Small temp compensated UPS for tiny server

Linear and non linear

Small temp compensated UPS for tiny server

Postby Doug Coulter » Fri Jun 20, 2014 5:27 pm

Ah Friday evening and I'm finished with this one. I'm working on a remote, wifi data acq and control server for a building about 30 yards from where I sit.
It used to be an office trailer, now it's just a guest house - but it's the one with plumbing, so it's still important - I have to ensure the water level in the cistern, know what the temperature is in there particularly in winter (so the plumbing doesn't freeze and so on), and I'd like to lean the tempco of the building vs wind, temperature difference and so on - both in the habitable area and in the crawl space with the plumbing and spiders. Fur6ther, I'd like to know the output curve of the particular woodstove in there, and how long it takes for heat at that end of the building to get to the guest bedroom. On top, it'd be nice to know how much water is in the cistern, and know whether to open the valve from the rain barrel (does it have good quality water? Is there enough room for it in the cistern, and so forth).

I decided on using a Chipkit WF 32 for this job, but there's a lot more on that elsewhere (look under software/embedded). The problem I just solved is how to power the thing, day in and day out - in a building that unlike this one, doesn't have it's own independent solar power system, and is merely fed with AC power from this one. Which can glitch, fail, or just be turned off from here for diagnostic reasons (what the heck is using power right now? Is the buried cable failing?). I don't want to lose data aq in those conditions - those are when I might well want it the most.

Gel cel battery to the rescue, in this case a 12v nominal 7ah nominal one. These have a tempco if you're going to use them as a float backup, so that the voltage from the charger changes with temperature correctly - else they die young, and of course, you don't fine out till right when its needed. Most UPS systems don't get this right, and it matters less as they are not in an unheated (usually) crawlspace either. But here that's where it is - with the water, plumbing, and spiders. So I need to tempco match the charger (which happened to be there already to run the plumbing pump) to the battery's desired float voltage for whatever the current conditions are over there, and for it to be reliable - once this is in, I don't want to have to fix it often.

There was already a supply with a 14v transformer, full wave rectified, with 15,000 uf filter there, for running a spot sprayer pump I use to pressurize the plumbing (yeah, I know - but it's cheap, simple, easy to repair, and rarely fails, even though it's noisy). It puts out around 23v no load, and drags down to around 16 v with the pump running, but in reality, it's all over the map. So I used an LM317 with some temperature control and a series diode to adapt that to the gel cel. Turns out that these AGM cells need about one diode/cell to compensate them, plus another to compensate the diode I put in series with the LM317 so losing AC power won't run down that little battery (and LM317's don't like backwards volts). I used a 317 rather than a higher rated part because I only had a convienient 1n5401 diode and I didn't want anything to burn out if AC was applied against a dead backup battery. It's always a question of balance in these things - the raw supply can easily fry all this without breaking a sweat (40amp bridge...14 amp rated transformer).

Now, I also wanted about 7.2v out of this to actually run the WF-32 and any ancillary arduinos from, so I added another stage to that - a little switcher I got from a chinese vendor on ebay that's real efficient, so that went on there too. Now I have terminals with nominal 13.5v, and 7.2v on top of my board to hook into the more-delicate computers and servo supplies for doing that valve work (or whatever else I dream up for this system, which might include some horsey 24v fans run at under-volt to move heat around and so on).

So, yes, it looks ranch-grade, because that was the intent. I was lucky to score a bunch of surplus aluminum plate a little over .25" thick, and have been using pieces of that for a lot of things where there might be a desire to spread heat around. It's super handy in another way - you can just drill and tap it to hold heavy junk on there, no nuts required (or cursing because you lose them).

So, the goal is to run this thing and friends (it'll be in a box when running), and in this pic, it's sitting on the schematic I drew on an envelope, which is about what stock trading advice mail is useful for- at best. Note for noobs - click the pix and they get full sized.
Server board and schiz

Here's the board I added to the basic plumbing power supply.
The board - this one didn't have or need solderable pads, just perf is fine, this is low-grade kinda stuff. Only needs to be reliable.

And, here's the whole mess on my messy bench just before I put it in. No pix of it in place as I didn't want to juggle the camera while fighting with the spiders and such (and the light is kinda crummy down there).
The whole mess.

This could go under alt energy or some kind of homesteading forum too, I suppose, but I have to choose, so it's here, being analog and all, at least mostly.
Yes, I really did use duct tape to stick the temp sensing diodes to the battery. I used gorilla tape on the other end. Both should just about be ready to fall off at about the time I'll want to install a new battery.
Perfection just wastes materials and manhours. For some things (fusor grids) it's worth it - not this.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Doug Coulter
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Joined: Wed Jul 14, 2010 8:05 pm
Location: Floyd county, VA, USA

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