OpenEnergyMonitor

Measuring DC electrical energy production from micro-renewables

Picture: is of the DC power meter on the right of the Arduino and the load controller on the left.

How it works and how to build it

The electronics and software detailed here measure quantites related to DC electrical energy production from micro-renewables. Including: Voltage, Current and Power.

It is best described in 3 main parts:

• The Electronics
• The Arduino based software
• The Computer based software

Note: Initial rough tests with this method seem good, I need to do some further calibration and more detailed tests to find out the performace and accuracy. As with the whole of the project this part is very much work in progress.

Electronics side

The electronics is divided in to 2 main parts: Voltage sensing and current sensing. Once both quantities are measured the power can then be calculated.

Here's the circuit shematic:

Note: I have made an error with the connection of the 2N3904 transistor which should be connected to the positive input terminal of the opamp not the negative one as I have in the circuit below. I will try to correct these diagrams soon. For more info have a look at the linear DC current sense pdf.

I came across the circuit design and a lot of good circuit examples of DC current sensing and advice on the linear technologies website here. The circuit above is half of the battery current monitor circuit which can be found in their DC pdf. Half is only needed since for measuring current flowing from a micro-generator to a load the current only flows in one direction. The battery monitoring circuit measures current flowing in two directions: current flowing to the battery - charging and current flowing from the battery discharging. I have built the battery monitor circuit so that I can use it for both applications if I need to. When Im measuring the total ouput of the turbine I only use half the circuit.

Here the shematic of the full battery monitor circuit. Click to enlarge.

Note: I have made an error with the connection of the 2N3904 transistors which should be connected to the positive input terminal of the opamp not the negative one as I have in the circuit below. I will try to correct these diagrams soon. 

Voltage measurment

The voltage is measured using a simple voltage divider to bring voltage within 0-5V Arduino input voltage requirments. A 12V wind turbine's voltage will vary depending on wind speed and on the load placed on the output. The output of the voltage divider is connected to analog pin 0 of the Arduino.

Im using resisor values of RVDa = 100kOhm and  RVDb=10kOhm for the voltage divider this scales the voltage down just under 11 times keeping it well withing the Arduino range.

Detailed design considerations

Current measurment

The current is measured by measuring the voltage drop across a low resistance current sensing resistor placed in between the positive terminal of the load and the positive terminal of the micro-generator (also known as high side current sensing).

This voltages is in turn amplified by the LT1495 op amp chip and the 2N3904 transistor. The LT1495 is quite an expensive chip, it would be good to find a cheaper one sacrificing on accuracy a little. The op-amp needs to be able to take above supply input voltages.

The amplified ouput voltage corresponding to the current going to the load is then connected to analog pin 1 of the Arduino.

The range of current that can be measured by the circuit is defined by Rsense, R_A and R_B. The output voltage needs to be again between 0 and 5V.

The Voltage out from the current sensing circuit is given by:

Taking the example of the 500W Hugh Piggot wind turbine. If the voltage at 500W is 12V then the current will be about 42Amps. I ordered my shunt from farnell and the cheapest one they had available that was rated above 42Amps was this one. Its resistance is 0.0005Ohms which dictates along with the voltage out needed the values of R_A and R_B. If used R_A = 1k and R_B = 220k which gives a voltage out of 4.62V at 42Amps.

Detailed design considerations

To build:

List of components required

1x Arduino

1x LT1495

1x 8pin DIL chip holder

1x 0.0005Ohm current sensing resitor (Rsens).

1x 220kOhm resistors for RB. (2x for battery monitor)

2x 1kOhm resistors for RA. (4x for battery monitor)

1x RVDa resistor

1x RVDb resistor

1x 2N3904 transistors. (2x for battery monitor)

1x stripboard

Wire, connectors...

Layout suggestion

Here are some closeups of the my board, if you need any guidance on layout.

Software side

On the Arduino

The Arduino reads in the signals connected to its analog input pins 0,1. It converts the analog signals to digital information which can be manipulated within the program uploaded to the Arduino.

The first step is to convert this raw analog input information into voltage and current values. The power can then be calculated simply by multiplying the voltage value with the current value.

The Voltage, Current and Power values are then sent along the USB to the computer.

This is all detailed in full in the Arduino sketch.

To implement:

1. Download the Arduino sketch here.
2. Compile and upload the Arduino sketch to the Arduino. For a 'How to' on compiling and uploading the sketch to the Arduino have a look here.
3. Check that values are being sent from the Arduino in the Arduino serial monitor.

On the Computer

The ArduinoComm java program reads the values sent from the Arduino. The program outputs the values to the terminal window. Terminal is then used to write the printed data to a file for storage. The data file can be opened simultaneously by KST for real-time graphing.

To do this:

1. Download the ArduinoComm java program here.
2. Compile the program. For a 'How to' on compiling and running java programs have a look here.
3. Run the program with $ java programName >tmp.dat. The addition of >tmp.dat at the end of the run command line writes the data outputed by the java program to a file for storage.
4. Open KST for graphing.
5. Configure KST to open the tmp.dat file.

Testing

5x 12V 10W Halogen bulbs powered by computer power supply

Using the wind turbine load/dump controller to switch on and off 5 halogen bulbs. Good way to compare measured result with multimeter readings.

Hugh piggott Wind turbines

The reason I needed a DC energy monitor.

North Wales wind turbine built on a course organised by myself and V3 Power.

Cardiff EWB wind turbine built as a training project with the Cardiff branch of Engineers Without Borders.

Both wind turbine are 12V DC machines. They can produce up to around 500W of power, about 40Amps.

For more information on Hugh Piggott and his wind turbine designs.

Pictures of the wind turbine up at a local festival in late July. It was the first test of the wind turbine energy monitoring:

Here's a video of it all working, sorry about the rushed and blurry video taking...

Monitoring the output of the wind turbine. from Trystan Lea on Vimeo.

Further Development

There are a couple of things with that arent finished with the above, the main one being a full characterisation of its accuracy and precision. But it should be in the range of the maximum number of Amps you want to measure divided by the arduino adc resolution 1024.

Update Feb 20: Myself and Suneil are going to building a second version of the above with USB datalogging a display and ethernet connection for a wind turbine installation so there will be an update soon probably in a couple of weeks. We are going to try these much cheaper shunts (Thanks goes to Matt Little for the suggestion).