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Old 06-09-2007, 11:28 PM   #21
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Well, Myron, they don't call it "Orygun" fer nothing.
Yep, I'm kidding. Actually my hands do not shake hardly at all, but my tripod weighs half a ton, so I don't use it as I should. Should get a carbon-foam one I guess.

My "other" scanner is an E. 2450 also, and it has done fine service, especially on medium format and some of the totally off the wall negative sizes from about a century ago. Other than that, I really ought to replace the Rolleiflex I dropped decades ago, nothing quite like it.

Then there is Eastern Oregon, a whole 'nuther world which I like just about as much but for different reasons. Fabulous stuff.
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Old 06-10-2007, 06:00 AM   #22
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Maybe you could explain what would cause a switching power supply to get hot when powered with a square wave, or dirty power source. After looking a couple schematics and thinking about it I can't think any reason for a large loss of efficiency.

I think the biggest objection to square wave or modified wave inverter output would be interference caused to radios and maybe some audio and video devices. Big old inductors, like motors will intergrate the square wave. Maybe a capacitive start motor could have trouble starting?

A switcher works the same way as a linear supply only the transformer and filtering elements can be much smaller because of the "switching". The linear part operates at a much higher frequency than a standard linear. The input still goes through an inductor which intergrates the square wave to some extent. Then into a bridge with filtering before the swiching circuit.
I can't explain because I'm not a designer. I'm always happy to be proven wrong.. In this case, back in about 1992, I was in charge of a bunch of Apollo workstations which had switching supplies... Some of them were in a part of the building where the main UPS wouldn't reach so we bought UPSes for them... The first time we lost power and they switched over to battery, the supplies started squealing real bad. By the time I got there, the back of the supply was hot. We took a scope to the UPSes and discovered they were basically crap, so we replaced them with these big-azz units that put out an almost perfect sine wave (a little teeny glitch on the trailing edge) and our problems went away...

Also, I've tried running a skil saw and a black&decker corded drill off cheap inverters. The drill just hummed, the drill bit sort of vibrated back and forth but didn't turn. On the skil saw, the motor hummed really bad and the blade turned slowly but that 'hot smell' came out of it almost right away...
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Old 06-10-2007, 10:59 AM   #23
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Maybe you could explain what would cause a switching power supply to get hot when powered with a square wave, or dirty power source. After looking a couple schematics and thinking about it I can't think any reason for a large loss of efficiency.
I'm not sure either, Byron, but since "cheap" usually means cutting corners I suspect a cheap switcher would probably use cheap capacitors.

Pumping joules into a lightly charged capacitor as the voltage rose above the rated voltage of the capacitor for a breif time wouldn't result in arcing across the dielectric until the dielectric is substantially energized. If one assumes a standard sine wave, perhaps you could take advantage of that and use a capacitor with a voltage rating below what you'd expect from a 115v RMS input source, and everything would work just fine until you hooked it up to a cheap inverter? If this were true the square sine wave of the inverter would spend more net time at the rms value from the input source, provide more time to pump more joules into the dielectric, allow it to reach a substantial charge level at which point the capacitor would be much more likely to leak across the plates.

Not an expert opinion . . . just a thought.

--Peter
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Old 06-10-2007, 03:09 PM   #24
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I don't want to push this much deeper into the theory which makes eyes glaze over in normal people....

--- electrical theory starts here ---

but there's one detail to clarify:
A transformer with a sine wave input will have a sine wave output. One fed a square wave will not make a nice sine wave out of it; instead, it produces a wild-looking series of spikes. The rate of change of current (and thus the rate of change of magnetic field) on the input side determines the output, so those steps in the square wave produce the spikes, and the constant-voltage spaces between them do zip. The current does not immediately follow the square wave voltage, due to the inductance of the input windings, so the output is a spike which decays as the current wanes and the field collapses. The key is that the time-derivative of a sine wave is another sine-wave; not so for square waves.

--- electrical theory ends here ---

Those interested in running motors from inverters should consider whether they are real AC motors (synchronous or induction), or "universal" motors with commutators. Small tools generally have universal motors, which don't care about frequency - heck, they run happily on DC. I don't know what bizarre things happen when square-wave power, capacitor starters, and various inductive bits get together in a specific appliance. I do know smoke coming out is bad, because we all know it's the smoke inside which makes things work (well, they stop working when the smoke comes out...)

I think that the old UPS as inverter source is an inspired reuse idea.
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Old 06-10-2007, 04:32 PM   #25
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I can't explain because I'm not a designer. I'm always happy to be proven wrong.. In this case, back in about 1992, I was in charge of a bunch of Apollo workstations which had switching supplies... Some of them were in a part of the building where the main UPS wouldn't reach so we bought UPSes for them... The first time we lost power and they switched over to battery, the supplies started squealing real bad. By the time I got there, the back of the supply was hot. We took a scope to the UPSes and discovered they were basically crap, so we replaced them with these big-azz units that put out an almost perfect sine wave (a little teeny glitch on the trailing edge) and our problems went away...

Also, I've tried running a skil saw and a black&decker corded drill off cheap inverters. The drill just hummed, the drill bit sort of vibrated back and forth but didn't turn. On the skil saw, the motor hummed really bad and the blade turned slowly but that 'hot smell' came out of it almost right away...

I wasn't doubting your experience, I was just trying to get an understanding. Thanks for responding. I talk to a co-worker tomorrow and see if he has any insite.
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Old 06-10-2007, 05:04 PM   #26
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Wow... there is some SERIOUS beanie propeller spinning in THIS thread!

We just got back from a camping weekend. In preparation for our trip to Australia this summer, I bought a 12" ultra-transportable Compaq/HP nc4010 laptop with all the goodies off eBay for the princely sum of $300. I also have a bluetooth GPS antenna and Microsoft and DeLorme maps. We used the whole shebang this weekend while we navigated from our house to Moraine View State Park in LeRoy IL on Friday night. It was a God send. I mention it here because I use a five or six year old 12v DC/120AC inverter to power the brick to power the laptop for a time significantly longer than it's 2 hour battery. It works like a champ, regardless of the shape of the juice.

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Old 06-11-2007, 08:44 AM   #27
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I discussed the heating problem with a square wave output inverter with a co-worker. The conclusion is that it's quite possible. How much is dependent on the input circuit of the switcher.

It seems to me it might be wise to stay away from square wave output inverters.
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Old 06-23-2007, 05:08 PM   #28
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There is a bit of "closure" (don't like that word much) on this subject for me.
When my wife can't answer my questions she tells me to "Google" it, which is what I did. A number of answers, some conflicting, but a pattern eventually emerged, and with the answers given here it started to make a little sense.

One site gave a list of items which would either not work or could be damaged by modified sine wave inverters. In it was "speed-controlled sewing machines" and I remembered that I was supposed to take the quilter's machine in for service, because it didn't run, or actually stuttered only at full speed off the inverter. Finally got the gears in my head turning.

I posed the question about this to the head geek at our local computer store, and he offered that as far as laptops go the battery is the item which will suffer the most (didn't make much sense to me). Some other items are questionable or won't work well at all. Then the opening: he said truckers use sine-wave inverters all the time with their laptops. Whether this was a lemming phenomenon or not I don't know, but they may be on to something.

His advice was to check for the "Samlex" brand, also sold by donrowe.com out of Monroe, OR. It turns out that they offer these things at far lower prices than through the RV stores and marine outlets.

Got a 300w model, installed it, ran the sewing machine. It was smooth, ran beautifully at even the lowest speed and acted as if it was on clean grid power. The net result is that it no longer matters what I connect up to this inverter: if it runs at home it will run just as well on this one.My wife is looking forward to using the quilter on camping trips when appropriate. It is her only addiction, so the problem could not be ignored. Thanks, all!
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Old 06-23-2007, 05:29 PM   #29
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For anyone interested in salvaging the inverter in an old UPS, one caution...

I had a look at a computer UPS of ours, which doesn't seem to have much run time anymore. It presumably need a new battery, and if that's not economically sensible, the inverter could be salvaged. The text on the bottom of the case (which I haven't gotten open yet - no screws) clearly states that it does not produce a sine wave; it even lists the (massive) harmonic distortion. My guess is that it's no worse than "modified sine wave" inverters intended for recreational use, and it is intended for use with computers and related electronics.
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