I figured, rather than letting these loose directly on the nodes themselves, I’d give them a try with a throw-away dummy load. For this, I grabbed an old Philips car cassette player that’s probably older than I am and hooked that up. I shoved some old cassette in.

The datasheet for the regulators defines the output voltage as: $V_{OUT}=1.240 \big({R_1 \over R_2} + 1\big)$

Playing with some numbers, I came out with R1 being a 2.7kΩ and 560Ω resistors in series, and R2 being a 330Ω. So I scratched around for those resistors, grabbed one of the MIC29172s and hooked it all up for a test.

The battery here is not the one I’ll use in the cluster ultimately, I have a 100Ah AGM cell battery for that. The charger seen there is what will be used, initially as a sole power source, then in combination with solar when I get the panels. It’s capable of producing 20A of current, and runs off mains power.

This is the power drain from the battery, with the charger turned on.

Not as much as I thought it’d be, but still a moderate amount.

This is what the output side of the regulator looked like:

So from 14.8V down to 13.1V. It also showed 13.1V when I had the charger unplugged, so it’s doing its job pretty well I think. That’s a drop of 1.7V, so dissipating about 600mW. Efficiency is therefore about 93%, not bad for linear regulators.

So, I’ve been doing a bit of research about how I can stabilise the battery voltage which will drift between around 11V and 14.6V. It’s a deep-cycle type battery, so it’s actually capable of going down to 10V, but I really don’t want to push it that far.

Once I get below 12V, that’s the time to signal to the VM hosts to start hibernating the VMs and preparing for a blackout, until such time as the voltage picks back up again.

The rise above 13.5V is a challenge due to the PicoPSU limitations. @Vlad Conut rightly pointed out that the M3-ATX-HV PSUs sold by the same company would have been a better choice. For about $20 more, or an extra$100 for the cluster, I’d have something that’d work 6-30V. I’d still have to solve the problem with the switch, but it’d just be that one device, not 6.

Maybe it was because they were out of stock that I went the PicoPSU route, I also wasn’t sure about power demands, I knew the CPU needed 20W, but wasn’t sure about everything else. So I over-dimensioned everything. Hindsight is 20:20.

One option I considered was a regulator in front of each node. I had mentioned the LM7812 because I knew of it. I also knew it was a crap choice for this task, the 1.5V drop, with a 5A load would result in about 7.5W dissipated thermally. So 20W would jump to nearly 28W — not good.

That of course assumes a 7812 would handle 5A, which it won’t.

LDOs were the way to go for anything linear, otherwise I was looking at a switchmode power supply. The LM2576 has similar requirements to the LM7812, but is much more efficient being a buck converter. If 1.5V was fine, I’d be looking for a 5A-capable equivalent.

The other option would be to have one single power supply regulate power for all nodes. I mentioned in my previous log about the Redarc DC-DC power supply, and that is certainly still worthy of consideration. It is a single point of failure however, but then again, Redarc aren’t known for making crap, and the unit has a 2 year warranty anyway. I’d have downtime, but would not lose data if this went down.

@K.C. Lee pointed me to one LDO that would be a good candidate though, and is cheap enough to be worth the experiment: the Micrel MIC29750. 7.5A current, and comes in an adjustable form up to 16V. I’d imagine if I set this near 13.5V, it’d dissipate maybe 2.5W max at 5A, or 1W at 2A. Much better.

Not as good as Redarc’s solution of course, and that’s still an option, but cheap enough to try out.

Of course, there’s always something there to throw a spanner in the works, and for me, it’s the PicoPSU.

It seems to work great, however, there is an Achilles heel with these things: they have a fairly narrow band of tolerable voltages they’ll operate at. Namely 10.5—13.5V.

Now, 10.5V is fair enough, but 13.5V? Typical lead acid batteries are 13.8V nominal voltage, and will get to 14.5V when charging. So I need some preregulator that will handle when the voltage is up around 13.5V or above, and drop it down just a little, passing through up to 2A (5A to be safe).

It still has to be stable when the current changes, “turned off” on these computers means a drain of about 200mA for the IPMI. So our operating parameters are summed up as 10.5—13.5V and 200mA—5A.

It needs to continue operating when the battery gets to ~11.5V.

So what are my options?

• LM2576 simple switcher? 12V in will produce 10.5V out.
• LM7812 has the same problem, and will chew more power.

A series regulator built on a zener/NPN might work. The voltage drop across the NPN ordinarily is going to be fairly low, however there will still be a drop of about 0.7V or so. That’s possibly “good enough”, since at 11.5V input, we should still see about 10.8V out which is within range.

Two diodes in series, with a relay to short them out when the voltage drops below 12V would work too. That’d need a comparator and voltage reference to drive the relay. It’s a cheap solution too.

Another prospect is a beefy DC-DC converter on the battery, so we don’t actually care what the battery voltage is, we boost it say to 15V then regulate it back down to 12V. A 30A-capable flyback or boost-buck converter would do it. This is more complex, and much more expensive to do off-the-shelf, so I think that’d be a method of last resort.