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emonTx SMT

The emonTx is the data-gathering heart of the system. In the most basic configuration, it uses just one current transformer. This can be added to with more current transformers, a voltage monitor, a pulse input and temperature sensors. It reads the data from the sensors, does calculations to convert the raw data into useful values and transmits the results to either the emonBase unit or the emonGLCD display, or both.

  1. Basic configuration: -one current transformer. Every few seconds, the program in the emonTx samples the mains current waveform. It takes over 50 readings for each cycle, and from that it calculates the true "rms" average value of the current. Knowing the standard voltage, it goes on to calculate the power and transmits this to the base unit and the display.
  1. Voltage and current: By adding a voltage sensor, it can read the true voltage and calculate a more accurate value for power. This also allows the direction of power flow to be found, which is useful if you have a PV installation that generates and exports power back into the grid. If you wish, you can also calculate more advanced data, i.e apparent power and power factor.
  1. Voltage and 2 or 3 current sensors. Adding extra current sensors allows you to monitor different loads around the house, e.g. how much power is used for heating water. If you have a PV installation, you could also measure the power being generated at any instant.
  1. Pulse Input: Adding a pulse input from your meter gives you a direct feed of the information that will ultimately be used to calculate your energy bill.
  1. Temperature Sensors: Adding temperature sensors (maybe inside and out) allows you to display the temperatures and to correlate your energy consumption with the weather.

Tech Spec

Powering the emonTx

Power Supply Limits

The AC-DC power supply circuit has been designed to supply about 20mA @ 3.3V, this is equivalent to the basic setup minus the LED's. If more current is drawn the AC voltage sample will be effected. Use of LED's should be limited to quick flashes one LED at a time when powering from AC-AC adapter.

The on-board AC-DC half wave rectifier circuit has an absolute max supply current of 45mA @ 3.3V - AC voltage sample will be effected if more than 20mA is drawn

Quick headcount approximate Max power consumption of various emonTx Setups:

Basic Setup Max

  • ATmega: 7.5mA
  • RFM12B: 14mA
  • Op-amps + switches: 1mA
  • 4 x LED's: 40mA

Total: 63mA

Advanced Temperature setup

  • Basic Setup: 63mA
  • 8 x DS18B20: 18mA - although temp sensor are powered from 5V line so not drawing from 3.3V MCP1702

Total: 80mA

Advanced RF Setup + Advanced Temperature

  • Xbee: 50mA / RN-XV Wifi:38mA /XRF:36mA
  • Advanced Temperature setup: 80mA


Ultimate setup: as above but plus Wiznet Ethernet (120mA) = Grand Total: 250mA

As it happens this is the maximum current output of the MCP1702 voltage regulator!

Note: RF module power consumption was when they are Tx'in, this won't be happening all the time.

CT Energy Monitoring

Current Only (Apparent Power)

Voltage and Current (Real Power)

Reading Directly from Utility Meters

Pluse Counting



IR Interface

Elster a100c

Temperature Monitoring

The emonTx has inputs to connect multiple DS18B20 one-wire digital temperature sensors. By default these sensor are powered from the 5V line (supplied from the USB) if powering the board from the 3xAAA battery holder the 3V and 5V power rails should be connected together to enable the sensors to be powered from 3.3V. This can easily by done by jumpering the 5V and 3.3V on the JeePort.

Caution: This jumper should be removed before a usb connection is plugged into the board. No voltage greater than 5.5V should be connected to the Aux input when DS18B20 temperature sensors are connected.

emonTx SMT Prototype V0.6 rev 1 Testing

Questions to Answer:

  • Can a 0603 resistor supply enough current to the LED's 1/16W=62.5mW = 18.9mA MAX @ 3.3V. Give a 1.8 Vf LED 10mA with a 160R resistor 16mA through Resistor = safe but will LED be bright enough?
  • Simulations show that if the emonTxSMT draws 20mA @ 3.3V then the power spikes through R5 and D1 will be about 1.5Wp at max mains voltage (16.76Vp) or 1.35Wp at typical mains voltage (15.5Vp).

The V0.6 rev1 prototype has got pads for a 1206 resistor & diode R23 and D1 which can do up to 1/4W at a push, mostly 1/8W. Standard SMT resistors go up to 1W (2512)

Changes to make.

  • The AC power circuit needs also to be fused to match peak rating of components - 140mA holding, 300mA tripping polyfuse in-line with D1 and R5. Farnel: 1800745 (random case size?) or 1345923 SMT 1812.
  • Working with I^=350mA the resistor will need to be able to dissipate 6W peak 4.2 RMS at peak UK pains voltage 16.8V. Should resistors be rated for the peak? 18x7.5mm for thru-hole 7W resistor Farnell: 1109319 56R 5%. Does this resistor need to be 1% of will 5% be ok? Farnell don't stock 1% 7W 56R, could go for 5W 1% 56R.
  • The rectifier diode and D1 and current limiting resistor R23 need to be swapped round?
  • Increase PTC fuse on micro USB to 1206, their half the price for same 500mA rating. 500mA tripping current.



RFM12B (Default)



Simple ASK RF

Wifi - RN XV


Wired Serial Network

Open-Source Design