EEVblog Electronics Community Forum
Products => Test Equipment => Topic started by: fastcooler on May 10, 2015, 08:14:06 am
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Hello,
I own a Rigol DS1052E oscilloscope and I would like to know if I can safely use it to measure Sine Wave and its FFT of 230 Volts main outlet domestic utility, please.
I wish to see the harmonic distortion of 3rd and 5ft harmonics.
Should I use probes at 10X? Is it enough to protect me, the probes and the oscilloscope itself, please?
I read that I should remove the ground from the power cable of the oscilloscope. Is it true?
Thanks!
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Hi,
Don't do that, the signal ground of the input is connected internally with the scope gnd et the safety earth of the power line.
You need a separate differential probe to do it.
Always remember that the gnd is connected to the earth...
Removing the earth ground from the power cable is a bad idea.
Lho
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Three-part reply:
(1) A look into the specs in the manual reveals that the DS1000E inputs are safe up to 300 V rms in Cat II, when using the included probes in 10:1 mode. (OK for measuring appliances connected to mains, but not for permanent installations and the mains distribution.)
(2) Do NOT disconnect your scope ground! This is very dangerous!
(3) If you have to ask these questions, do NOT mess with mains voltages. Do NOT perform the measurements you were asking about.
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Thanks for your replies.
So my question is: How can I measure with my oscilloscope the 230V main power utility in my home, please?
I also own a Peaktech isolation transformer:
http://www.peaktech.de/productdetail/kategorie/trenntransformator/produkt/p-2240.html (http://www.peaktech.de/productdetail/kategorie/trenntransformator/produkt/p-2240.html)
Can I use it to be safe?
Thanks
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I also own a Peaktech isolation transformer:
http://www.peaktech.de/productdetail/kategorie/trenntransformator/produkt/p-2240.html (http://www.peaktech.de/productdetail/kategorie/trenntransformator/produkt/p-2240.html)
Can I use it to be safe?
Yes. Using that is a very good idea....
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Simply use a measurement transformer between mains and oscope input !
Cheap, tiny, and accurate (at least for a 8-bit DSO ADC :
http://www.yhdc.com/en/product/521/ (http://www.yhdc.com/en/product/521/)
http://www.ebay.co.uk/itm/TV19-Current-Type-Voltage-Transformer-0-5mA-Input-Blue-/290913310413?pt=LH_DefaultDomain_3&hash=item43bbc8d6cd (http://www.ebay.co.uk/itm/TV19-Current-Type-Voltage-Transformer-0-5mA-Input-Blue-/290913310413?pt=LH_DefaultDomain_3&hash=item43bbc8d6cd)
It is a 1000:1000 current transformer : you have to use a resistor in series with the primary, and a burden on the secondary. BW, according to datasheet : 20Hz-20KHz, and excellent linearity.
And you will be able to observe the harmonics, measure voltage.
I just built a power meter, and got a second one to build a poor man's "diff" voltage probe, for DMM and DSO
A ten grams transformer vs a 10Kg one !
This image is taken from two sources (blue vs yellow) : the TV19 vs an isolation transformer. have a look to the signals and the phase error : the traces are identical !
(https://www.eevblog.com/forum/testgear/max-voltage-for-rigol-ds1052e-and-230v-main-power-utility/?action=dlattach;attach=151244;image)
(https://www.eevblog.com/forum/testgear/max-voltage-for-rigol-ds1052e-and-230v-main-power-utility/?action=dlattach;attach=151246;image)
(https://www.eevblog.com/forum/testgear/max-voltage-for-rigol-ds1052e-and-230v-main-power-utility/?action=dlattach;attach=151248;image)
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I also own a Peaktech isolation transformer:
http://www.peaktech.de/productdetail/kategorie/trenntransformator/produkt/p-2240.html (http://www.peaktech.de/productdetail/kategorie/trenntransformator/produkt/p-2240.html)
Can I use it to be safe?
Yes. Using that is a very good idea....
I am not so sure. I assume the idea is to measure the harmonic content at the output of the isolation transformer, instead of directly at the mains outlet. Measuring at the output of an isolation transformer will be safe as long as you use a battery-powered, "floating" measurement device (and do not touch both transformer outputs at the same time with different body parts!). But as soon as you connect the probe's ground lead of your mains-powered scope to one of the transformer outputs, it is not isolated any more, and the other output will be dangerous to touch.
If you have a little "wall wart" that produces low-voltage AC output, that would provide a safe alternative. Most wall-warts include a rectifier and provide (regulated or unregulated) DC; those are not suitable. But 9V AC transformers are used for some consumer electronics, typically for deriving positive and negative supplies inside the appliance itself.
(As a side note, I am not sure how valid a harmonic measurement at any transformer's output would be. The transformer will act as a low-pass filter, but I have no good idea how much it will do to the first few harmonics.)
Are you sure you need to measure the harmonics? What for? Letting it be is the safest and easiest approach here...
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"If you have a little "wall wart" that produces low-voltage AC output, that would provide a safe alternative"
that *WILL* provide a crappy signal, and generate distorsion and harmonics : (blue trace = small power supply transformer)
(https://www.eevblog.com/forum/testgear/max-voltage-for-rigol-ds1052e-and-230v-main-power-utility/?action=dlattach;attach=151252;image)
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"If you have a little "wall wart" that produces low-voltage AC output, that would provide a safe alternative"
that *WILL* provide a crappy signal, and generate distorsion and harmonics : (blue trace = small power supply transformer)
Ah, thanks -- I suspected it would affect the signal, but it's good to see some actual data.
Your power monitor looks well built. But if the 1:1 measurement transformer is used in an improvised measurement setup, with a grounded scope on the secondary, it would still provide the same limited safety as the big isolation transformer, right? Output voltage remains 230V; the main advantage is better signal quality, due to the defined frequency characteristics.
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No ! It is perfectly safe. Your oscope is grounded, and you measure an isolated floating signal. There is no return path from oscope ground to live, neutral or ground. It is isolated and floating, there is no difference between this transformer and a PSU transformer, except there are made for current mesurements, and not for power.
You can find these transformers on industrial class ammeters (open core), and on oscope probes (better quality and high BW, far more expensive).
Read some pdf about current transformers, you will find a lot of information on the web.
You have a resistor in series with the primary, here, it has to let 5mA come in the winding for the rated input (here : 230V)
The secondary is isolated, and produces 5 mA for 5mA input (1000:1000 ratio). You use it exactly as you use any current transformer. With a 500R burden, you get 500*0.005 = 2.5Vrms, isolated from mains.
You can adjust the input resistor, and the burden in order to get what you want (but recommanded bruden is 500R)
Remember : a current transformer *MUST ALWAYS* be loaded with a burden, otherwise it will behave as a current source : infinite voltage (in theory !). It could destroy itself or bite you. For this reason, current transformer modules (Adafruit and others) are sold on a PCB with a preinstalled burden.
As long as the burden is connected, there is no risk. You get isolated low voltage. For the tests, I used a burden directly soldered on the transformer pins (current and voltage sensors) to avoid mistakes at breadboard level. The transformers (for current 1000:1 and voltage 1000:1000) were in a plastic case, away from the breadboard.
As long as the transformer is loaded, you get only very low voltage. They are disigned for this usage.
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Ah, OK -- I had missed the series and load resistors. So, when your scope screenshots show 100V/div, that's only because you have defined a probe ratio of (close to?) 100, to scale 2.5V rms to 230V rms on the scope display?
Need to find a project that gives me a reason to play with current transformers; I had not come across these yet...
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I also own a Peaktech isolation transformer:
http://www.peaktech.de/productdetail/kategorie/trenntransformator/produkt/p-2240.html (http://www.peaktech.de/productdetail/kategorie/trenntransformator/produkt/p-2240.html)
Can I use it to be safe?
Yes. Using that is a very good idea....
I am not so sure. I assume the idea is to measure the harmonic content at the output of the isolation transformer, instead of directly at the mains outlet. Measuring at the output of an isolation transformer will be safe as long as you use a battery-powered, "floating" measurement device (and do not touch both transformer outputs at the same time with different body parts!). But as soon as you connect the probe's ground lead of your mains-powered scope to one of the transformer outputs, it is not isolated any more, and the other output will be dangerous to touch.
I assumed he meant 'safe for the scope'. You can safely connect the scope probe clip to that transformer without worrying about making big sparks or blowing fuses.
You're right though, as soon as you connect the ground clip the other wire becomes one of the four horsemen of the apocalypse, raining death and destruction on all who touch it. Keep one hand in your pocket.
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I had no time to answer in details... (repairing Mss Mushroom's car : a dry solder in the screen wiper relay : a 555 and a few components in a crappy french car !)
The oscope images show a comparison between a "high" voltage (230V is called low voltage where I live), and the signal given by the TV19.
So there are two signals :
- 1 - true 230Vrms signal, comming from an isolation transformer. This isolation transformer is a high tech DIY model : two identical 150VA 230V/15V : 230->15->15->230. This way, I get 230V with a total isolation from mains and ground. This signal is connected to the oscope through a 100:1 probe (an extremely accurate probe made with two resistors : 1M and 100K + 10:1 probe). This gives a safe 230V (just avoid touching the two wires at the same time...) , and the oscope is tuned for a 100:1 probe.
- 2 - signal from the TV19 : primary series resistor = 3x 270K 0.25W resistors, giving 90K, and a 470R burden. It is connected to the isolation transforemer output (230V)
The goal was a comparison between input and output signals, and an evaluation of the distorsion using the fft function (harmonics, exactly what you want to do). I don't konw why there only odd harmonics, and I'd like to know ! Do you have the answer ?????????
I also wanted to compare the results to a commonly used voltage sensor : small mains PSU transformers. I found a sweet point with 5VA or so transformers. But the tiny TV19 gives far better results for a fraction of the price (and the weight) !
I know it is not politically correct, but if you want to see more comparisons, I recently answered such a question on Arduino forums... I'd just like not having to reload images ! But if a moderator ask me to do it, I will do it ! link : http://forum.arduino.cc/index.php?topic=310930.30 (http://forum.arduino.cc/index.php?topic=310930.30)
You will see more oscillograms, and what a transformer can do to a pseudo sine. [EDIT] the image server seems to be down :(
If you do not understand my circuit, I will draw one... Fill free to ask. But I am lazy https://www.youtube.com/watch?v=uflsjU95lRM (https://www.youtube.com/watch?v=uflsjU95lRM).