OWON HDS242 Line Voltage Measurement, Or Not

[MrAl]

Hello there,

Yesterday i tried to measure the line voltage coming into the house with the said scope.  10x probe of course.  I got a strange reading.
It's not a sine wave, and the voltage is lower than in most places but it's always been like that, so that part of it is not the fault of the scope.
However, it measures 150v max and -144v min, which cant be right.  That would mean a net DC current flows in every thing we plug in that has a transformer in it as the DC current ratchets up when there is a non symmetrical wave like that feeding it.

So any ideas what would cause this, or it is really the scope?

I've seen some other reviews on some much more expensive ($1000+) OWON scopes and was quite disappointed.  I am starting to think maybe this is a company that i should stay away from.

[electr_peter]

Hi,

It's not a sine wave, and the voltage is lower than in most places but it's always been like that, so that part of it is not the fault of the scope.

Mains has a sine (-ish) waveform shape with some distortion, this is expected.

However, it measures 150v max and -144v min, which cant be right.  That would mean a net DC current flows in every thing we plug in that has a transformer in it as the DC current ratchets up when there is a non symmetrical wave like that feeding it.

Think about measurement setup and possible errors involved. Assuming you are using 50V/div setting (5V/div 1x input + 10x probe attenuation) with 8 vertical divisions and 8 bit ADC, vertical sensitivity is 400V/256 = 1.5625V. Error is in the order of 4 LSB. On top of that, scope has some inaccuracies in scale and offset, this easily explains possible error. Also, AVG waveform value should be better criteria than (MAX + MIN)/2.
IOW: scope is not well suited to accurately measure small DC offset on large AC waveform. HDS242 has built-in DMM, you can use that in DC mode instead.

[MrAl]

Hi,

It's not a sine wave, and the voltage is lower than in most places but it's always been like that, so that part of it is not the fault of the scope.

Mains has a sine (-ish) waveform shape with some distortion, this is expected.

However, it measures 150v max and -144v min, which cant be right.  That would mean a net DC current flows in every thing we plug in that has a transformer in it as the DC current ratchets up when there is a non symmetrical wave like that feeding it.

Think about measurement setup and possible errors involved. Assuming you are using 50V/div setting (5V/div 1x input + 10x probe attenuation) with 8 vertical divisions and 8 bit ADC, vertical sensitivity is 400V/256 = 1.5625V. Error is in the order of 4 LSB. On top of that, scope has some inaccuracies in scale and offset, this easily explains possible error. Also, AVG waveform value should be better criteria than (MAX + MIN)/2.
IOW: scope is not well suited to accurately measure small DC offset on large AC waveform. HDS242 has built-in DMM, you can use that in DC mode instead.

Ok thanks and i believe you are correct i did not think this through.
It just struck me as strange because i never saw that with a CRT scope.
Yes i know i could use the meter and get better results, but i wanted to use the scope itself.
And yes, the waveform of our line was never perfect i mentioned that and i measured it with a CRT too and it's distorted.  I thought that was strange too but then it was always like that in this neighborhood.

The other thing is i dont really need a better scope and since i payed just around $100 USD for this thing i guess i cant complain (ha ha).
I was bugged about the bandwidth being only around 24MHz but one of the most important things i had to measure with it so far required a horizontal time base setting of just 1 second per division (big chuckle) which means even a 1MHz scope would have done the job, and actually i did use a 1MHz digital scope a while back to measure the same thing and it worked ok.  The scope pics look very similar.  [EDIT] Actually that was a 1MSa/s scope with 100kHz bandwidth ha ha.

The PC software is very limited though, only allows you to upload the waveform to the computer for viewing and storage.  No real time scope display on the PC.  The portability is just great though if i get another scope i am going to make sure it is either battery operated or can be USB operated using a USB power pack used for charging phones.  Cant beat that kind of isolation from the line either.  I see a lot of scopes that run on USB power now too.

[wasedadoc]

It is not the most +ve and the most -ve values which determine the net dc current component. It is the difference in the areas under the curves of the +ve and negative half cycles. If the distortion is different on those half cycles it is still possible for the areas to be equal even though the absolute values of their peaks are not.

[Martin72]

As ever, pictures saying more than 1000 words can do...

[MrAl]

It is not the most +ve and the most -ve values which determine the net dc current component. It is the difference in the areas under the curves of the +ve and negative half cycles. If the distortion is different on those half cycles it is still possible for the areas to be equal even though the absolute values of their peaks are not.

Hi,

Yes that's very true, almost, but the waves also look similar above and below zero so i suspected something askew.  It could be just hard to tell though.  I could take some screen shots i guess but i dont think i will sweat this issue too much more.  It's a low end scope i have to live with that.  It does what i need though so it's also not the end of the world.  I didnt want to shell out more moola right now and i needed a portable scope so i grabbed that one.  It was supposed to be an overnight shipment too so i would have gotten it fast, except they messed up the shipping so i had to wait three days.

The line voltage in this area (central NJ) is the worst i have ever seen since i lived in New Jersey.  The voltage fluctuation by itself is nuts.  Sometimes as low as 80vac when it should be 120vac.  Lucky it is 90vac or above most of the time.  In cooler months it is 105vac or above, in summer it goes lower.

[MrAl]

As ever, pictures saying more than 1000 words can do...

Yes thanks, and maybe i'll take a few screen shots if i repeat this measurement.  It's always good to know your test equipment well.
As i mentioned before, the scope is absolutely isolated from the line when it runs on batteries alone so that's a big plus sometimes.

[orb]

In another topic you mentioned that your owon hds242 doesn't have autocalibration option. Which firmware version do you have?
In my hds242 there are also similar distortions in the min/max values ​​on the sine wave (transformer). Glitches decrease significantly after calibration, about 0.5h (or more) from start.

Try to repeat the measurement on a "cold" and "warmed up" scopemeter.

Here are my results (hds242, fw: 4.1.0, probe x10; AC transformer "220V -> 12V", but in my country the standard voltage is 230V).



Cold (not calibrated):


After 30 mins of use (not calibrated):


After 32 mins of use (calibrated):


After 1h of use (calibrated once again):


Then I turned off the scopemeter and did not change the settings. After nearly 3 hours I turned it on and pressed "pause" button:

[MrAl]

Thanks for the reply.

The version reported on the scope itself is:
V8.0.1

Either they fixed the drift issue or they just dont care about it anymore, but if you have any ideas i'd be happy to hear about them.
I suppose it could check it for drift by using a stable input known to stay solid over time and temperature.

I wonder though if it only happens on the higher voltage scales.  I'll have to check the lower voltage ranges with the frequency generator.

[orb]

I don't have any more ideas at the moment.

I wrote to the seller about this strange behavior of hds242, I will probably send the scopemeter back, see what the result will be. Drift is repetitive. Reminds me of your situation. Perhaps (?) the manufacturer "hid" this defect with newer firmware, but it appears in the "min/max" values. As you can see, the offset in my hds242 is 2V on a scale to 10V, which is quite a lot (the last photo, min/max: -16/20V, but should be: -18/18V). The offset is also clearly visible on the oscillogram. Even owon "reported" it with message "CH1:DC [offset symbol] 2V", although I didn't set anything like that.

[MrAl]

I don't have any more ideas at the moment.

I wrote to the seller about this strange behavior of hds242, I will probably send the scopemeter back, see what the result will be. Drift is repetitive. Reminds me of your situation. Perhaps (?) the manufacturer "hid" this defect with newer firmware, but it appears in the "min/max" values. As you can see, the offset in my hds242 is 2V on a scale to 10V, which is quite a lot (the last photo, min/max: -16/20V, but should be: -18/18V). The offset is also clearly visible on the oscillogram. Even owon "reported" it with message "CH1:DC [offset symbol] 2V", although I didn't set anything like that.

Hi,

Oh yes i still have to check mine at the lower voltages.  I dont remember seeing anything but i wasnt actually looking for that yet.
I hope it's not too bad.  Maybe later today i will be able to test it again.
Thanks for the info.

[evlo]

Did someone here tried how it works with imeter?


I do actually use it for measuring in remote locations and looking at the measurement trough BT, so having oscilloscope in the sw for data logging would be nice.