Monday, April 28, 2014

Solar Power in the Desert

I had the opportunity to assist a friend in the building a off-grid solar system in Arizona. I spent about four days in the desert on the project. He purchased all the components separately and the frame for the solar panels was built from scratch using 3X3 and 2X2 steel tubing as well as 1X2 steel "C" channel. There was a lot of digging, welding and concrete mixing to lay the footings, the whole array will be 64 feet long. I had to leave on Saturday and we had completed the frame for one of the four solar arrays. Each array is 10 panels. The panels are 230 watt 24 volt type. So one array is about 2.3 kw. The panels are wire in two banks of 5 panels in parallel. This will provide a target voltage of between 120 to 180 volts DC depending on the temperature of the panels. A rough line drawing of the components are below.

When completed the four arrays will deliver 9.6 kw. Each of the four panel arrays will have its own MPPT solar charger. The charge controllers will be configured for 24 volts and the battery array will be eight (8) 12 volt 200 Ah batteries wired in a series parallel for 24 volts. So that will be a 800 Ah capacity at 24 volts or 19.2 kwh of storage. With an average of 5 hours of solar collection a day in this part of Arizona the array can deliver 48 kwh per day. This should provide full charging of the storage batteries as well as plenty of capacity during the day to run loads while charging (Air conditioners and swamp coolers in the  summer). The site is a 100 acre ranch with many power needs including pumping water for a 60 acres of citrus orchard, running a house, shop and an RV.

Saturday, April 19, 2014

Hellschreiber Contest

I have been experimenting with the Hellschreiber mode lately and looking at the simplicity along with the human element to this digital "fuzzy" mode. I joined the Feld Hell Club (and got a cool membership certificate).

They ended up having a contest this weekend  (Saturday) from 2000 to 2200 UTC.

I listened and called CQ on all the recommended Fled Hell frequencies, however I only heard stations on 20 meters. I was only able to make two contacts (W0IZ in Colorado @665mi and K7SCX also in Colorado @838mi) . K7SCX's QRZ page listed his last 15 QSO's and I was listed, pretty cool. Anyway, the mode is very fun and easy to operate with on the air. I ended up calling W0IZ after the contested because his signal was so strong throughout the contest and I wanted to let him know. We had a nice QSO and at the end he told me my signal was very strong there as well. What is interesting about that, is he was running 100 watts and I was only running 5 watts! Fun stuff. I need to do experiments with Feld Hell using micro controller beacons as well. -73

Sunday, April 13, 2014

Voltage and Current measurements with the INA219 Sensor

Now that the Distributed Power Node (DPN) project is progressing on the software side I thought I better look into how the power management sensor will work for measuring the health of the battery. I got one of the Adafruit INA219 break-out boards back in December and had not had a chance to try it yet.

I wired it up exactly like the guide on Adafruit site and it worked with the library and sample code fine. I modified the code to sample every minute instead of every 2 seconds and wrote the output to the serial monitor in CSV format so I could cut and paste it into a text file for use with Excel. I connected a 12 volt 8.5 Ah SLA battery to a 400 watt power inverter running a 15 watt CFL and monitored the voltage and current for 2 hours or 120 samples. This is the results as plotted by Excel.

I was not happy with the current measurement since they varied by 500 ma from sample to sample. The voltage was not as bad. I was going to do some sample averaging in code and then thought that perhaps the power inverter is a "noisy" load and was causing the up and down readings. I decided to re-test with a static load of just a power resistor. I had just connected the power resistor load and had a couple of samples on the screen that seems much more stable and I reach over to check the heating on the load resistor and accidentally shorted the battery at the load. I notice the readings were way off now but it was still running so I powered it all down and checked everything. There was a slight smell of burnt resistor in the air so I checked the shunt resistor on the INA219 and it was open. The strange thing though was that the bus voltage was reading 32 volts constantly now. I could see the current reading being wrong since the shunt was open but the bus voltage is not dependent on the shunt. I made a temporary shunt with wire just to test it again and the current reading was still bad along with the bad voltage measurement. I think the large transient must have destroyed the op amp circuits on the chip but the I2C interface components survived because the Arduino can still talk to the board. This sensor design does provide good isolation from the MCU so I think the moral of the story is that if the battery would have been fused, it may have survived. I will need to study protection methods for the DC outputs anyway so this was a $9.95 plus shipping lesson!

Monday, April 7, 2014


Finished populating the 2M TX PCB, and found that the PICAXE is loading the oscillator in some way that kills it's oscillation with a common power supply but works with separate supplies. I plan to try using separate regulators and see if that works. I can have 6 volts for the transmitter and 5 for the MCU