Running an O Scope on Inverter

[MrAl]

Hi,

Anyone here ever do this? This is like a 12vdc to 120vac inverter to run an oscilloscope.

As you now, the inverters dont put out a sine wave but a square wave instead. I've never tried to run the scope on the inverter.

[Added Note Later]
Just to reduce confusion, the kind of inverter i am referring to here is what is normally called a modified sine wave converter which is really a square wave with plus and minus excursions each half cycle being 'on' for 80 percent of the time and 'off' for 20 percent of the time, and plus or minus 145v when on. This is NOT the same as a square wave inverter that puts out a regular plus and minus square wave with no 'off' time.

 

[audioguru]

CHEEEPO old inverters produce a square-wave. GOOD modern inverters produce a voltage regulated sinewave.

 

[MrAl]

Hello audioguru,


Well thanks for pointing that out. But what i want to use is a cheeepo (he he) because that's all i have at the moment and i would like to run it from 12v so i can use it on the car or other places distant from home without buying anything else.

 

[ccurtis]

Can't say that I've tried it, but if you have a Variac, or isolation transformer, it will smooth things out.

 

[audioguru]

A heater or incandescent light bulb works from a squarewave inverter. But the peak voltage of a squarewave inverter is 0.707 times lower than the peak voltage from a sine-wave inverter.

Electronic products rely on the high peak voltage of a sine-wave from the mains so your oscoilloscope might not work or it will not be accurate.

 

[Mr RB]

What type and age of scope? New digital ones will use regulated offline SMPS supplies and should be fine with most input voltages and frequencies.

Old analogue scopes likely have a mains transformer and may have issues with a squarewave inverter, but then again might work ok.

 

[MrAl]

A heater or incandescent light bulb works from a squarewave inverter. But the peak voltage of a squarewave inverter is 0.707 times lower than the peak voltage from a sine-wave inverter.

Electronic products rely on the high peak voltage of a sine-wave from the mains so your oscoilloscope might not work or it will not be accurate.

Hi there audioguru,


Well actually the peak of a pure sine wave inverter (or just the line voltage) is 170v, and 0.707 of that is 120v (the nominal AC voltage, but the 'modern' square wave inverters have a peak that is higher than 120v, it's really the mid point of the peak and the nominal, which is (170+120)/2=145 volts. So the peak of my inverter is 145 volts and the duty cycle is something like 0.80

So the scope would see a 145v square wave with around 80 percent duty cycle.

 

[MrAl]

What type and age of scope? New digital ones will use regulated offline SMPS supplies and should be fine with most input voltages and frequencies.

Old analogue scopes likely have a mains transformer and may have issues with a squarewave inverter, but then again might work ok.

Hi MrRB,

Yeah this one is a fairly old model. Im thinking now that it has a transformer inside to generate the required voltages. Probably rectified and regulated power supplies inside. So i guess the main issue would be the transformer seeing the 145v peak 80 percent duty cycle square wave. I guess the harmonics could overheat the transformer, but maybe if it was run for only a short time just to get a few readings then shut off.

That's one thing i dont like about 'modern' cheep inverters is that you never know what is going to work and what isnt until you try it. I hate that

I've also read on the web that no one is ever sure until they try it too, or someone else tried it and told them.

I guess i could check the power requirement too and see if i could develop a small filter for the output of the inverter. That might not be too bad of an idea as this would then work with other small questionable devices. Inductor probably has to be somewhat largish though and a load always tied to the output...geeze probably be worse off then without it

 

[audioguru]

Well actually the peak of a pure sine wave inverter (or just the line voltage) is 170v

correct.

0.707 of that is 120v (the nominal AC voltage0

Correct.

but the 'modern' square wave inverters have a peak that is higher than 120v, it's really the mid point of the peak and the nominal, which is (170+120)/2=145 volts. So the peak of my inverter is 145 volts and the duty cycle is something like 0.80

WRONG!
The peak of the squarewave must be 120V or heaters and incandescent light bulbs and maybe motors will burn out.
Maybe you are thinking about a "modified sinewave" (actually a modified squarewave) inverter.

 

[Mr RB]

Hi A.G. yes the "squarewave" inverters are almost always a "modified squarewave" that has a pause between power pulses. Making it hard to calculate, because you don't know how many degrees the pause is and it can vary from product to product.

To MrAl; I would just try it and see. The makers of the inverter have surely considered that a lot of users will run smallish mains transformers from their inverter, so it will probably be close enough to spec to work fine without harm.

And you could always do a heat test comparison, ie testing for X minutes/hours with each power source and checking how hot the scope gets.

 

[audioguru]

I think dimmers and motor speed controls do not work properly when fed from a "modified sinewave" inverter.
A product with a power transformer might make a loud BUZZZZ when fed from a "modified sinewave" inverter.

 

[MrAl]

Hi A.G. yes the "squarewave" inverters are almost always a "modified squarewave" that has a pause between power pulses. Making it hard to calculate, because you don't know how many degrees the pause is and it can vary from product to product.

To MrAl; I would just try it and see. The makers of the inverter have surely considered that a lot of users will run smallish mains transformers from their inverter, so it will probably be close enough to spec to work fine without harm.

And you could always do a heat test comparison, ie testing for X minutes/hours with each power source and checking how hot the scope gets.

Hi again MrRB,

Yes, and how that you mention it, i've used it with small wall warts and it worked ok. Little voltage difference on the output, that's all.

The product i have does indeed perform the way i specified, 145v peak with 80 percent duty cycle, and that's verified with scope pictures.

 

[MrAl]

correct.


Correct.


WRONG!
The peak of the squarewave must be 120V or heaters and incandescent light bulbs and maybe motors will burn out.
Maybe you are thinking about a "modified sinewave" (actually a modified squarewave) inverter.

Hello again,

The peak is 145v and the duty cycle is 80 percent. That's a modified square wave which i referred to as a 'modern square wave inverter output'. They also call that a modified sine wave. So yes i am aware that a 50 percent duty square wave (plus and minus voltages) can not have a 145v peak, but a modified square wave (or modified sine as they call it) does have a 145v peak with 80 percent duty cycle (80 percent duty cycle here refers to one have of a cycle, which would be 40 percent on, 10 percent off, then 40 percent on negative, then another 10 percent off).

Also, as verified by scope pictures, my product does 145v at 80 percent duty cycle as i mentioned previously, which again that 80 percent refers to 40 plus,10 off,40 minus,10 off to make up the whole cycle.

 

[audioguru]

In your FIRST post you mentioned a square-wave inverter, not a modified one. There is a HUGE difference.

 

[MrAl]

In your FIRST post you mentioned a square-wave inverter, not a modified one. There is a HUGE difference.

Hi again ag,

Sorry if i confused you. I figured when we hear the word 'inverter' with the phrase 'dont put out a sine wave but a square wave instead', that anyone would put two and two together and realize that it was a modified sine wave inverter that is so, so common in today's market place.

But anyway, now you know for sure what i meant

 

[audioguru]

I have never used an inverter because I never needed one.
Inverters sold in the stores near me do not say what is their output waveform. They might be old-fashioned square-wave or newer modified sine-wave.

Most inverter projects on the internet are old fashioned square-wave.

 

[MrAl]

Hello again audioguru,


Ok, well i apologize if i misled you with my less than overly detailed original post. Perhaps i'll modify it now. Thanks for bringing this up as others might see it the same way you did.