Dec 252017
 

So some spare time today… I decide to construct a new I/O module to fix up the mistakes made with the previous iteration.  Mainly:

  • the TVS diodes… going for one with a higher clamp voltage so it doesn’t smoke when 12V is applied
  • switching to a 4-pin connector on the output side, with pins for 0V, GPIO, DRAIN and +12V
  • fixing the pin-out on the input side so it matches the PCB.
  • rather than having jumper leads to make the boards separable, we’ll make one monolithic board that plugs into all 8 channels simultaneously with one long connector.

For the TVS diodes, I ordered some TPD2E007 in SOT23… thinking those would be a reasonable size for hand-soldering.

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Now… how the bloody hell am I going to solder these little tiddlers?  I had thought SOT-23 was about twice that size.  Never mind, can’t un-buy them.

The circuit is pretty much identical to what came before.  My MOSFETs and 4.7nF capacitors seem to have gone walkies, not sure where.  No doubt the arrival of replacements will summon them back.  I decided to use SMT for many parts on this build.

0805 resistors and veroboard aren’t a bad combo really, just have a sharp blade handy to cut the track where needed, and the resistor can straddle the gap made.

For the TVS diodes, the common pin is to ground, so I made a bus bar running vertically down the PCB and scoring the tracks either side.  The common pins could be soldered to that, and the two I/O pins would straddle the division between each track.  Aside from me getting some parts off-by-one at first, this went well.

The zener and schottky diodes of course, being through-hole, went on the other side of the PCB.

I still have to locate where my MOSFETs have gone, and I think I found some 12 ohm resistors (through-hole).  I can use some 0805 1k resistors for the MOSFET gates.  So that’s some MOSFETs and 4.7nF, probably 0805 size capacitors that need ordering in the new year.

Dec 252017
 

So, I’m home now for the Christmas break… and the fan in my power supply decided it would take a Christmas break itself.

The power supply was purchased brand new in June… it still works as a power supply, but with the fan seized up, it represents an overheating risk.  Unfortunately, the only real options I have are the Xantrex charger, which cooked my last batteries, or a 12V 20A linear PSU I normally use for my radio station.  20A is just a touch light-on, given the DC-DC converter draws 25A.  It’ll be fine to provide a top-up, but I wouldn’t want to use it for charging up flat batteries.

Now, I can replace the faulty fan.  However, that PSU is under warranty still, so I figure, back it goes!

In the meantime, an experiment.  What happens if I just turn the mains off and rely on the batteries?  Well, so far, so good.  Saturday afternoon, the batteries were fully charged, I unplugged the mains supply.  Battery voltage around 13.8V.

Sunday morning, battery was down to 12.1V, with about 1A coming in off the panels around 7AM (so 6A being drained from batteries by the cluster).

By 10AM, the solar panels were in full swing, and a good 15A was being pumped in, with the cluster drawing no more than 8A.  The batteries finished the day around 13.1V.

This morning, batteries were slightly lower at 11.9V.   Just checking now, I’m seeing over 16A flowing in from the panels, and the battery is at 13.2V.

I’m in the process of building some power meters based on NXP LPC810s and TI INA219Bs.  I’m at two minds what to use to poll them, whether I use a Raspberry Pi I have spare and buy a case, PSU and some sort of serial interface for it… or whether I purchase a small industrial PC for the job.

The Technologic Systems TS-7670 is one that I am considering, given they’ll work over a wide range of voltages and temperatures, they have plenty of UARTs including RS-485 and RS-232, and while they ship with an old Linux kernel, yours truly has ported both U-Boot and the mainline Linux kernel.  Yes, it’s ARMv5, but it doesn’t need to be a speed demon to capture lots of data, and they work just fine for Barangaroo where they poll Modbus (via pymodbus) and M-bus (via python-mbus).