emonTx SMT update > Introducing emonTx V3

If you're a regular follower of our blog you might remember towards the end of the summer of last year (2012) we posted up a design of an emonTx SMT (pre-assembled using surface mount electronics).

Since that original post progress on the new design has been rather slow (I've been trying to document progress on this forum thread). Slow progress has mainly been due to the success of the shop! Order fulfilment, stock organisation and administration has been taking up most of our time. However, we have built a prototype emonTx SMT with the original design from Aug 2012. The functions tested so far have worked well. Although we are yet to do a proper accuracy test, however we are confident that it will perform well as it's based on the Atmel energy monitor app note.

emonTx SMT prototype
The original emonTx SMT was designed with high accuracy and flexibility (everything but the kitchen sink on one PCB!) in mind. However being a complex design with many components assembly costs will be high. At some stage in the future we would like to get it manufactured but we feel that a lower cost less complex board with comparable performance to the current emonTx but in SMT form and nicely cased will be more suitable as a next step for us. The performance of the current emonTx is adequate for most monitoring applications and with real power monitoring performance is better than most domestic energy monitors currently on the market.

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Introducing the emonTx V3:


emonTx V3 prototype CAD design

The form factor is the same as the emonTx SMT design. It's been designed to fit in a wall mountable extruded aluminium case with laser cut plastic fascia for the openings. 

emonTx V3 case mockup

The energy monitoring parts of the emonTx V3 are the same as the current emonTx but using SMT components. However unlike the original emonTx (V2) we have put effort into designing the PCB with best analogue performance in mind, time will tell if this will result in improved performance:


emonTx V3 analogue performance design considerations 

We have added an extra 'high sensitivity' reduced range CT channel to be used to monitor up to 4.5kW (@240V). Reducing the monitoring range by using a higher value burden resistor increase the sensitivity when monitoring lower power levels. This channel will be perfect for monitoring single circuits or appliances e.g. solar PV output, heatpump power etc.

Like on the emonTx SMT the emonTx V3 will have the option to be powered from a single AC-AC adapter using a AC-DC half-wave rectifier circuit (see blog post part1 and part2). This circuit has been carefully designed so that it does not effect the quality of the AC sample wave form. Being able to use only one power adapter to power the unit and simultaneously provide an AC voltage sample to be used for Real Power, Vrms, Power Factor and frequency measurements will reduce the cost and overall embodied energy of the system.

The 'heart' of the emonTx V3 is provided by an Rfu. An Rfu is a tiny little Atmega328 with a radio in an SMT XBEE footprint (the final design will also have thru-hole Xbee pads). 

Ciseco RFu - ATmega328 plus RFM12B 
Ciseco RFu - ATmega328 witth SRF radio

The RFu has been designed and manufactured in the UK by Ciseco. The radio on the Rfu can be an RFM12B or SRF. We will be using the RFM12B, but it's nice to have a 'drop-in' option for a different radio. Using the Rfu makes the emonTx V3 more modular and it will allow us to manufacture the emonTx V3 easier and cheaper. As with the current emonTx the ATmega328 will be programmable using an FTDI and the Arduino IDE.

A barebone emonTx V3 prototype PCB arrived from PCB Train over the weekend, if all goes well we should have a working prototype by the end of the week. Fingers crossed! 

emonTx V3 prototype PCB


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