I got a APC Smart-UPS 700 that somebody was throwing out. Works, only it had bad batteries. The question: what output does it have -- "modified sine", or sine? A quick lood at the APC web site didn't reveal the answer, though it says not to run laser printers, so I suspect modified sine.
kell wrote in news: snipped-for-privacy@k79g2000hse.googlegroups.com:
I never understood that term 'modified sine' as a substitute for sine. It sounds like garbage. If you want a sine, and HAVE a sine to modify, the last thing you do is modify it.
What they probably mean is a rectangle wave that has three states, positive, zero, or negative, and is switched between these with a timing that makes a waveshape which tries to approximate (very crudely) a sine. It's not a bad idea, as it halves the voltage step change that would happen if a square wave was used, is just as efficient, and is probably easier to filter. I've used one on various things that have their own switchmode PSU's without trouble, including a lab DPSS laser, but it's always a risk trying it with something untested.
All of my Smart-UPS units output a sine wave. Back-ups are square wave.
- Mike
Thanks for the info.
Plug in something with a transformer to it, like one of those big heavy wall warts or a stereo reciever. Modified sine wave will normally make the transformer buzz much louder than usual, while a pure sine wave will cause the normal soft hum you get plugging it into the wall.
A 'modified sine wave' is not a three state square wave, although you are on the right track. It is actually what you might describe as a "poorly digitised" sine wave - that is each of the half cycles of the wave is made from two or more voltage steps. It looks on a 'scope like the result you would get if you ran a pure sine wave through a 2 bit - (not the american phrase for something that's crap, but maybe that 's appropriate as well !! ) & sign A-D converter. It is a common waveshape for the output of transformerless DC-DC power converters, and is called 'modified sine wave' because the voltage steps, whilst being clunky and crude compared to a true sine wave shape, never-the-less occur with a timing and voltage position, such that you could draw a true sine wave through the mean of them.
My UPS 700 has a dirty great transformer in it, and as far as I am aware, it outputs a true sine wave, as did all of the UPS's in this series.
Incidentally, I think that I have now gotten to the bottom of my one's intermittent failure when it either self-tests, or needs to actually provide backup power. As several people have suggested, it seems to be the spade connectors inside the thing - in my case, the ones to the batteries. When I obtained it, it had been standing for a long time, and the gel cells were knackered. The replacements that I bought had slightly smaller terminals, and it seems that the spade connectors don't make well to them, and when the converter kicks in, and tries to draw the many amps from the batteries that you forget that these things pull, the connections just can't cope. I actually watched the voltage at the back side of the connectors drop to about 22v, whilst the voltage at the actual battery terminals remained up at almost the nominal 27.4v that you normally read there. The unit then immediately detects a battery problem, and starts whistling furiously at you, and flashing its lights, including the 'battery fault' one.
Arfa
"Arfa Daily" wrote in news:ycz2i.13388$ snipped-for-privacy@newsfe4-gui.ntli.net:
Thankyou. I agree. I was half asleep not wanting to wake up, and dreamed of stuff that I want solved, as I often do... I realised that two schmitt triggers (one each side of zero volts) with thresholds set to respond to a small sinewave signal might be an easier and cheaper way to get the waveshape required, than to model it with fast timers/dividers, so a true modified sine would be made. My error was in assuming that the sine was wanted, but in this case it isn't, because switching for efficiency is more important.
It is most likely a modified sine wave unit.
This doesn't have anything to do with the type of output coming from the UPS. A laser printer's fuser module will trigger sudden high current demand when it turns on. This surge will be hard on the inverter and may exceed the capacity of the UPS. Even if the UPS can handle the load, the operation of a laser printer's fuser will shorten battery runtime quite drastically.
William
Hi!
The circuitry to generate a true sine wave is probably more expensive than circuits to produce an acceptable approximation of one.
More expensive UPS units have true sine wave generation capability. I have an APC Smart UPS 1500 that is equipped with such circuitry.
I've never had problems with a modified sine wave UPS, but some switchmode power supplies start "singing" when running from one. When attached to a UPS that outputs a true sine wave, these power supplies stay quiet, even when on battery.
William
wm snipped-for-privacy@hotmail.com wrote in news: snipped-for-privacy@l77g2000hsb.googlegroups.com:
I thought so too, at first, hence my remarks about making one just to throw away its virtues, but that might not be the case. A Wein Bridge made using something other than a filament lamp might make a slightly inferior sine wave, but in this case that doesn't matter, and the frequency is fixed so you don't need expensive tuning pots, or have to worry about amplitude and shape changes while tuning changes. That means a very small cheap sine osc, which crossing zero and driving a Schmitt triggers on each supply pole, can be cheaper than trying to model the timing with faster oscillators and dividers and the logic required to set the timing of transitions. It certainly uses less parts, and has a higher accuracy despite needing only two voltage dividers to set thresholds.
Modified sine is somewhere between square and sine. A complete square wave has the disadvantage of not having enough voltage for lamps or electrical equipment. If you make it 120 VAC RMS and its a square wave, then the peak voltage is much less than a 120vAC RMS sine wave. You can make a stepped sine wave using changes on the transformers taps, which is efficient. You can also make a sine wave using switching technology. Making a sine wave with a linear amp makes a lot of heat.
greg
snipped-for-privacy@pitt.edu (GregS) wrote in news:f2fh81$gh8$ snipped-for-privacy@usenet01.srv.cis.pitt.edu:
It does... This interests me, how is a switching system arranged to get close to a sine shape without the linear amp? I can imagine that as you can derive switching points from a Schmitt trigger/comparator following a real sine signal to get two poles around zero to get the kind of wave that Statpower use in their inverters, using two main switching slements, you could have more than two switches to get finer resolution, but at what point does it become too awkward, and how many switches are optimal for close-to-sine output at reasonable cost and high efficiency? And is it humanly possible to make a more lengthy and cumbersome sentence than the previous one?
Generating the true sine wave is easy, that's not the problem. The difficulty is in efficiently amplifying it to drive the load.
Here is how a 120 volt true sine wave inverter using a high frequency switching supply works:
You make a high frequency transformer isolated switching supply that can generate up to 150 volts or so and filter the supply so that attenuation at
120 Hz is minimal. You modulate the regulator with a full wave rectified sine wave at the desired output frequency. The switching supply will output a voltage which looks like full wave rectified line voltage. You use MOSFETS in an 'H' bridge configuration to commute this into plus and minus voltages for the load on alternate half waves. You can also have two switching supplies with one supplying the positive half and the other the negative half. This simplifies the commutation circuitry.Dave
"David" wrote in news:nGJ2i.6242$ snipped-for-privacy@newssvr11.news.prodigy.net:
I think I get it, you mean it uses the high speed switching to allow small filter caps on the power output? I guess if so, this is also the basis for those amps for PA that do something similar to track an audio wave with arbitrary signals in it.
Yes, The switching frequency is 100s of KHz and small caps are used. You actually need a small load on the output to get the desired waveform.
The so called class 'D' audio amplifiers use a form of pulse width modulation with positive and negative pulses followed by a low pass filter. Inverters can also be designed this way, but the ones I have seen use the method described earlier.
David
In message , kell writes
No idea on the waveform type I'm afraid but the laser printer thing is because it just isn't powerful enough, some fusers and motors in laser printers can draw *lots* of current. The APC I have has a label that states no laser printers on less than a 1500VA UPS.
-- Clint Sharp
Modifies sine wave' is an expression so that those who know only by feeling will then 'feel' they know reality. The answer to your question is in numerical specifications - especially its THD number. Without that number, anyone can subjectively claim a modified sine wave and a sine wave as same. Only other way to answer your question is using an oscilloscope. A neat little trick, using a 'big transformer' appliance, will identify the 'worst of the worst'. But still, some modified sine wave outputs may not create detectable noise in that test.
Three different procedures to answer your question. Without underlying 'whys' or numbers, no reply has provided a reliable answer. Any answer without the 'whys' or numbers should be considered speculation.
Meanwhile, what are you trying to accomplish? If UPS batteries are bad, then the entire discussion of modified sine wave is moot. What is the original objective? What are you trying to solve?
I see. Thank you.
Boy, I'm detecting a lot of negativity there ! I thought that most of the answers and discussions that have gone on in this thread so far, dealt with what we are talking about, pretty neatly and comprehensively. The original question was quite straightforward, and the basic answer is quite simple, not really requiring an in depth examination of THDs or any other numbers. The term "modified sine wave" is regularly used by UPS and other inverter manufacturers to describe what is readily understood by everyone to imply a digitally derived approximation to a sine wave, with rather less steps than would be desirable in an ideal world. How many steps, and what those steps represent in terms of distortion figures relative to a true sine wave, is neither here nor there in understanding the concept of the term.
Arfa
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