Checking my Inverter with an Oscilloscope (1 Viewer)

[DBK]

In this thread I described fitting a budget inverter:

Fitting a 1500W Inverter

In this thread I asked for comments on a 1500W pure sign inverter. As the comments were favourable I went ahead and ordered one. :) https://www.motorhomefun.co.uk/forum/threads/thoughts-on-this-inverter.155627/ When it arrived I tried it for size in my chosen spot in the PVC. This is very...

www.motorhomefun.co.uk

The inverter works fine and hasn't destroyed anything plugged into it so far. But I was never sure if it really did produce the "pure sine wave" it claimed.

Until today.

I recently bought a small LCD oscilloscope to check the output of a radio I had built. Sadly, it couldn't reliably measure RF frequencies, I think there is an intermittent fault in the probe, but it will measure lower frequencies so I tried it on the inverter in the 'van.

It is saying the peak voltage is 344 which if it is a sine wave works out at 243 volts rms - a little high but I dont know how accurate it is.

But the main point is the waveform looks smooth which is reassuring and I needed be worried about what I plug into it.

I'm returning the 'scope and probably won' t buy another as I've got my radio working now - but it was useful for checking the inverter.

 

[TheBig1]

Peak voltage will always be more the the RMS or average ac voltage

 

[Lenny HB]

Not bad for a cheap inverter better than I would have expected but still not a pure sine assuming the jagged edges on the waveform are not the scope.

I have a similar inverter I have been threatening for a few years to drag my scope out of the loft to check it, might just do that now.

Although mine powers everything I use on it OK, e-bike charger, washing machine and occasionally use it when I need a fast charge on my phone and of course for the electric kettle when on the Chunnel.

However if I connect the laptop charger to it, the mouse pad goes haywire but its OK with a USB mouse plugged in. And the boss's Crapple pad goes all jumpy, not a problem as both normally powered by 12v chargers.

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[DBK]

Not bad for a cheap inverter better than I would have expected but still not a pure sine assuming the jagged edges on the waveform are not the scope.

The jagged line is just the inverter - it looked the same on the mains.

 

[kevenh]

The jagged line is just the inverter - it looked the same on the mains.

That looks like the trace from a digital scope rather than an analogue one.
Did you mean "The jagged line is just the 'scope - it looked the same on the mains."

 

[Richard n Ann]

Try it under load. It was Lenny's suggestion when I was talking about my sine wave. Not got round to it yet.

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[Lenny HB]

The jagged line is just the inverter - it looked the same on the mains.

You mean the scope.
Before you return the scope worth seeing what the waveform is like under load.

Edit:
Richard and Ann

 

[DBK]

You mean the scope.
Before you return the scope worth seeing what the waveform is like under load.

Edit:
Richard and Ann

Good point, i did plan to do that - but forgot this morning. Ill have another go tomorrow.

 

[DBK]

That looks like the trace from a digital scope rather than an analogue one.
Did you mean "The jagged line is just the 'scope - it looked the same on the mains."

Yes, my typo, it is just how the display looks.

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[Gromett]

Good point, i did plan to do that - but forgot this morning. Ill have another go tomorrow.

Just came to suggest the same. Seen many youtube videos of tests like this where they produce a perfect sine until even a sniff of work is applied. The more work the worse they got on the budget ones due to under spec FETs and really crappy inductors some with only a passing resemblance to a core in them.

 

[andy63]

Hi John.. im just looking at the settings in your photo above..
Does the AC refere to the channel coupling and if so why..I thought it was more normal to use dc coupling unless you wanted to get rid of any dc component in the trace..
I have a little digital 2 channel scope and can't recall ever using ac coupling ..hence the question..
Andy

 

[DBK]

Hi John.. im just looking at the settings in your photo above..
Does the AC refere to the channel coupling and if so why..I thought it was more normal to use dc coupling unless you wanted to get rid of any dc component in the trace..
I have a little digital 2 channel scope and can't recall ever using ac coupling ..hence the question..
Andy

Another good point. I was just relieved it showed something on the display.

I only got it to display RF signals once and couldn't get it it to again so when it showed the 240 volt signal I was reasonably chuffed. I can't remember the details of button pushing performed now but I'll give it another go tomorrow.

This will involve playing with an uncovered main plug so if you don't hear from me again you can draw your own conclusions.

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[kevenh]

Hi John.. im just looking at the settings in your photo above..
Does the AC refere to the channel coupling and if so why..I thought it was more normal to use dc coupling unless you wanted to get rid of any dc component in the trace..
I have a little digital 2 channel scope and can't recall ever using ac coupling ..hence the question..
Andy

For one thing, AC Coupling removes any DC offset so for looking at an AC voltage signal {a DC component} wouldn't help.
I'd have to rattle brain cells for more - prob. helps the 'scope trigger better too :?

edit: for clarity

 

[kevenh]

Phew. Didn't write anything too out there: Difference Between AC & DC Coupling Oscilloscopes

edit: to cure a linked site oddity

 

[DBK]

Phew. Didn't write anything too out there: Difference Between AC & DC Coupling Oscilloscopes

edit: to cure a linked site oddity

Thank you, that link is a really useful explanation. I guess it all boils down to the recognition you won't find an oscilloscope much use for DC. By definition they are for AC but some AC's are applied to a DC bias at some stage in a circuit.

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[kevenh]

Thank you, that link is a really useful explanation. I guess it all boils down to the recognition you won't find an oscilloscope much use for DC. By definition they are for AC but some AC's are applied to a DC bias at some stage in a circuit.

Depends on your trade.
As an electronics engineer looking at mostly TTL circuit components so I was DC coupled >90% of the time.

 

[Gromett]

you won't find an oscilloscope much use for DC

You haven't done much digital electronics then

 

[Lenny HB]

Even with an AC signal its useful for checking DC offset, particularly when working with audio.

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[andy63]

This will involve playing with an uncovered main plug so if you don't hear from me again you can draw your own conclusions.

I'm going to dig out my little oscilloscope or rather its instructions..I know you are using the 10x attenuation but im still not sure it will be suitable for mains connection..
Its just I seem to remember reading in my instructions not to use it on 240v mains even though the max voltage it quoted on a channel was well in excess of 240..
I never did look into why..I just followed the instructions..
So if you care to wait before you attempt electrocution I'll have a check and get back..

I've just remembered a use for ac coupling..
Alternator output on a vehicle..it shows a ripple and hence the integrity of the rectify pack diodes..
Andy..

 

[Jake Royd]

This post caused me to idly check eBay.
That sell little LCD scopes from about £15 plus.
Been decades since I used one.

 

[Sandancers]

I'm going to dig out my little oscilloscope or rather its instructions..I know you are using the 10x attenuation but im still not sure it will be suitable for mains connection..
Its just I seem to remember reading in my instructions not to use it on 240v mains even though the max voltage it quoted on a channel was well in excess of 240..
I never did look into why..I just followed the instructions..
So if you care to wait before you attempt electrocution I'll have a check and get back..

I've just remembered a use for ac coupling..
Alternator output on a vehicle..it shows a ripple and hence the integrity of the rectify pack diodes..
Andy..

Probably as the probes won't be safe to use on 240v. If you think of the probes used on a Martindale tester they have finger guards etc. Whereas the scope probes probably don't.

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[andy63]

Probably as the probes won't be safe to use on 240v. If you think of the probes used on a Martindale tester they have finger guards etc. Whereas the scope probes probably don't.

It could be but but the probes supplied have finger guards and a shielded hook and a x10 attenuation setting so in all those respects I reckon they are fine for 240v..
I'm only guessing but I think it will be more to do with grounding and the possibility of damage from faults there...

 

[kevenh]

Depends on your trade.
As an electronics engineer looking at mostly TTL circuit components so I was DC coupled >90% of the time.

Quoting myself to add that even in the 10% of the time I was troubleshooting analogue circuits I’d be DC coupled as a) these analogue signals would usually have 0v as a reference point or b) as Lenny HB mentioned if the signal did have an offset to 0v that would be filtered out by using AC coupling.
Offsets were often a fault condition in TV signals so best not filtered out

 

[Lenny HB]

Its just I seem to remember reading in my instructions not to use it on 240v mains even though the max voltage it quoted on a channel was well in excess of 240..

When checking mains we used to disconnect the earth from the scope.
Not a problem with an inverter output as it is isolated.

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[andy63]

I've checked John.. the reference to not connecting to mains was in the instructions for an automotive scope and not the little handheld digital one so I'm not really sure now.. as the automotive scope is a pc based... that may make some difference..just don't know..
I'm sure you will be fine as you have already done it..
Andy

 

[DBK]

I've re-run the test with the inverter under a 1200W load and apart from the voltage dropping a bit to around 231 volts RMS the waveform looks OK to me.

 

[SIFTA SAM]

I've re-run the test with the inverter under a 1200W load and apart from the voltage dropping a bit to around 231 volts RMS the waveform looks OK to me.

View attachment 451776

Why not do a comparison check against the 'mains' 230v?

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[autorouter]

Does this digital scope have a spectrum analyser function? Once the signal is digitised, it's 'only' a software function. You could look at the value of the second, third etc harmonics (100Hz, 150Hz etc) and compare it to the mains waveform.

 

[autorouter]

This will involve playing with an uncovered main plug so if you don't hear from me again you can draw your own conclusions.

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Does this digital scope have a spectrum analyser function? Once the signal is digitised, it's 'only' a software function. You could look at the value of the second, third etc harmonics (100Hz, 150Hz etc) and compare it to the mains waveform.

 

[DBK]

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Does this digital scope have a spectrum analyser function? Once the signal is digitised, it's 'only' a software function. You could look at the value of the second, third etc harmonics (100Hz, 150Hz etc) and compare it to the mains waveform.

No, sadly it doesn't.

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