Sunday, March 29, 2015

Dummy Load - Watt Meter

One of the first pieces of test equipment I decided to build was a QRP dummy load.
The first one was just twenty 1k  1/2 watt resistors on perf-board.  I added a diode RF detector and used a DVM  and a conversion table to measure power output.  Looking at the formula for voltage vs. power and the resulting table, it looked like it would only take a voltage divider to scale down to 5 volts max for an  Arduino  analog input.

I had recently tested a 128 x 128 pixel color TFT display that interfaced with an Arduino Pro-Mini without having to use level conversion.  I designed and etched a small circuit board, that fit in an Altoids tin along with a 9 volt battery.

With a 128 pixel wide display and a  dummy load that should handle 10 watts continuously, I decided to go with a 12 watt scale.  From the tables I found that 20 watts would give me a peak voltage of  just under 45 volts.  Giving a good safety factor I used a voltage divider ratio of 11 for the Arduino input.  The detector diode I used is also rated at 45 volts, so everything should be fine up to about 20 watts peak.   During testing I did blow a couple of diodes when I went over about 25 watts on voice peaks, but the Arduino was not damaged.
To make it easier to use while aligning equipment I wanted a bar graph display along with the digital readout.  To indicate that the power is over the  rating of the load I change to color of the bar graph to red when over 10 watts.  Under 1 watt I display the power in mw. and in watts over 1 watt.  After scaling to the screen, I found I also had room for Peak and Average power readings.  To reduce digit bobble on the power reading I take the average of the last 8 readings, for the Average value I use the last 64 readings.  The average and peak values are reset after about 10 seconds of no power input.

 For anyone who wants to build one, I  put a copy of the schematic, board artwork and Arduino code in a public dropbox folder.



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  2. Hi DuWayne

    Nice project....the 128 x 128 displays are kind of hard to find now...any chance of updating to a more common display ???


    1. The problem with different displays is the size. The 128x128 are the only one that will easily fit in the Altoid size tin.
      There are 2 versions available through eBay, alibaba and other suppliers. Difference is the driver chip. Both drivers are available from Adafruit

  3. OK I'll look at those..



  4. Very informative article. Great post, thanks for sharing .if you want to Wattmeter in Mumbai