You're using an inverter, you say? (1 Viewer)

[e9999]

This is a little 500W guy I was given a while back for travel, from a "reputable" brand. I only used it for lighting and never bothered to check it until just now. But I had to put it on a cable modem in a pinch and saw some odd things happening. So, I checked it (open circuit) :

I've seen worse, but it ain't great either.
But to be fair, it did indeed say somewhere on the box in small print "Modified Sine Wave", so caveat emptor and all that...

Good to keep in mind if you have some basic inverters laying around.

Dang, now I have to check my other ones too... Those are "Pure Sine Wave" ones, thankfully, but who knows. Should check my generator too, but that's a Honda, so I'm not too worried.


added: I found a YT vid where a guy compared the THD of the mains vs the EU2000i and the Honda was better, at 1% THD vs 3% for the mains (with load).

 

[peesalot]

Sweet, in laymens terms your 500w inverter is functioning improperly ? What does the screen show ?

 

[e9999]

well, not improperly as in defective, just not producing a very good output because of its type of design (typical of less expensive inverters). Basically, they are just chopping a DC input into squarish bits by brute force, add them up in a pattern, and serve them to you like the above.

That scope screen shows the shape of the voltage produced by the inverter. (I used a transformer cuz I didn't want to put 170V on my scope, but that should not change the shape.) The inverter output shape should ideally be a clean sine curve, smooth and rounded, with no sharp points and flats like in the above. (Google "Mains voltage waveform" or something like that and check the Images, if you're curious.) A smooth sine waveform is what you get out of the Mains and also out of a good Pure Sine Wave inverter.
In fact, I did put another one of my inverters on the scope today, out of curiosity, a Pure Sine Wave one, and I got a very nice smooth curve, with only a couple of very minor blips, nothing like the above. Huge difference. But of course the Pure Sine Wave inverters have much more sophisticated electronics and are more expensive.

Real life bottom line: you may want to avoid using a Modified Sine Wave inverter like the one in the OP for electronics you value, and use instead a Pure Sine Wave inverter (or a good generator) for those applications. For crude stuff like light bulbs and the like, it does not matter (well, for incandescents at least, for LEDs I'm not too sure).

 

[peesalot]

Thank you, that is very understandable. So the Modified Sine Wave inverter supply's voltage but it changes radically which is NOT good for components. So a Pure Sine Wave has a smoother change, curious if the high & low of the two types of inverter are close to the same ? Also, would the jerky changes of the Modified inverter cause more heat ? I did find out that a Starlink on a good PSW inverter(400w) will work fine unless you forgot to turn cold protection off, when the antenna heater kicks on it shuts down the 400w inverter. Curious, you said you didn't want to put 170v into osiliscope, where was that 170v coming from ?

 

[e9999]

The voltage is set by the inverter, and it could be anything really but in my experience they are usually pretty close to 120V RMS, both types. I'm running this very computer right now off a 300W PSW inverter, and it is giving me 121V RMS under a 50W load at the outlet.

The waveform above is a bit better than a pure square wave but not by much. I don't know about heat per se. I am under the general impression that the abrupt changes in voltage can cause weird spikes of sorts in electronics.

Your inverter shutting down could have to do with overdraw with the heater and/or low resulting input battery voltage. I would put a meter on it and check how much it draws.

The 170V is the output amplitude of the inverter when it is giving out 120V RMS. Our typical US supply at 120V RMS is actually 340V peak to peak.



Totally unneeded but also totally free bit of nerdish related info: the kind of waveform above is why you want a True RMS voltmeter to measure the output of your inverter or you'll get a funky misleading number.

 

[peesalot]

I was wanting to understand this because I had a friend with same set up save the inverters we were using. Starlink, at Rubicon, on 400w inverter. Mine is a 400w pure sine wave that came with my Traeger, it works well. We both experienced the cold protection , that shut down inverters. His shut down way faster than mine. We have similar supply, good batts with solar so I am confident it was not a supply/low volt issue. I never did look at the specs on his inverter just know it shut down when starlink heater kicked on, mine would show a red indicator, then it went to audible tone, which I shut it off before it's internal systems did, i knew what was coming due to the alarms which I was unaware the little inverter had so that was cool. Anyway, ever since I was wondering what the differences are.
Also, my Starlink stayed connected once online where as his would hook, then drop, hook, then drop, I wonder if that was due to a jerky wave.

 

[e9999]

the 500W MSW inverter I am talking about in the OP was connected to a cable modem and a router with only about 40 watts of draw total, so should have been plenty big but it was nevertheless shutting down unexpectedly, which is why I checked it with the scope. It is possible that your friend's issues had something to do with a bad waveform. But could also be that his is just less forgiving of overdraw if the heater sucks too much power -it that is what is happening (I don't know anything about Starlink systems)- .

 

[AgentOrange]

Makes me happy I went with the psw inverter I have now. Wasn't too sure what it meant, just that it was better for my electronics. Thank you for the explanation

 

[e9999]

Yup, better for electronics for sure.

P, ask your friend what kind he's running.

What is the nominal (or actual if you know) power draw of the Starlink thing? And what is that heater for?

Keep in mind that the battery may be good but if the wiring from the battery to the inverter is too skinny or too long, the voltage at the inverter itself could possibly be too low for the inverter to run properly when there is a significant current draw.

 

[peesalot]

The Starlink takes the 110 ac and changes it back to 53v dc, the heater is an antenna warmer for freezing conditions.

 

[e9999]

The solution to all our issues is simple:
Connect the battery to the inverter and then use the inverter to power a charger that you connect to the battery to charge it back up...
Presto, free energy forever! No more electric bills!

 

[peesalot]

Thats a continous loop with only draw being parasitic loss, trick question.

 

[Mrtwooooo]

I've got a 700w Wagan Tech PSW inverter installed in my 80. I have not really pushed it hard at all but the specs on paper seemed good to me. Also claims it can run at its full rated power continuously. Then I notice that the Renogy brand PSW inverters are basically an exact copy and I don't see them as having a great reputation for they electronics.

Although I have been happy with my Renogy suitcase solar panel .

 

[e9999]

from a quick search, it does seem like -at least for the non-main label chinese inverters- the difference in cost between MSW and PSW is not as large as it used to be. Don't know for main brands. But that probably means there is not a good reason to buy a new MSW inverter any more. Especially, since we are now awash in electronics. Fortunately, all the waveforms I've seen from labelled PSWs seemed pretty good.
That said, my post was intended to remind folks to be careful when using inverters and fancy electronics, and many of us probably still have some older ones around.

added: note also that the frequency is 0.5% off. This may not be insignificant for some applications. It is in fact crazy bad compared to the Mains frequency stability.

 

[e9999]

I should add that you can now buy a digital oscilloscope that is likely suitable for this kind of low speed stuff for around $50. Astonishing!

 

[e9999]

so, nobody else has an even funkier waveform they want to show off...?

 

[AirheadNut]

If my oscilloscope was working I'd check the 1000W modified sine inverter in the LC and the 2500W modified sine inverter in the shop truck. Sadly, my oscilloscope is not working and I can't seem to get around to fixing it! Not entirely sure what's wrong with it either, looks like either a semiconductor rectifier or an op-amp has failed.

 

[e9999]

Tektronix for one had entire series of scopes some years back that are notorious for leaking capacitors after some years (more like 30 than 10 I think). There is lots of info about how to fix that online. But it can be a big tedious job. I've read about some guys changing 100 caps. Fortunately, back in the day, when the US still made stuff, they were fantastic about providing schematics and service manuals so there is that available. I have one of their scopes that flat out died and another on the way out...
I was at a garage sale a few days ago, there was an old HP scope there that nobody wanted. I could have gotten it for free and used it for this kind of job easily enough.

 

[ntsqd]

I have an old o-scope. Given where it came from it was likely a good one in it's day. Were it only that I knew how to use it.

 

[e9999]

go for it! Easy. And great fun! (OK, well, for the nerdy few of us...) One important rule to remember, though, is never connect the ground side of the probe to anything that is not truly the same ground or not truly floating. Also, always be careful to note the max voltage allowable depending on the input impedance. If you can set it for 50 ohms, say, you can't put a lot of voltage on it. Either one of these situations is otherwise likely to result in release of magic smoke.