[Beowulf] Newbie Question: Racks versus boxes and good rack solutions for commodity hardware

Geoff Jacobs gdjacobs at gmail.com
Mon Dec 15 16:33:23 EST 2008


Lux, James P wrote:
> \  True, the power
>>> supplied to the motherboards is in several voltages 12V and under, but
>>> believe it or not you can kill yourself with 12V, and starting a fire
>>> with 12V is even easier.
>> You can actually do a good job of TIG welding with 12V.
> 
> Hmm.. I think not.  The voltage across the arc while welding might be 12V,
> but the open circuit voltage will be higher, and almost all TIG rigs use a
> HF/HV circuit to stabilize/start the arc (basically a small tesla coil)..

You can do scratch start TIG with just DC. Pipeline welders do this all
the time for the root pass. Good quality TIG supplies can output a
square wave, so you've got continuous HF, but this is mostly used for
aluminum (which is a common application, as you know).

> I used to work in a special effects shop (physical effects, not computer
> generated), and we had lots of welders around.  The classic challenge for a
> good TIG welder is to weld aluminum foil (or gum wrappers)..

You graduate to foil by doing lots of coke cans.

> The owner used to maintain that for offroading, one needed two batteries,
> not as a spare, but because you'd need 24V to do decent stick welding (e.g.
> To fix a broken frame or suspension component in the field) with a
> coathanger and jumper cables.  We called him on this stunt.. Got two car
> batteries and jumper cables out and handed him the coat hangers..

Many off roaders have a second, modified alternator in their Jeep for
this purpose. Holding an arc with no flux is a testament to the guy's
skill, but I still wouldn't trust the welds. They're likely to be very,
very porous. They didn't create all those rod formulations for fun.

> You can do it, but it isn't pretty, nor what one might be proud of.  But
> with 12V.. No way..
> 
> Granted, TIG has a shielding gas, which the coathangers and batteries do
> not, but I'm pretty sure you'd have a tough time striking the arc and
> holding it stable with just 12V. Once you get a puddle going and the
> electrode is hot, thermionic emission helps, but....

Never said it was ideal, or even easy. TIG makes it much easier to
maintain the arc gap, and the current required is much, much lower. Try
to do SMAW welding with the power levels a TIG supply uses using 1/8"
rod and you'll get nothing but bubble gum.

> On the other hand, 12V is just fine (if not overkill) for resistance or spot
> welding.  A microwave oven transformer with a single turn secondary of
> copper tubing works great for that.
> 
>> Commodity ATX power supplies will have a grounded frame. Mounting the
>> power supply to the pan will work quite well.
> 
> Except I use foam double stick tape, which is an insulator, so you need
> another wire to ground it. I guess you could screw the PS to the baking
> sheet, but that takes more time to drill holes, etc. One sheet metal screw
> for grounding vs 4 screws in just the right pattern.. One is easy hackery vs
> the other is precision machining.

Just gang drill a single hole for grounding. That way, precision isn't
really an issue, but you've got not-bad continuity.

One site that I found a couple of years ago when I was debating building
my own TIG unit was this:
http://www3.telus.net/public/a5a26316/TIG_Welder.html

Convert a standard 225 Amp AC "buzz box" to a good quality DC or square
wave output, suitable for any metal which can be TIG welded. Very neat
project.

How does this relate to Beowulf? How else are you going to build your
own telco rack?

-- 
Geoffrey D. Jacobs
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