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Forum Index : Electronics : Modified sinewave inverter risks
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Haxby Guru  Joined: 07/07/2008 Location: AustraliaPosts: 418 |
I want to use my mains powered DSO in my car while driving to monitor the hybrid system current, voltage etc. I have a "modified" sine inverter to power the DSO. Is there a risk of blowing up the scope with this type of inverter? |
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phil99 Guru Joined: 11/02/2018 Location: AustraliaPosts: 1773 |
During power outages I have run all my electronic gadgetry on a "modified sine wave" inverter (in fact it's output is almost square) without problem. However the inrush current of one old laptop power brick is high enough to trigger the inverters overload protection. This probably isn't good the power brick's main filter capacitor either. The solution is to put a large iron cored inductor in series with the supply, as used in many desktop computer power supplies. The type of inductor used in old 40W fluorescent lights might be suitable. |
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| Davo99 Guru Joined: 03/06/2019 Location: AustraliaPosts: 1577 |
I have run Lap and desktop computers off Square wave Inverters for long periods and without problem. My unqualified Conclusion is if something has it's own power supply regulation/ filtering as everything seems to do, it's fine. That said, I Imagine this does not draw much power and Sine wave Inverters are very cheap now anyway. 150-300W unit should not cost a lot. The other thing that would come to mind is if you have a UPS on anything in your house. Would probably power it a decent amount of time and you may be able to improve the run time with a little boost converter to put some power from the car in the batteries as you use it. Most of the smaller UPS units I have come across are 24V. I have a few Ups units I used to run from car batteries after I extended the leads out the machine. Only thing is you may have to start them off the mains before you leave home or they Might boot off the inverter and then disconnect it as they don't seem to like square wave outputs but all my APC UPS units used to boot from them. I got 3 of the little Cheap square wave inverters out the other week and they were all dead which surprised me. I was sure they were working when I stored them about 5 years ago. They all just sat there and screamed and then shut down. Booted up one of the little UPS units stored in the same box and it was fine. Have to buy myself a little 12V inverter, everything I have left now is 24. |
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wiseguy Guru Joined: 21/06/2018 Location: AustraliaPosts: 990 |
Personally I wouldn't risk it with my CRO. The issue with modified sine wave (read as altered duty cycle square wave) is the instantaneous peak currents. A 240V sinewave has a relatively gently changing voltage that rises to a peak of 340V over 5milliSecs. which is what the equipment power supply was usually designed for & is expecting. The modified square wave is an instantaneous rise from zero to 340V in essentially a few microseconds. The peak currents are many times greater than would occur with a sinewave input and could be detrimental to the units power supply. Phils suggestion of a series choke would certainly improve the issue, but I personally still wouldnt do it. It is relatively cheap to buy a small sinewave inverter and with your skills, a cinch to build one. I have been tasked a few times to repair some electronic items that failed with modified square wave supplies, it was usually current limiting/sense resistors and capacitors that fail (sometimes with other parts taken out at the same time). Edited 2021-07-18 11:43 by wiseguy If at first you dont succeed, I suggest you avoid sky diving.... Cheers Mike |
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| Davo99 Guru Joined: 03/06/2019 Location: AustraliaPosts: 1577 |
What sort of gear was this in? I don't know beans but I would be looking at a proper inverter for the cost/ risk factor. I also wonder if the squarewave supply could affect the reading the machine gives? Quick look on fleabay shows pure sine waves are available in Oz from $65. I have a couple of the Jaycar Brand inverters I bought used and they are excellent. Not cheap to buy new but defiantly have the quality in my experience. |
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| wiseguy Guru Joined: 21/06/2018 Location: AustraliaPosts: 990 |
The only two items that come to mind (probably because they were more recent) a power supply brick for a laptop and a power meter that used a simple charge pump power supply with a small series current limiting resistor. The zener in that unit also showed signs of overheating but it didn't fail. A camera charger also snuffed it but I didn't investigate/fix it (it wasn't mine). I'm slowly learning when in groups if someone says do you know anything about TVs/ microwaves /amplifiers etc etc I say nothing and if I'm looked at say no. I inherit a lot less problems that way. If at first you dont succeed, I suggest you avoid sky diving.... Cheers Mike |
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CaptainBoing Guru Joined: 07/09/2016 Location: United KingdomPosts: 1985 |
it shouldn't be a problem, but I DO have a problem with the manufacturer's description of these things as modified Sine, it should be considered a modified square. Imagine a typical square wave with a dwell around the centre point on both transitions and you have it. About as "Sine" as my foot! It is a marketing ploy to describe them as such. Most stuff will be OK on such a power waveform but traditional linear PSUs (i.e. with a transformer to take mains down to a more manageable voltage) can get warm because of the dwell at full power on the flats at the top and bottom of the waveform. Will likely be OK but watch for it getting warm. |
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InPhase Senior Member  Joined: 15/12/2020 Location: United StatesPosts: 178 |
I don't have an answer, but I did kill a tool battery charger and TV by using an MSW inverter. |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
I have become that way myself now, and I no longer do repairs, it only leads to trouble. Someone buys a $199 piece of Chinese electronics that craps out, and they expect if it only cost that much in the first place, it should be fixable for no more than $20. So I spend two days chasing down some elusive fault, sourcing a part, and make it live again. Then months later, it craps out again. And its all my fault. Not even the importers or local agents try to do warranty repairs these days, its cheaper to just hand the customer another brand new unit. Cheers, Tony. |
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| Davo99 Guru Joined: 03/06/2019 Location: AustraliaPosts: 1577 |
I can well see the problem. It's the same with cars. People buy some POS KNOWING it has a problem then complain when they go to buy the expensive/ hard to get part or the pile of parts the car needs, that " The car only cost $xxx. " Heard that 1000 Times. If they have owned it a while, The predictable line is " It's a good Car". Yeah, Really?, Tell me what's good about it exactly... apart from the fact there is air in the bald Tyres? If I want to dig the knife in, I stand back, looking at the thing from end to end in deep contemplation then look at the owner and say with as much innocence as I can muster, " Hard to believe this was new once isn't it?" 50% of them won't get it and go on to tell you how much they paid for it and it's life history as it anyone else would give a damn. Like wise, they have some irrational conception that because the vehicle is a worthless POS, The labour should either take a quarter as long or be a 1/4 the normal rate because they bought some garbage that isn't worth having. |
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| CaptainBoing Guru Joined: 07/09/2016 Location: United KingdomPosts: 1985 |
I gave up the repair game years back... my Rubicon moment was a blazing row I had with a customer (which also resulted in me leaving that gig). Spent ages chasing through a lightening strike to a very nice expensive cordless phone (Panasonic, hey it was 1995). Fixed. When the customer came to pick up the phone, he got the repair docket and a little placcy bag of the bits I had replaced (usual). He then queried the bill claiming "I know something about electronics and those components only cost a few pennies each". I'd had a bad day at the hands of an inept colleague. The customer was one of those annoying educated floppy oxygen-thief types who'd get a sparkie in to change a light bulb. I just flipped, I asked the customer why he felt entitled to belittle my skills and decide my time was worth nothing and by the way he now had a perfectly working device that was beautifully cleaned (was filthy when it came in) and all for a fraction of the cost of a replacement. Then informed said colleague that he was her customer and walked out. Some might say I was a bit of a drama queen, but I had a years of this sort of customer-is-always-right BS and justifying what I charged for my time (which was less than a plumber would for the same). Wow, did it feel good to heap the sum of my frustrations on that entitled pr*ck as the millstone lifted from my shoulders and I walked to my car. Wife was not so keen when I arrived home unemployed but I was back working a week later doing controller design work for a little outfit - that, I enjoyed and no customers to talk to and a bit more money - bonus! I keep schtum now-a-days. Edited 2021-07-19 19:01 by CaptainBoing |
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| noneyabussiness Guru Joined: 31/07/2017 Location: AustraliaPosts: 506 |
Oh heck yeah, I've had to scold the wife a few times about bragging I know a little ( enough to be dangerous as they say) about electronics... cause you always get that 1 that will ask " I have this piece of crap 30 dollar stereo thats 10 years old and treated like crap, but its not miraculously still working, can you fix it?? " oh for free of course... Its sad , but as you said warp... only leads to trouble ... I think it works !! |
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| Davo99 Guru Joined: 03/06/2019 Location: AustraliaPosts: 1577 |
A one off incident can seem like an over reaction when taken out of Context. In the big picture of repeated and ongoing frustration and having to deal with idiots, the reality is different. I have learned NOT to be patient with idiots and nip stupidity in the bud because I also learned a LONG time ago there are customers you are far better off without. Trying to be nice and have patience with them only results in more frustration in them taking more Liberties. I was doing Underwater photos of kids at a learn to Swim school some years back in a very well heeled area. A conceited " look at me" type woman came up and scoffed at the price of the photos which were 5 images on a CD for $30. She looked at the price board said You must be making a killing, and pointed out loudly in front of the waiting people that a CD only cost $1! I excitedly said, "Oh no, I buy them by the 1000 and pay much less than that" while I reached behind me and Picked up the camera's with the underwater housing and flash etc and dropped them loudly on the plastic table and said " But that lot alone is worth over $20K and just a part of the gear I use over all". I reminded her that she was under no obligation to buy anything if she couldn't afford the pictures, totally up to her if she wanted them or not. The laughter from the crowd behind took the smug look off her face. I'm glad it worked out for you in the end. I have done similar things myself and thought later I should not have done that only to fall on my feet and have the only regret of not blowing a fuse sooner. :0) I coined a phrase way back which I have said to people many times: " I don't have to work hard to earn nothing, I can sit on my fat arse in front of the TV with my feet up and earn the exact same amount". Dealing with the public is one of the most frustrating things an intelligent person can ever have to endure in my book. |
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| wiseguy Guru Joined: 21/06/2018 Location: AustraliaPosts: 990 |
Thanks for the stories they all ring a very familiar chord with my experience. One annoying neighbor came into my garage carrying a VCR under his arm and said Mike I am wondering if you could help me out. I looked him in the eye and said certainly, which way did you come in ? From the scowl he didnt appreciate my caustic humour - I had seen it in a book about 8 years previously and just waited for some one to use that exact phrase, today was the day ! If at first you dont succeed, I suggest you avoid sky diving.... Cheers Mike |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
I guess its life experience eventually turns many of us into grumpy old buggers hahaha. Cheers, Tony. |
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| Volhout Guru Joined: 05/03/2018 Location: NetherlandsPosts: 3496 |
Don't use a MS convertor for anything other than glow bulbs. You are asking for trouble. Technically all DSO's have switch mode power supplies. These are capable of running from AC, and even DC (although not advertised). Typically there is an inrush current limiter in the DSO power supply to meet the agency requirements. This works on a power-on basis. After the DSO has started , the inrush limiter is switched off (minimize dissipation). After the limiter is switched off the DSO has a low impedance for (fast) transients. If you apply a sine wave, the transients are controlled (sine wave has controlled dv/dt). If you apply a DC voltage, there are no transients anymore, only maybe some ripple. But if you apply a 50Hz/60Hz square wave of modified sine wave, transients are huge. This will result in a/ high stress on the components in the DSO (lifetime impact) b/ high stress in the modified sinewave convertor (lifetime impact / shutdown) The DSO may likely only consume few hundred watts max. A sine convertor for few 100 watts is not really expensive. You could use the trick proposed above with an inductor in series, but make sure you have MOV or TVS diodes on both sides to protect both convertor as well as DSO from the spikes. The DSO may already have protection, when it meets some agency standard, but I am not sure about the output of the modified sine wave convertor. PicomiteVGA PETSCII ROBOTS |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
That is a great insight ! Thank you. Cheers, Tony. |
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