Apr 232016

Well, I finally got busy with the soldering iron again. This time, installing the regulators in the cluster nodes and in the 26-port switch.

I had a puzzle as to where to put the regulator, I didn’t want it exposed, as they’re a static-sensitive device, so better to keep them enclosed. It needed somewhere where the air would be flowing, and looking around, I found the perfect spot, just in behind the CPU heatsink. There’s a small gap where the air will be flowing past to cool the CPU, and it’s sufficiently near the ATX PSU to feed the power cabling past.

I found I was able to tap M3 threads into the tops of the heatsinks and fix them to the “front” of the case near where the DIN rail brackets fit in. So from the outside, it looks all neat and tidy.

After installing those, I turned my attention to the switch. Now I had an educated guess that the switch would be stepping down from 12V, so being close to that was not so critical, however going above it would stretch the friendship.

Rather than feeding it 13.1V like the compute nodes, I decided I’d find some alternate resistor values that’d be closer to 12V. Those wound up being R1=3.3kΩ and R2=390Ω, which gave about 11.8V. Close enough. It was then a matter of polarity. The wiring inside this switch uses a non-standard colour code, and as I suspected, the conductors are just paralleled, it’s the one feed of 12V.

Probing with a multimeter revealed the pin pairs were shorted, and removing the PSU confirmed this. I pulled out the switch mainboard and probed around the electrolytics which had their negative sides marked. Sure enough, it’s the Australian Olympic team colours that give away the 0V side.

I’ve shown the original colour code here as coloured dots, but essentially, green and yellow are the 0V side, and red and black are the +12V side. So I had everything necessary. I grabbed a bit of scrap PCB, used the old PSU as a template for drilling out the holes, used a hacksaw to divide the PCB surface up then dead-bugged the rest. To position the heatsink, I drilled a 3mm hole in the bottom of the case and screwed a 10mm M3 stand-off there. Yes, this means there’s an annoying lump on the bottom, I should use a countersunk M3 screw, I’ll fix that later if it bothers me, I’ll be rack-mounting it anyway.

On the input to the regulator, I have a 330uF electrolytic capacitor and 100nF monolithic capacitor in parallel, on the output, it’s a 470uF and a 100nF. A third 100nF hooks the adjust pin to 0V to reduce noise. I de-soldered the original PSUs socket and used that on the new board. It fits beautiful. 100-240V? Not any more Linksys.

So now, the whole lot runs off a single 12V battery supply. The remainder of this project is the charging of that battery and the software configuration of the cluster.

At present, the whole cluster’s doing an `emerge @system`, with distcc running, and drawing about 7.5A with the battery sitting at 12.74V (~95W). Edit: Now that they’ve properly fired up, I’m seeing a drain of 10.3A (126W). Looks that’s going to be the “worst case scenario”.

Mar 272016

Probably going to be easier than expected. I popped open the cover to see whether it was like the much older Netcomm switch we have.

Sure enough, Linksys do it the same way in the LGS326AU:

This PSU is an open-frame PSU made by Asian Power Devices, Inc. Model NW-20A12-BAAB. Not sure if there’s someone here who knows more about this particular PSU.

It appears it’s the one 12V feed split in two. Red/Black are negative, green/yellow are positive. I’ll double check this. I think with a nice big inductor/capacitor as an in-line filter, this can be hooked straight up to the 12V line, perhaps with a small amount of zener overvoltage protection. It appears that the voltage is further rectified on the switch mainboard:

Mar 272016

Well, I managed to get some parts yesterday. The plugs I bought don’t quite fit, so I’ll have to buy some different ones. Apparently the ones I got are for a 2.1mm inner pin, the ones I need are a 2.5mm inner pin.

No problems there though, they sort-of jam in there, and I can use them for other projects. Jaycar have changed things around and most of the bits are behind the counter — might as well go through online crowds if I’m not able to inspect the part close up. That’ll be a Tuesday job.

I’ve just put up a photo of the last two nodes being tested and bootstrapped, along with a shot of the partially-wired up wiring loom. I sort-of whacked that together to prevent the wrong parts from coming into contact. There’s a 30A fuse on the battery feed and individual 10A fuses for each machine. I could use 5A fuses too.

So far, I’ve now built kernels on all 5 boxes, and all seem to be performing well, if there were going to be issues with a box, I think they’d have shown up by now.

I also cracked open one of my old 24-port switches (an unmanaged Netcomm 10/100Mbps), and was pleasantly surprised to see the PSU was a separate module, putting out 3.3V 4A. I’ll crack open the Linksys and see if the same is true there, if so, this could be a much better option than trying to convert 12V?240V?some low voltage. If the step to 240V can be avoided, I might as well.