As a precautionary measure, a 5A fuse should be placed in series with the appliance. The voltage across the power resister can then be passed to a low pass filter to remove line transients and noise. The chosen filter was a Sallen-Key 2 nd order active filter with a cut-off point of 100Hz [4]. This should be sufficient to retain the power signal while removing most of the transients. The output from the filter can then be passed to a simple Op Amp circuit to achieve a gain that will distribute the resulting voltage across the 5V range of the ADCs used on the microprocessor. Finally, the switching of the adapter can be accomplished relatively simply using a relay. A dry reed relay that switches 240V AC using a 5V input was selected. This relay needs to be placed in series with the appliance. 4.4 Microprocessor The microprocessor used for both the client and server must be capable of controlling the power lines network chip. As mentioned before, there is no need for fast communications. Therefore, the processor need not be fast and an 8-bit microprocessor should be sufficient. For the client, there is an additional need for Analogue to Digital Converters (ADCs) in order to translate an analogue signal from the power meter into a digital signal. It is possible to use external ADCs so this requirement will serve to mainly save board space. - 20 -
Home Automation, Power Lines and the Internet Quenten Alick The chosen microprocessor must also have both an SPI (Serial Peripheral Interface) and an SCI (Serial Communications Interface). The SPI is needed to be able to communicate with the Intellon P300 network chip and the SCI interface is needed to communicate with the PC that will be running the web server. In order for the SCI interface to be able to communicate with the PC a RS232 transceiver must be used to convert the voltage levels from TTL to RS232 and vice versa. The MAXIM MAX232 RS232 interface should be able to perform this task with a minimum of extra circuitry. There are a number of processors from the Motorolla range that are suitable. The processors considered were from the MC68HC05 and the MC68HC11 range. The use of a MC60HC05C series would have been ideal since the Intellon development board already used one. The only drawback would have been the need to provide external ADCs since the MC68HC05C does not provide any internal ones. This would require more PCB space, making the board more expensive. However, the cost of purchasing another development kit for the microprocessor made this option non-feasible. Even if a development kit could have been found, the individual cost of $55 per microprocessor was also rather high. If this option had been taken, the cost would have risen about $310. This left the choice with the various MC68HC11 processors for which there were suitable development kits available. The microprocessor used in the development kits was the MC68HC11A1. This is an 8-bit processor with ADCs and a SPI and SCI interface and so meets the requirements. The 256 bytes of RAM should be sufficient to store temporary variables, however the 512 bytes of EEPROM are insufficient to store data from the power meter. There are several options available to address this problem. Firstly, the MC68HC11A1 development boards come with 8k of parallel SRAM, which is more than sufficient to store the required data. If necessary, the system could be demonstrated using the development boards. The preferred option would be to - 21 -
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The Internet Power Line Adapter - University of Queensland

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