Solar Cluster: Rack taking shape

Well, it’s been a while since I last updated this project. Lots have been due to general lethargy, real life and other pressures.

This equipment is being built amongst other things to host my websites, mail server, and as a learning tool for managing clustered computing resources. As such, yes, I’ll be putting it down as a work expense… and it was pointed out to me that it needed to be in operation before I could start claiming it on tax. So, with 30th June looming up soon, it was time I pulled my finger out and got it going.

At least running on mains. As for the solar bit, well we will be doing that too, my father recently sent me this email (line breaks for readability):

Subject: Why you're about to pay through the nose for power - ABC News
 (Australian Broadcasting Corporation)
To: Stuart Longland
From: David Longland…

Hi Stuart,

This is why I am keen to see your cluster up and running.  Our power 
bill is about $300 every 3 months, a lift in price by 20% represents 
$240pa hike.


Umm, yeah… good point. Our current little server represents a small portion of our base-load power… refrigeration being the other major component.

I ordered the rack and batteries a few months back, and both have been sitting here, still in the boxes they were shipped in, waiting for me to get to and put them together. My father got fed up of waiting and attacked the rack, putting it together one evening… and last night, we worked together on putting a back on the rack using 12mm plywood.

We also fitted the two switches, mounting the smaller one to the lid of the main switch using multiple layers of double-sided tape.

I wasn’t sure at first where the DIN rail would mount. I had intended to screw it to a piece of 2×4″ or similar, and screw that to the back plane. We couldn’t screw the DIN rail directly to the back plane because the nodes need to be introduced to the DIN rail at an angle, then brought level to attach them.

Considering the above, we initially thought we’d bolt it to the inner run of holes, but two problems presented themselves:

  1. The side panels actually covered over those holes: this was solved with a metal nibbling tool, cutting a slot where the hole is positioned.
  2. The DIN rail, when just mounted at each end, lacked the stability.

I measured the gap between the back panel and the DIN rail location: 45mm. We didn’t have anything that was that width which we could use as a mounting. We considered fashioning a bracket out of some metal strip, but bending it right could be a challenge without the right tools. (and my metalwork skills were never great.)

45mm + 3mm is 48mm… or 4× plywood pieces. We had plenty of off-cut from the back panel.

Using 4 pieces of the plywood glued together and clamped overnight, I made a mounting to which I could mount the DIN rail for the nodes to sit on. This afternoon, I drilled the pilot holes and fitted the screws for mounting that block, and screwed the DIN rail to it.

At the far ends, I made spacers from 3mm aluminium metal strap. The result is not perfect, but is much better than what we had before.

I’ve wired up the network cables… checking the lengths of those in case I needed to get longer cables. (They just fit… phew! $20 saved.) and there is room down the bottom for the batteries to sit. I’ll make a small 10cm cable to link the management network up to the appropriate port on the main switch, then I just need to run cables to the upstairs and downstairs switches. (In fact, there’s one into the area already.)

On the power front… my earlier experiments had ascertained the suitability of the Xantrex charger that we had spare. The charger is a smart charger, and so does various equalisation and balancing cycles, thus gets mightily confused if you suddenly disconnect the battery from it by way of a MOSFET. A different solution presented itself though.

My father has a solar set-up in the back of his car… there’s a 12V 120W panel on the roof, and that provides power to a battery system which powers an amateur radio station and serves as an auxiliary battery. There’s a diode arrangement that allows charging from the vehicle battery system.

In an effort to try and upgrade it, he bought a Redarc BCDC1225 in-vehicle MPPT charger. This charger can accept power from either the 12V mains supply in a vehicle, or from a “12V” solar panel. The key here, is it relies on a changeover relay to switch between the two, and this is where it wasn’t quite suitable for my father’s needs: it assumed that if the vehicle ignition was on, you wanted to charge from the vehicle, not from solar.

He wanted it to switch to whichever source was more plentiful, and had thought the unit would drive the relay itself. Having read the manual, we now know the signal they tell you to connect to the relay coil is there to tell the charger which source it is plugged into, not for it to drive the relay.

The plan is therefore:

  • use a 240V→12V AC-DC switch-mode power supply to provide the “vehicle mains” DC input to the charger.
  • measure the voltage seen at the solar input with a comparator and switch over when it is above some pre-defined voltage (use hysteresis to ensure it doesn’t oscillate)
  • use the output to drive a P-channel MOSFET attached to the “vehicle mains”, which drives the relay.