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Modified sine wave inverter

  1. May 6, 2011 #1
    I recently bought an inverter that connects to a 12 volt battery and gives out 220volts AC current. It is rated at 1200W and it produces modified sine wave. The main question I am trying to solve is: Is it safe to run my laptop and dsl router on this modified sine wave? I have done a LOT of reading online about modified sine wave and I came up with VERY contradicting opinions! Some say it is totally suitable for almost all home appliances that the average user may use it for (including TV, radio, laptop, wireless router, stereo..etc). Others say it is only safe to run very basic things (no electronic circuits), like a light bulb, heater..etc.. Some report that they have used all sorts of electronics (including the things I listed above) on modified sine wave for years with no problems at all.. Others report that they hooked up their laptop and minutes or hours later..poof... it was burned!!! I totally understand the difference between sine wave and modified sine wave (or modified square wave if you wish). I also totally understand that pure sine wave is much better (because it is what ideally electronics were designed to work on)... But this is my situation: I already bought the inverter. I live some place where they don't have pure sine wave inverters... I have a LOT of periods where there is no electricity (sometimes for up to 4 hours at a time). And I REALLY need to use my laptop and wireless dsl router and to charge my phone.. Before reading about the topic (when I first bought the inverter a few days ago), I just plugged everything into the inverter and everything (laptop, a desk lamp, my wireless router, my cell phone charger) all ran PERFECTLY normally (as they do on regular electricity).. No difference in performace at all. I would say, I have run them for at least 10 hours (2 or 3 hours at a time) and I have checked the adapters/transformers (the thing that transforms from 220V ac to the DC that the various items require) (specifically my laptop adapter which transforms from 220v AC to 19V DC, my wireless router adapter 220v AC to 22V DC and my phone charger... ) I have periodically checked them and they do not seem to be any hotter than they normally are when I use them on normal AC power (from the grid). I also used a multimeter to check the DC voltage that is coming out of each of them, and it is identical whether using sine wave AC (from the grid) or modified sine wave (from my inverter).. Also I put them to my ear to see if there is any buzz on modified sine wave... Only the laptop adapter (transformer) gave off a very low buzz, but I actually had to put it against my ear to hear it, so it is very low.
    I also read that some sound equiptment will have 'noise' if run off a modified sine wave.. I did the following experiment: I hooked up a radio and I got the following results: If the radio is attached very close to the inverter there is some noise, however, when I put a long extension cord and then plugged in the radio (so the radio was physically far away from the inverter) it didn't have noise (same level of 'static'/noise on modified sine wave from inverter and on sine wave from the grid)...

    So after presenting my case, can I assume that I can safely run these items (router, laptop, charger) off the modified sine wave inverter? Again I repeat, getting a true sine wave inverter is not an option for me, so either I use my current one or I sit in the dark and wait for the electricity to come.

    Also, if I decide to run any other appliance of the modified sine wave, how do I know it is safe? Can I use the following logic (which I used above): If it is not heating up and not producing any weird noises, and is functioning correctly, can I assume it is safe to run it off the modified sine wave inverter?

    I would truely appreciate any information you could provide me.
  2. jcsd
  3. May 6, 2011 #2
    Light bulbs do work fine on MSW so you can sit in the light if nothing else does.

    But the real answer is most likely "it depends". Most switch-mode power supplies should deal with MSW just fine. The issue is usually either the harmonics causing some small parts to experience excessive current, or for the noise to leak through filtering intended to block power frequencies (e.g. 50 or 60 Hz). Motors are likely to have an issue with the harmonics. I've heard CFLs do.

    There are a lot of ways to clean up the AC, some of which can stress the inverter with more current, and some of which can divert the higher frequencies to a dummy load (big resistor that would get hot or a light bulb).

    The buzz on audio equipment is likely due to the same reason you have hum on audio equipment, particularly microphone pickup. That is because a grounded wire is carrying the current. If you have a split phase inverter that really does 110 volts between each of two inverter sections that in series will give you 220 volts (sine waves in sync), you could ground it between the sections and that would reduce the hum/buzz issues with audio. A few devices might have trouble with this kind of power, and screw in light bulbs that have an exposed metal for one of the conductors will be a safety risk when connected at 220 volts. But the risk is only 110 volts exposure relative to ground.
  4. May 6, 2011 #3
    MisterX, I am not sure what the total harmonic distortion is for my modified sine wave inverter. The maufacturer does not specify. As for getting a sine wave inverter, let us just say that for the sake of discussion this is not an option and I want to keep using this one.

    Shaperen, thank you for your reply. I know that light bulbs will work.. Sitting in the dark is not the issue. I am more concerned with running my laptop and my wireless router. So for the situation that I described (and after providing all the 'test' results about no overheating and the devices running well and the DC voltages given by the adapters/transformers of the various devices being the same on sine wave and modified sine wave), can anyone reassure me that I am safe to continue using these devices with the modified sine wave inverter?

    Any information (concerning my specific uses on my inverter) would be highly appreciated. Also I have a question: If using these things on my modified sine wave inverter will 'shorten' the life of these devices (laptop..etc.) are we talking about something significant? Or is it just something in theory? I mean, if for example it takes 5% off the life span, then I can live with that.. But if we are talking about cutting the life span down by 50% then that is significant! Btw, Sharperen, you mentioned 'cleaning' up the AC.. I didn't quite understand how I would do that.. You mentioned using a light bulb. So I just connect it in addition to my other equiptment (laptop, router..etc..) and it will take away some of the frequencies? Please explain how I would do that.
  5. May 6, 2011 #4
    I can't really assure that without knowing the exact design of the PSU that will be used, as well as the exact waveform of the inverter. The general knowledge is that switch-mode PSUs tend to work fine, as do resistive loads. Transformers will need a somewhat higher rating in most cases. Motors may or may not run hotter depending on design. Filtering applied to the AC power lines can help enough to make a difference in some cases, although this tends to be an expensive solution.

    If things run hotter, that can shorten the life much like the ambient temperature being as much as the heat rise would shorten it. If you run your laptop in 45C weather, you can expect it to fail sooner than if you always ran it in 25C weather. If running it on modified sine wave causes it (the PSU) to be 20 degrees hotter, that would be about the same life shortening. It might still be a little different if that heat source is some small component that is individually more hot now.

    Cleaning up the AC is basically filtering out the harmonics from the voltage steps that make the fake sine wave. It depends on how coarse or steep those steps are as to how effective that can be. Getting a waveform of the inverter output could tell us how much this might involve. If you have just 2 or 3 steps per quarter cycle, it could be hard to clean up. If you have 20 or 30 steps per quarter cycle, it would be easier. The "pure" sine wave inverters are likely just MSW inverters with a very high number of steps followed by a good filter that removes virtually all harmonics.
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