From: John De Armond Newsgroups: rec.outdoors.rv-travel Subject: Re: Cheap inverters Message-ID: <48ja5vgnicceg97lhj0rf3837vva19s0ou@4ax.com> Date: Thu, 20 Feb 2003 17:10:21 -0500 On Wed, 19 Feb 2003 09:41:37 -0700, Alan Balmer <albalmer@att.net> wrote: >On Tue, 18 Feb 2003 05:40:13 GMT, John S <camper123456789@yahoo.com> >wrote: > >>In article <6gr15vg98mbfq1oph696oct65e18869201@4ax.com>, will@epix.net >>says... >>> The so-called car adapters contain a small inverter/converter designed >>> for the light load of a computer. The dc input is inverted to a ac >>> voltage which is then (via a small transformer - transformers do NOT >>> work on dc) boosted up to a higher ac voltage and then converted to >>> the higher dc voltage the 'puter requires - usually around 18v. >>> >>> Will Sill >>Will - I have suggested before that if you want to fake being an >>engineer, then stay out of technical conversations. Your problem is >>that you are too clueless to know that you are clueless so when you >>open your mouth like this, you drop the ball. The real problem isn't that he tries to play being an engineer. It's that he plays a really BAD one. >Will's description is not incorrect, though it may be out of date. >DC-DC converters have been built this way for years, and can be quite >efficient. Because the oscillator is typically fairly high frequency, >a low-current transformer can be very small. He is completely wrong. Inverters have never operated in the manner he describes. The very first inverters used radio vibrators to mechanically switch DC alternately into one winding and then the other of a specially designed transformer. This transformer was designed not to saturate with the large DC component and not heat too much from the odd harmonics that accompany the square wave switching. A bit later, inverters used simple blocking oscillator designs in which an aux winding supplied the correct phase signal to the transistor that switched the DC on and off. This was a unipolar design with a DC current in the transformer of about half the total current. The very early Tripp-Lites used this very inefficient design. A bit later, the design evolved into bi-polar switching where two transistors driven by two separate "tickler" windings switched the 12vdc alternately. This greatly reduced but did not limit the DC bias. The frequency was determined by the transformer characteristics and the load. Still later, the design acquired a simple 555 IC timer-based 60 hz timebase to stabilize the frequency and to further reduce the DC flowing in the transformer winding. This is the last style that was popular before the advent of the switchmode inverters. The 2KW UPS I built for dixie.com's little data center used one of these inverters from Tripp-lite. It weighed over 100 lbs. In all of these, low voltage, high current DC was switched periodically into the primary of a very heavy iron cored transformer whose secondary produced the nom 120 VAC square wave. >As you point out, modern low-power converters generally use IC-based >Buck conversion. I don't know about other aspects, but price is an >advantage. Modern inverters have glomed onto developments in switchmode power supplies such as are in the PC. The architecture is quite similar. The development of high power FETs and small but high power ferrite core transformer designs are the enabling technologies. The architecture is thus. The incoming 12 volts is chopped at a high frequency - 20khz to >1mhz - and applied to a ferrite core step-up transformer. The output is rectified and filtered and results in about 145 volts DC. In many designs, especially if it is desired for the neutral to be ground referenced, there are two of these inverters, one for the positive swing and one for the negative. This high DC voltage is switched at a 60 hz rate to produce the output. In most low end inverters, the switching is simply on-off and produces a square wave. There is a zero crossing dead time so that the wave more resembles a sine wave. An output low pass filter will further improve the waveform. Some slightly more expensive inverters will switch two different voltages, one after the other, to produce a multi-step waveform that is even closer to a sine. With some low pass filtering, the THD is typically <5%. The high end units such as the true sine wave inverters and the inverters built into variable speed generators like the Honda EU series produce the sine wave by synthesizing it using a pulse width modulation scheme to switch the high DC voltage. The switching rate is typically 20khz or greater. Since each half cycle of the output is made up of hundreds of modulated pulses that closely approximate a sine and because there is invariably some low pass filtering on the output, the THD is very low. Honda claims less than 1% and from I've seen on my instruments, I'd believe it. John From: John De Armond Newsgroups: rec.outdoors.rv-travel Subject: Re: Costco Inverters [and campground power] Date: Sat, 27 Sep 2003 17:43:35 -0400 Message-ID: <mlvbnv8aes2qvnrj488b56964thb53ab0k@4ax.com> Jim, I've been hearing this rumor about printers at least since the original HP laserjet, what? maybe 1985 or so? Back then the issue was with UPS's, as the cheap inverters didn't exist. Here's my actual experience: In about '85 I built an uninterruptable power supply for my computer lab, not liking the prices of commercially built units. It consisted of a 14 volt, 200 amp linear regulated power supply, a large bank of marine batteries and a Tripp-lite 1000 watt square wave inverter. Not pseudo sine or modified sine. Plain old square wave. With one exception everything I tried ran just fine on this inverter. That included the computers (an original IBM AT for a file server, monitors, modems (I expected trouble there but didn't have any) and an original HP laserjet and then a Laserjet II. The only thing that I tried that didn't work was a microwave oven. The reason is the voltage doubler that drives the magnetron depends on the line voltage's peak voltage (about 170 volts.) The square wave inverter output a square wave with a peak voltage of 120 volts. The original pseudo-sine inverters used two different high voltages, switched in sequence to partially synthesize the wave. It would usually involve an approx 120 volt signal and a higher voltage one. The sequential switching generated a stepped waveform. Things like microwave ovens that require the high peak voltage might or might not work, depending on what the high voltage is. The current generation of super-cheap inverters use a single, typically 145 volt DC source that is switched as a square wave but with some zero crossing delay. That is, the positive voltage is turned off and then some time elapses before the negative voltage is turned on. About the only things that don't work on these are the transformerless battery chargers such as many cordless drills use. some audio equipment will buzz a bit but most don't. John On Sat, 27 Sep 2003 12:06:06 GMT, "Jim Walker" <walkerjd1@worldnet.att.net.net> wrote: >I agree that the warning to use true sine wave inverters is overdone. I >have used the cheap one for years on a laptop mainly because I didn't hear >the warning before I began. I now know that some laptops have built in >protection that takes care of the problem. My Thinkpad is one of those with >the protection. A thread a while back went into this and a couple of >computers did not work well with the non true sine wave inverters. I am >not sure of the brands now, sorry. A Life On Wheels seminar went into this >and they concluded that some computer printers would not work with non true >sine wave inverters. This is practical experience from some full timers. >Again, it could be only some printers and not all. My conclusion is that >the appliances are generally protecting themselves. I will not go for the >true sine wave inverter unless I absolutely have to. From: John De Armond Newsgroups: rec.outdoors.rv-travel Subject: Mendelson's surplus 1000 watt inverter Date: Tue, 11 Nov 2003 23:28:56 -0500 Message-ID: <nrb3rvkrspah15ajlnlolq8f766j9kq47k@4ax.com> I mentioned last week that Mendelson's had a decent price on 1000 watt inverters and that I'd ordered a couple. They came in. Someone asked me about the waveform. I've done some testing. The results are as follows. The inverter is smaller than the Vector unit. One of the smaller 1000 watt units I've seen. My first reaction upon removing the cover was "holy sh*t, where are all the parts!?!" I saw the usual 4 high frequency transformers, a few transistors along each side of the board and some filter capacitors but nothing else. Then I noticed a small daughter board. Upon casual examination it appeared to have only one IC. Then I looked underneath. A high density surface mount board. Ahhhh, that's where all the works are. It looks like whatever chicom engineer designed this thing intended for one daughterboard to serve a whole line of inverters. On the main board of one inverter there were signs of the blue smoke leaking out. On the other, none. Both have had power transistors changed out. The original used some sort of polymer heat sink sheet but the repair tech used thermal grease. Obvious which ones had been reworked. Photos for the following discussion are here: http://bellsouthpwp.net/j/o/johngd/files/rv/1000%20watt%20inverter/ To evaluate the unit I hooked it via short jumpers to a fairly large AGM battery* I had on hand. I loaded the inverter with a 600 watt ceramic heater. I used a Fluke 97 ScopeMeter to observe the output. Upon powering the unit, the fans run for a few seconds and turn off. The unit is totally silent. The photo of the no-load waveform shows the usual stair-step square wave with dead time at the zero crossing. Same as all the others I've looked at. When the 600 watt load is applied, the fans start within seconds. The photo of the waveform shows a bit of sag at the peak of the square wave but not enough to affect the RMS value very much. The rise time of the square wave provides a good indication of the potential for buzz in audio equipment and magnetic devices such as fan motors. The photos show first the no-load rise time and then the 600 watt load rise time. The 1.2 uS rise is fairly fast but is also fairly typical of the breed. The rise time stretches out to a leisurely 3.5 microseconds with a 600 watt load. This is slow enough not to buzz most devices. Of most concern to me is the fairly low RMS voltage, 112 volts, and the fairly low frequency, 55 hz. Of the two I'd worry more about the lower voltage affecting voltage sensitive devices like microwave ovens. The low frequency will cause fans and line-operated digital clocks to operate more slowly than normal. I connected the second inverter and got 120 volts and 61 hz. There are several pots on the daughterboard so perhaps this variability can be adjusted out. One of the things I like about these inverters is that it looks fairly easy to parallel them, with all the logic being on that daughterboard. Another thing I like is that the DC power is distributed to the 2 sets of 2 internal inverters that run in parallel via heavy bent copper bus bars. The Vector, along with every other inverter I've seen in this class rely on a combination of circuit board traces and stamped sheet metal bus bars. This is probably the core cause of the Vector's significant sag in output voltage vs load. These numbers seem bad but compared to other inverters I've looked at, they're not. One inverter that I'd used for years without problems yielded 50 hz when measured! The waveform stayed clean when subjected to an inductive load (fan) which is good. Another inverter that I've used quite successfully on motor loads showed a drop out (double peak) on each half cycle on the scope. As my old Tripp-Lite square wave (not pseudo or rounded but bone-jarring square wave) inverter demonstrated, most devices will run on almost anything that resembles an approximately 60 hz bipolar wave. The Costco inverter is probably a better deal at $100 (anyone near a Costco care to buy one for me if I send you the money?) but for those of us far away from Costco stores, this Mendelson's unit is a pretty good deal. I like mine enough that I'm going to remove the Vector unit from my MH and install this one instead. with the smaller voltage sag, it should provide a few more volts to the microwave oven. John *New, code date 2002, 80 amp-hour AGM battery that I purchased at the Lawrenceville, Ga hamfest for $25. Actually I bought several. Typical of what one can find at a hamfest. I bought a similar set last year to go on my server's UPS. From: John De Armond Newsgroups: rec.outdoors.rv-travel Subject: CostCo Inverter first impressions Date: Sat, 22 Nov 2003 01:35:54 -0500 Message-ID: <fc0urvsff25t6u3q5ovhi0dqvs2i1emki7@4ax.com> Peter Crowl was kind enough to answer my request to buy and ship me a CostCo 1000 watt inverter for evaluation. Thanks again, Peter. I will not have the time before Thanksgiving to wring this thing out but what I saw in my initial observations are important enough to mention now. Below is my last note to Peter I sent a little while ago. Executive Summary: It appears that you get what you pay for and that this Xantrex inverter isn't very good. More info after Thanksgiving. On Sat, 22 Nov 2003 00:18:51 -0500, "pcrowl@earthlink.net" <pcrowl@earthlink.net> wrote: >> My first impression is that >>Xantrex made a special version for costco with every penny of cost squeezed >>out. For instance, they soldered the main fuses to the PCB instead of >>spending a couple of pennys on spade lug sockets. > > Wouldn't be the first time that was done. This business about the fuses >being soldered doed not impress me - and sure won't come replacement time! > I look forward to the review on RORT! My words were prophetic. I hooked the thing up and it was dead as a doornail. those *(^&^ soldered-in fuses were blown. 3 40 amp ATC fuses. I quickly realized that I'd have to remove more than a dozen screws holding the transistor clamps in place to get to the bottom of the board to unsolder those fuses. I cheated and bridged the gaps with solder after prying the fuse bodies off. The inverter then worked and appeared undamaged. I bet someone hooked it up backwards, blew the fuses and then 'returned' it for a refund. Anyway, I'm not terribly impressed with this unit. It is of the design that floats both the hot and neutral above earth ground. In this case about 55 volts. Some surge arresters and switching power supplies will behave badly. One of my first tests is to plug in a "100 watt equiv" Lights of America CF lamp just to see if everything is working. On this inverter the lamp strobes badly, probably indicating the output is not symmetrical. This is the first time I've encountered strobing so I'm going to have to investigate further after Thanksgiving. One of the important things that has been de-contented is input filter capacitors. These prevent ripple from backing out the DC side by supplying the surge current to the high voltage inverter on each half cycle. My Vector has perhaps a dozen large electrolytics scattered around the board. The Mendelson's surplus unit has probably half a dozen larger ones. This one has 2. The result is that with a 600 watt load on the inverter, there is almost half a volt of 60 hz ripple on the AGM battery terminals. I'm going to postulate that this much ripple would cause 12vdc fluorescents to strobe and would cause hum on audio appliances. Not good at all. This is all the result of perhaps 30 minutes of poking around with the scope and voltmeter so consider my comments preliminary. I'm thinking that this is a case of one getting what he pays for. I have enough different inverters in hand now that I think I'm going to do a formal comparison test after Thanksgiving. IT'll be a lot of work but I think worth it. I wish I had one of those inverters that Flying J sells under the Shakespeare brand but I'm not willing to pay the inflated price for one. I think that what I have will be sufficient. John From: John De Armond Newsgroups: rec.outdoors.rv-travel Subject: Re: 12-volt Fluorescent Lighting Date: Wed, 25 Feb 2004 01:25:48 -0500 Message-ID: <0fco309gf74ktsd1bv7bsi40cf8lrr9la6@4ax.com> You are quite simply wrong, Ralph. I have tested numerous modern inverters - and I bet I know more about testing them than you do - in recent times and I've yet to find one that is as bad as 90%. The efficiency is almost independent of load when the load is >10% or so. The reasons are many. The modern architecture. The modern CMOS components. Modern design techniques. Let's take a look at the Vector 1kw inverter that I currently have on my bench, testing for long term durability. The architecture is thus: (12v to 180vdc inverter) -> (high voltage storage) - > (power FET 60 hz driven choppers [several parallel stages]) -> LC high pass filter -> 120VAC outlet. First the inverter. Where the heavy lifting is done. It uses multiple parallel power MOS-FETs to reduce the on resistance to minimize loss. It also uses more FETs as synchronous rectifiers. SRs avoid the 0.2 to 1 volt drop see across conventional rectifiers. The switching frequency is high - 40khz - to further minimize losses and reduce the size of the components. The inverter's overall efficiency is remarkably improved at less than full load with a simple feature - the DC/DC inverter does not run all the time. It is switched on and off, with fairly high hysteresis, to maintain a setpoint voltage on the energy storage caps. All easy to do, given processors that cost 35 cents a pop in quantity. The high voltage storage consists of multiple aluminum electrolytic caps. This is actually a weak point with this particular design. The caps apparently have a fairly high ESR and/or dissipation factor, as they get very warm during full load operation. The output stage is again several power FETs in parallel for each polarity of the 60 hz output. The standard stair-step waveform is used to synthesize to some degree, a sine wave. The "sustain" vs "peak" duration ratio is varied according to load to keep the RMS value near 120 volts and to maximize efficiency. Because of the DC/DC inverter's switching on and off at a fairly slow rate, probably 2-5 hz, measuring the efficiency of one of these critters is a bit more complicated. Driving the inverter from a DC power supply or battery/charger combo requires data logging and averaging to determine the true input power. I used that method in the beginning, using my National Instruments datalogger. After I acquired a 7 farad, 15 volt capacitor, the testing got much simpler. The cap is large enough to smooth the input draw, at least for 1000 watt class inverters. Simple DC instruments can then serve. On the output I use only bench grade (5.5 and 6.5 digit Fluke and Keithley) meters with data logging (IEEE-488) and lab quality F.W. Bell transducers. The results agree quite favorably with old fashioned analog (iron vane or electrodynamic) lab instruments that I own. With datalogging, I don't have to stand there and write down the numbers. The results also agree very well with that of an old-fashioned rotating disc power meter. Without taking a walk to my lab to fetch my notebook, something I won't do for a pissin' contest like this, I can recall some interesting numbers. The efficiency remains >90% until the load drops to <20 watts. With no load applied, the inverter draws an AVERAGE of about 15 ma. The Vector spec is 30 ma, if I recall correctly. That it does, but only when the DC/DC inverter is topping off the energy storage caps. Another remarkable little inverter is the Vector 70 watt unit built into an oversized cigarette lighter plug. Here: http://tinyurl.com/5zhg I have probably half a dozen of these little inverters. When I first ran one through my lab I wasn't terribly impressed. Full load efficiency <80%. Then I realized why. The crappy cigarette lighter plug was getting quite warm, indicating a good deal of wasted energy. I opened the unit up and soldered wires directly to the inverter board. The efficiency was then what I expected, >95%. So little heat produced that the case barely gets warm. Back to the original question, my suggestion would be similar to "Q". Use compact fluorescent lamps where practical and conventional fluorescents, preferably with electronic ballasts, elsewhere. Instead of one inverter, I'd use numerous of those little vector inverters, one on each switched circuit. That way there is zero no-load loss. Awhile back I bought one of these Thin-lights to evaluate: http://www.campingworld.com/browse/skus/index.cfm?skunum=6083 I'll have to say that I was impressed. The inverter design is extremely clever and appears to be of conservative design. I had a couple of problems that kept me from buying any more. One is the cost. I just can't see $40 for a 20 watt fixture (they claim 30 watts but that is incorrect.) Two is the color temperature of the light. They only come in cool white which has a blue cast and to me, at least, makes the RV interior feel like a hospital room. Warm white, or even better, incandescent white (about 2000degK color temperature) tubes are available but not from Thin-light. One of the main reasons CFs have been so well accepted is that the phosphor used duplicates the color spectrum of incandescent lights. The light is familiar and warm. People give no thought that they're now using a "fluorescent lamp". CFs work equally well in an RV where there is space to use a conventional fixture. The cost advantage is large. That little 70 watt inverter is $20 from Northern. I've seen them for $16 and change at WallyWorld. It can easily drive 4 15 watt CFs. That would be $4 per lamp. I can get CF lights at Sam's for $19 for 5 of them. That's about $4 each. That would be $8 per lamp total. Plus the cost of a fixture if necessary. On Mon, 23 Feb 2004 17:11:42 -0800, Ralph Lindberg <n7bsn@callsign.net> wrote: >In article <ilvk30t2frpkpvtrmr1kvbgmfk8ou0r57u@4ax.com>, > Alan Balmer <albalmer@att.net> wrote: > >... >> >> Nevertheless, the very inexpensive Vector VEC024 (400W continuous) >> inverter sold by Sam's Club is speced at 90% efficiency. This is >> probably at maximum load. No-load draw is 0.4A. > >While that is the spec, have you measured one? > >The last time we did, it wasn't nearly as impressive. If you drop the >load off 100%, the efficiency drops way off. Let's say you are loading >it at 20W (typical lighting), the efficiency is typically less then 50% From: John De Armond Newsgroups: alt.energy.homepower Subject: Re: Inexpensive 24 or 48 volt inverters Date: Sat, 09 Aug 2008 21:33:28 -0400 Message-ID: <s1hs94hiqht6fdta9krk0r2rdtl266jj97@4ax.com> On 9 Aug 2008 23:41:59 GMT, phil-news-nospam@ipal.net wrote: >| for 48 to 120, there are telecom inverters. Lucent and Major Power make >| them, and they're very expensive but available in that power range. > >And they expensive because? > >1. 48 volts is harder to deal with than 12 volts. Actually 48 volts is EASIER to deal with at a given wattage than 12 volts. For power FETS, what almost every inverter uses, the conduction losses are proportional to I^2R. A 48 volt inverter draws but a quarter the current of a 12 volt inverter of the same size. All else being equal, that equals 1/4 the number of FETs and 1/16th the thermal losses. It's rare to find a power FET with a voltage rating lower than 60 volts anymore so the same device will probably be used at both 12, 24 and 48 volts. The lower voltage simply requires more of 'em. The economy of scale of 12 volt inverter manufacturer is more than offset by the reduction in power devices and the reduced heat that has to be disposed of. That's why, with the help of some folks who actually know what they're talking about, I've been able to find high voltage inverters in the same price range as 12 volt one. JOhn |
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