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..