Oscilloscope measure directly AC power - Page 1
Forum section: electronics / beginners

author: salviador
author_rank: Regular Contributor
timestamp: March 15, 2014, 02:22:20 pm
content: Hi,Hi have a analog oscilloscope Not isolated and I have a DUT Not isolated.I want measure directly AC but I have 2 problem:1-ground earth loop2-Hight voltageI want buy this product: B&K Precision PR-60http://it.mouser.com/ProductDetail/BK-Precision/PR60/?qs=sGAEpiMZZMs4AFwHyzhTm5WY9tLur3EIcqiGm063PbU%3dMy question is, with special probe (PR-60) connect in the oscilloscope (x100) attenuation, Can I measure directly AC power without isolation transformer?

author: johnk
author_rank: Contributor
timestamp: March 15, 2014, 02:51:31 pm
content: Yes you could, but before you do so you should really know what you're doing. Also Dave made a video exactly about this topic. ("How not to blow up your Oscilloscope")I don't know if its the best tool for the job because it's just rated at Cat II.But don't trust me alone - I have no experience with probing mains.

author: electronics man
author_rank: Frequent Contributor
timestamp: March 15, 2014, 03:00:41 pm
content: Quote from: salviador on March 15, 2014, 02:22:20 pmwhy do you need too, wont a good multimeter do?

author: facumedica
author_rank: Contributor
timestamp: March 15, 2014, 03:11:06 pm
content: Check this video that Dave made:Chao!

author: salviador
author_rank: Regular Contributor
timestamp: March 15, 2014, 03:16:42 pm
content: Quote from: electronics man on March 15, 2014, 03:00:41 pmExample use when I test Switching circuit, if measure voltage and frequency of drain mosfet in flyback circuit.Or in triac AC , or in inverter for high power brushless motor ecc ecc.

author: BravoV
author_rank: Super Contributor
timestamp: March 15, 2014, 03:24:14 pm
content: Quote from: salviador on March 15, 2014, 03:16:42 pmIf that is the case, I experienced similar needs in the past, safer options are using hand held battery powered & fully isolated scope or differential probe, yeah, sadly none of my suggestions are cheap.

author: electronics man
author_rank: Frequent Contributor
timestamp: March 15, 2014, 03:33:03 pm
content: why dont you isolate your DUT with an isolation transformer.

author: Rigby
author_rank: Super Contributor
timestamp: March 15, 2014, 03:44:53 pm
content: use two probes instead of one. don't use the ground lead on either. display the difference between ch1 and ch2 using math.

author: johnk
author_rank: Contributor
timestamp: March 15, 2014, 04:43:51 pm
content: @Rigby: An oscilloscope can not measure el. potentials. I think there would be no useful readings if you leave the ground leads unconnected.

author: Nerull
author_rank: Frequent Contributor
timestamp: March 15, 2014, 04:49:46 pm
content: An oscilloscope always measures potentials, that's how they work. They measure a voltage, relative to ground, over time.Not using a ground lead means that the ground path to the DUT goes through the electrical outlet instead of the ground lead, and makes the probe much more likely to pick up noise, but for some signals that's not a big problem. You would never use that for high speed data signals, or tuning analog circuits, but if all you want to do is plot high voltage AC its not a problem, and so long as you respect your scopes voltage ratings there is no risk of blowing something up by connecting a ground lead to the wrong spot.

author: BravoV
author_rank: Super Contributor
timestamp: March 15, 2014, 05:17:03 pm
content: This below video is much better, a basic explanation for differential measurements using scope.Using dual probes with both ground clips connected has weakness on CMRR. Again, as I mentioned before, to have serious measurement like this, a differential probe is much better, but the best still to use an isolated handheld scope especially working with high voltage/energy circuit.I own a 1KVA isolation transformer, and it is not an ideal solution and also error prone which is dangerous if working with mains rated voltage circuit. I ended up buying a used Tek handheld scope which has a total isolation between channels, below is the design illustration on how is the isolation in the Tek handheld scope.

author: johnk
author_rank: Contributor
timestamp: March 15, 2014, 05:26:40 pm
content: Quote from: Nerull on March 15, 2014, 04:49:46 pmI think not. Oscilloscopes measure potential differences / voltages. Also they don't measure a voltage relative to ground (voltages don't have a reference point), but a potential relative to ground (or whatever the "ground"-lead is connected to).But you're right. Your video proofs it (neat bit of information btw.). I think this works because the two ground-leads of the probes are connected internally and the two potentials at the probe-tips stay in relation to each other through that - I haven't thought about that before.

author: Nerull
author_rank: Frequent Contributor
timestamp: March 15, 2014, 05:56:10 pm
content: Erm, voltage and potential are the same thing. Voltage is a unit used to measure electrical potential.In almost all cases - battery powered or isolated scopes being the exception - the ground clips on a scope are directly connected to earth ground inside the scope, and to each other. The ground lead on a non-isolated scope does not function like a multimeter, where it defines one of the reference points used for measurements, but to provide a direct, low impedance connection between the reference ground in the scope and a ground near the signal being measured. For high frequency signals this is very important, but for 60Hz mains it doesn't matter much at all. Without a clip the scope, and its ground reference, are still grounded and if the DUT is also grounded than the scope probe will still be able to measure any voltages in the DUT. In the video I posted, had he been using a ground-referenced power supply rather than a 9v battery, he could have measured its voltage with only one probe, without a ground clip, because both the power supply and scope are grounded already.That is why you have to worry about where you connect ground clips in the first place.

author: The Electrician
author_rank: Frequent Contributor
timestamp: March 15, 2014, 05:58:11 pm
content: Quote from: salviador on March 15, 2014, 02:22:20 pmYou have another problem. Most analog oscilloscopes can't do extensive trace math. To measure power, you must sample a voltage with one channel and current with another channel, and multiply the two with trace math. Digital oscilloscopes can usually do this, but not analog scopes. Typically, analog scopes can add traces, or subtract by inverting one channel, but not multiply.Also, the current measurement must be isolated, so you need two differential probes, not just one, or the current measurement could be done with a clamp on probe, and that gives you isolation for the current.See this thread where I made some power measurements using a Tek scope with 4 isolated channels built in to the scope:https://www.eevblog.com/forum/projects/led-light-bulb-meaurements/msg207417/#msg207417

author: Nerull
author_rank: Frequent Contributor
timestamp: March 15, 2014, 06:01:17 pm
content: Given the wording of his post, I suspect 'power' is a language issue. His later posts states he's after voltage and frequency measurements, not voltage*current.

author: The Electrician
author_rank: Frequent Contributor
timestamp: March 15, 2014, 06:06:48 pm
content: What say you, Salviador, do you really want to measure power, involving multiplication of voltage and current?

author: johnk
author_rank: Contributor
timestamp: March 15, 2014, 06:13:01 pm
content: Quote from: Nerull on March 15, 2014, 05:56:10 pmhttp://en.wikipedia.org/wiki/VoltagePS: voltage = electrical potential difference != potential.Sorry for the off-topic discussion.

author: Nerull
author_rank: Frequent Contributor
timestamp: March 15, 2014, 06:52:02 pm
content: No one was talking about anything other than potential differences, as pure potential is completely irrelevant.Scopes are grounded. They measure ground-relative voltage. They can take ground-relative voltage measurements using the probes without a ground clip, so long as you aren't too worried about noise. Where does anything other than a potential difference come into play?

author: BravoV
author_rank: Super Contributor
timestamp: March 15, 2014, 06:54:08 pm
content: Quote from: The Electrician on March 15, 2014, 06:06:48 pmQuote from: salviador on March 15, 2014, 03:16:42 pmRequoting the OP, I believe he is going to measure various high voltage spots mixed with low voltage ones like mosfet's pins, D or S on high voltage, G for low voltage, or switching controller, triac triggering, flyback input and output at the transformer and etc.If that is the case, pretty sure he will need a fully and "proper" isolated scope + fully isolated high voltage probes. No cheap solutions I'm afraid.

author: salviador
author_rank: Regular Contributor
timestamp: March 16, 2014, 10:59:43 am
content: Quote from: The Electrician on March 15, 2014, 06:06:48 pmYes is an error of language, ... but Your post has been very useful, I like to understand how to measure all in high voltage (voltage, current, frequency, power)

author: artserg888
author_rank: Newbie
timestamp: January 20, 2016, 01:48:16 pm
content: What is max input voltage for Hantek DSO1152S?I have some doubts because on the body of oscilloscope is written 400V,in the manual:- CAT III 150V (1x)- CAT I and CAT II 300V (10x).

author: Kokin
author_rank: Newbie
timestamp: March 04, 2016, 06:33:12 pm
content: Hi everyone,I know that I double posts of measuring mains AC, but to be clear I'd like to summarize all options.First of all, my equipment: RIGOL 1054Z. standard probes CATII 300VMy main AC: 230V 50Hz (at least should be), and at home I have system of two wires L + PEN (TN-C), PEN wire is split in my basement to PE (go to ground), N (go to main grid).And sockets in walls have three pins plug (see picture below) where PEN wire is connected to right hole and ground pin.Reason of measuring AC:- Measuring my mains grid AC - I have some trouble with my grid (when I'm using computer or 3D printer lights blinking).- Another reason is to measure how triacs work - I have now some mini project with four triacs which control 1kW heaters each and sometimes, after few hours control unit stops and on LCD I see different signs (like Chinease signs), I control them with different time on/off, longer then 2sek ON (no phase control).=>For this reasons and for me own safety I'd like to know all posibilites to measure such things.For now:- I watched Dave EEVblog yt movie "How not to blow up your osciloscope"- "Oscilloscope operation differential voltage measurement"- and many other movies about measuring voltage.Options:a) I can buy new probes with higher voltage resistant => CAT II 600V DC + peak AC (230V * square root (2) = 325V, much lower than permisible probes max voltage). and not use ground probes wire+ cheap - 15-25$/probe,- not grounded oscilloscope- I can only measure difference between point and ground.b) Differential measuring - use two channels and plug ground wires of probes together.- I need new probes as in point 1. but cheap- not grounded oscilloscope- I can measure difference between two point - as like between transistor or triac - drop voltage.c) I can use isolation transformer.+ I can measure any point of my circuit -- more expensive then new probes CAT II 600V.- when I want measure my main AC there is not a problem - I can buy smole power transformer. Bigger probles is when I want to power up my heater in my project where I have over 4kW of power - it is too expensive for me now.d) insolated differential probes or new oscilloscope with insolated channels- more expensive then insolated transformer but too expensive for me now+ most safety solutionQuestions:1. When I use isolation transformer (1:1) and I have some speakers in input AC, the waveform will be different at input and output? Output waveform will be more smooth?2. In option C) (isolation transformer), can I plug ground wire of probes to one any wire of transformer output - differential measure? When I've insolation transformer ground or positive wire of probes can be pluged into any points of circuit?I would like to start from buying new probes (1:10 CATII 600V or higher) and test my mains AC directly - no isolation transformer and without pluged ground wire of probes to anything.Then set up differential measurement with my RIGOL 1054Z and two new probes and test triacs.Could you tell me is in this connetion some problem?? (I know that this is on my own risk of injury my self, but I want to avoid to burn out something - oscilloscope or some fuses at home).

author: tggzzz
author_rank: Super Contributor
timestamp: March 04, 2016, 07:41:08 pm
content: Read professional information about how to do this safely.Do not rely on non-professional statements made here - they may be subtly wrong or you may misinterpret them.Seehttp://circuitslab.case.edu/manuals/Probe_Fundamentals-_Tektronix.pdfhttp://www.tek.com/document/primer/abcs-probeshttp://info.tek.com/www-xyzs-of-oscilloscopes-primer.htmlhttp://in.tek.com/dl/3AW_19134_2_MR_Letter.pdfhttp://www.tek.com/dl/51W_10640_1.pdfFor an overview of why those are essential reading, and for more information, seehttps://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/

author: tautech
author_rank: Super Contributor
timestamp: March 04, 2016, 07:59:46 pm
content: The first but not the last cost of entering the scope world is the scope, then one must acquire the right accessories to complete the tasks ahead.IMHO option D, Differential probes will serve you best for this task and also offer isolation for tasks in the future.