In news:hoklde$jmg$1@news.eternal-september.org,
John Doue typed on Sat, 27 Mar 2010 12:07:41 +0200:
> On 3/27/2010 12:24 AM, Barry Watzman wrote:
>> The advice to at least be cautious is good.
>>
>> I teach PC Repair, Upgrading & Maint. at a local college and I am an
>> engineer who has worked for power supply companies (which also made
>> UPS'). The output of MOST of the CONSUMER grade UPS's is a
>> discontinuous
>> square wave. For one of my classes, I connected an oscilloscope to a
>> UPS (APC) output and I was kind of shocked at what I saw (I knew it
>> wasn't a sine wave, but I was expecting a better approximation).
>>
>> I can't give good guidance on what you can safely use it with, beyond
>> saying that switching power supplies and resistive loads should be
>> ok. To your comment "Square wave would be a different story, of
>> course"
>> ... THAT IS WHAT IT IS, except that between the positive and
>> negative halves of the square wave there is a period of NO output.
>> In other words, it's a Positive square wave pulse, then nothing then
>> a negative square
>> wave pulse, then nothing .... repeated.
>>
>> Not what I was expecting, but that's what it is. There are only 3
>> levels; zero, positive and negative.
>>
>>
>> John Doue wrote:
>>> On 3/25/2010 2:45 AM, Billsey wrote:
>>>> Those who insist that Consumer UPSs produce sine wave output,
>>>> please check
>>>> your egos at the following doors:
>>>>
>>>> http://tinyurl.com/yc22fn4
>>>>
>>>> and
>>>>
>>>> http://tinyurl.com/y8zqc8z
>>>>
>>>> All others would be well advised to listen to the manufacturer's
>>>> warning and
>>>> NOT use UPSs for anything but computer equipment.
>>>
>>> What incident triggered your decision to post this advice? I am yet
>>> to have any difficulty feeding APC UPSs output to my video
>>> equipment, ranging from Reel Tape recorder, Cassette records,
>>> Amplifiers, VCRs and TVs (I live in an area where the power is
>>> frequently restored less than a second after a cut; very effective
>>> at destroying equipment). Of course, this also includes computers,
>>> ranging for full size desktops to laptops.
>>>
>>> I am not as qualified as Barry, but I believe that any equipment
>>> that incorporates a regular stepdown transformer should be able to
>>> handle this kind of output. Square wave would be a different story,
>>> of course. Of course, the yellow flag "Use caution, Wet floor" left
>>> everywhere
>>> in hotels to protect somes a..es might be adequate, but until you
>>> specify in which circumstances an equipment fed with UPS output
>>> failed, this kind of post does not appear very helpful to me.
>>>
> Hi Barry,
>
> Could you post somewhere a copy of the output you saw from an APC UPS?
> Have you tested this way several models? If as you say - and I do not
> doubt you - it is squarely (forgive the pun, too tempting) a square
> wave with a no output in between, I am shocked too.
Hi John! No this is a very common output from inverters (UPS uses
inverters) to convert DC into AC. As it is easy a cheap to build. I know
they were using this back in the 70's when I was getting my EE degree.
And that waveform that Barry speaks of has a name and I can't remember
what it is called right now.
And without getting too technical, the reason why it rest at 0 volts 120
times a second (assuming 60 hertz here). Is because 120V@60 hertz also
passes through zero 120 times a second. And you might say so what? Well
transformers and motors, etc. gets a big break around 70% of the time.
So what does this mean? Well current causes heat. Feed it a true square
wave and you have current flow virtually 100% of the time. Thus it
generates heat at nearly 100% of the time.
But transformers, motors, etc. are not designed to handle current
without any breaks at all. Well you can, but that doesn't help all of
the transformers, motors, etc. already out there in use. Thus creating a
wave like what Barry saw, keeps the temperatures pretty close to what
they would be if used on standard AC.
And this works fairly well instead of a pure sine wave. Works for
transformers and motors. Works for rectifiers and filters, switching
power supplies, lamps, etc. Frankly, I can't think of anything that runs
on AC that wouldn't work okay with this.
And while what Barry had seen is what is commonly called a stepped
square wave (probably incorrectly). And I know of no inverter (or UPS)
that actually generates a true square wave output. If they do exist,
they would be especially bad for transformers and motors. As nearly 100%
current flow would cause a lot more heat and their life would be much
shorter. Although there is a way around this too. That is if the square
wave output voltage was lowered by 30% or more. This would keep the heat
down, but it wouldn't work well for devices that needs the full amount
of voltage to operate. Motors for example might not even start up.
> As most people here I guess, I did not wait for Billsey post to read
> the warnings of UPS manufacturers: they have been there for as long
> as I can remember. But one point bothers me:
>
> What is the fundamental difference between a computer (laptops
> excluded for obvious reasons) and most consumer electronic equipments
> which would make UPSs acceptable for the former and not for the
> latter? Computers do have motors (hard disk, motorized trays) as VCR,
> tape recorders and CD players do , and LCD displays, as TVs do. That
> point escapes me, but admittedly, I do not have or claim to have any
> in-depth knowledge in electronics beyond what I learnt, almost
> centuries ago! I would very much appreciate if you took the time to
> explain this, since this would go a long way complementing common
> sense and avoiding blinding compliance to self-protecting
> manufacturers mentions.
I believe I explain this above without getting too technical. But the
motors and such that you are talking about in computers, tape equipment,
etc. are actually running on DC and not on AC. As the AC hits the power
supply and the power supply converts it to one or more DC voltages.
There are some odd exceptions and once in awhile saying like a very old
tape deck might actually run the motor off of AC and not from DC. Or the
panel lights are actually 120 volt bulbs or something, but it is rarely
done this way anymore.
> I personally never have had any problem with any of my various audio
> and video equipment that could be even remotely related to a UPS (I
> only use APC UPSs), and I have never had people I know report any
> such problem. Since usual consumer grade UPSs only provide power in
> case of sudden loss, and then for a very limited time, there would
> have to be a severe incompatibility to cause damage.
Well there are zillions on devices that run off of AC power. So I can
see some putting out a warning that the wave forms are different and
there is a small chance that an UPS can hurt a very small number of
devices out there. And has I stated above, if there is no rest time in
the output, transformers, motors, and even lamps doesn't get that
current break 120 times a second. Well lamps increases resistance the
hotter they get, so they should be okay. But transformers and AC motors
for sure won't get that time to cool down. Thus they just would get
hotter and hotter and at some point burn out.
> One other way to look at the issue might be that, UPS manufacturers do
> not bother to improve the quality of their outputs ... knowing
> perfectly well that it is *acceptable* for most equipments. The
> improvement of output quality would not be cost effective considering
> they would still not be able to claim perfect compatibility with any
> equipment, which would be foolish in any case.
> --
> John Doue
Well UPS and inverter manufactures sometimes use a disclaimer don't use
for other devices. And other devices sometimes have a disclaimer saying
not to use them with inverters or UPS as well. All I can say is there is
no set standard output for inverters and UPS. As they can use whatever
waveform they wanted too. So I can see manufactures getting a little
nervous about this, can't you?
And if you or anybody else gets nervous, I would be checking the
temperature of the device you are worried about. And to be especially
watchful of the temp of motors and transformers. As these would likely
have the most problems. But the waveform that Barry had seen have been
shown to work just fine (people have been using it for decades without
problems). If you want to get fancy, some UPS and inverter manufactures
use tiny stepped square wave output which looks and acts virtually just
like a real sine wave output. But I wouldn't bother unless you have some
device that absolutely won't work without a real sine wave output. And
while there are zillions of things out there that runs off of AC, I
can't think of one actually that would have a problem. But I can't say
100%, but it has to be very close to 100%.
--
Bill
Gateway MX6124 ('06 era) 1 of 3 - Windows XP SP2