Measuring high voltage signals with a TDS 540D scope

Hi there,

I recently got a Tektronix TDS 540D scope as a gift so i could properly capture signals for all my tinkering needs.

Now i started capturing some high voltage AC signals (using a standard clamp probe, signal to signal and GND to Earth), and i came across the problem that i just can't seem to scale them down enough to fit them on the screen.

The max. i can set it to is 10V/Div, which does not suffice.

The connectors are rated at 400V / 1MOhms so it's within the specifications.

I haven't figured out all the details of using this scope yet but i think i got the gist. However i can't seem to figure this one out so I'd be glad if someone could help me along :slight_smile:

A lot of scope probes have a switchable 1 / X10 attenuation. With that switched in your 10V / Div becomes 100V / Div.

Grumpy_Mike:
A lot of scope probes have a switchable 1 / X10 attenuation. With that switched in your 10V / Div becomes 100V / Div.

Oh yeah, i also have a x10 probe but it only has a pinpoint needle tip, the one i am using has clamps but they go directly to signal/gnd, without any attenuation.

So there is no way other than using a probe with attenuation? (More/better probes are on my shopping list anyways, but i was trying to figure out if i could achieve this from 'within' the scope)

So there is no way other than using a probe with attenuation?

No.

A lot of scope probes have a switchable 1 / X10 attenuation. With that switched in your 10V / Div becomes 100V / Div.

Mike I don't post much but your response in this thread is alarming

Working with HV AC isn't to be taken lightly and I am astounded by your suggestion to use a probes attenuation to drop mains voltages with no consideration of the scopes ground connection back to the mains earthing conductor !!

What would you then suggest about a tripping RCD? to remove the earth? its the realm of cowboys and chancers Michael

Then your last post with a definitive No is basically telling the OP theres no way to make a 1:10 attenuator for a 1:1 probe?

The connectors are rated at 400V / 1MOhms so it's within the specifications.

You are forgiven for being fooled into thinking you can measure mains voltages with a scope because the input of the scopes is in range

Working with mains requires special precautions because a scopes ground clip referenced to mains earth can be a very bad thing you can blow up your scope very easily

If your working with mains voltages taking measurements etc then you need an isolation transformer its not optional, then to measure the mains the proper way is to use a differential voltage probe, they are as expensive as a cheap scope

Thats the proper and safe way of doing it, anything else and your asking for trouble

Hi,

Now i started capturing some high voltage AC signals (using a standard clamp probe, signal to signal and GND to Earth), and i came across the problem that i just can’t seem to scale them down enough to fit them on the screen.

The clamp, does it clamp over the mains current carrying wire without making electrical contact?
If so you are measuring current, as long as you use the clamp and you can safely use a x10 probe on the output of the clamp is not at AC mains potential.
Can you post a picture of the clamp before trying anything else, a name plate picture would help as well.

Tom… :slight_smile:

Mike I don't post much but your response in this thread is alarming

Come off it.

Thats the proper and safe way of doing it, anything else and your asking for trouble

Total rubbish.

Then your last post with a definitive No is basically telling the OP theres no way to make a 1:10 attenuator for a 1:1 probe?

No it is not. If you would engage your brain for a moment the OP was asking:-

i was trying to figure out if i could achieve this from 'within' the scope

To which I answered no. There is nothing that can be done from within the scope.

Scopes are connected to ground and the mains is referenced to ground there is no need to do anything with the ground. You can simply measure the mains by connecting one end of the probe to it. I have done it many times in the past without problem.

Working with mains requires special precautions because a scopes ground clip referenced to mains earth can be a very bad thing you can blow up your scope very easily

You can blow up electronics of any voltage if you go connecting the ground to the wrong place.

What would you then suggest about a tripping RCD? to remove the earth? its the realm of cowboys and chancers Michael

That is an insult and a total misunderstanding about measurement. Yes you can remove the ground from a scope and operate it quite safely like that. It is not being a cowboy or a chancer it is knowing what you are doing.
Maybe it is because I am old and not brought up with the over arching nanny health and safety culture but I do not subscribe to the world view that you must protect everything as if people were idiots.
Yes working with mains requires that you know what you are doing, and it can be dangerous if you don't but do not paralyses yourself or go into back somersaults trying to prevent something stupid because people can always out stupid any precautions.

What would you then suggest about a tripping RCD?

There is no way you will trip an RCD by taking a scope measurement, do you actually know what you are talking about?

Come off it.

Come off what?, are you seriously going to try and defend recommending using a scope probe to drop mains voltage? is this seriously where you are going here? your advice is really bad and is dangerous in fact theres no argument here

Total rubbish.

What is that your argument? no counter point or justification its just total rubbish Mike has spoken and thats final?

To which I answered no. There is nothing that can be done from within the scope.

Mike you are trying to move the goal posts, sorry but read his post he asked is there no other way and you seem to think its a definite no!!! my brain is fully engaged

Scopes are connected to ground and the mains is referenced to ground there is no need to do anything with the ground. You can simply measure the mains by connecting one end of the probe to it. I have done it many times in the past without problem.

Mike you are exposing your lack of knowledge you seriously havent got a clue have you, RCD's trip when you try and do what you are doing because you put the load in parallel with the 1M input impedance and you get earth leakage thats why you should use a floating scope and that doesn't mean disconnecting the earth that is what cowboys do

I have sped up the motorway many times does that mean its risk free?

That is an insult and a total misunderstanding about measurement. Yes you can remove the ground from a scope and operate it quite safely like that. It is not being a cowboy or a chancer it is knowing what you are doing.
Maybe it is because I am old and not brought up with the over arching nanny health and safety culture but I do not subscribe to the world view that you must protect everything as if people were idiots.
Yes working with mains requires that you know what you are doing, and it can be dangerous if you don't but do not paralyses yourself or go into back somersaults trying to prevent something stupid because people can always out stupid any precautions.

Mike you deserve to be insulted here because what you are advising is dangerous and not recommended, I have seen your style Mike constantly berating n00bs and making out like you are some kind of Guru, sorry Mike but this is well above the level of decoupling or fitting pull ups what you are saying isn't right and someone needs to point it out to you but given the amount of arguments I have seen you involved in I would be amazed if you admit that you are wrong and I can see where you are going with the veiled insults I won't get in an argument with you but I will debate this and I will show you why you are wrong so leave the pathetic arguments at the door and debate

I am not even going to argue with removing an earth from a scope to get a measurement its a ridiculous thing to do, its so rough I can't find the words but any professional engineer wouldn't dream of it they would buy some differential voltage probes

There is no way you will trip an RCD by taking a scope measurement, do you actually know what you are talking about?

Yes Michael I know what I am talking about, I know much more than you do about this because its what I do, I spent a long time of my working life working on mains, I was an electrician for a long time and I am very experienced and extremely qualified. I now work in commercial electronics development professionally every day and I am also a PhD student in power electronics, I build mains voltage power converters and I have a lot of experience in this area.

I have spent a lot of time and money getting my equipment and I know exactly what I am talking about, if you connect an earth referenced scope to a circuit to measure mains then you get earth leakage its so blindingly obvious I can't believe you are arguing (well I can actually), if you have an RCD then it will trip as soon as the load you are measuring is below a certain impedance

Do the math and work out how much impedance it takes in parallel with 1M to give 30mA leakage @325V

Don't take my word for it though go and educate yourself

Dave Jones on what I am talking about (shock horror @the name of the video!!)

To think you are back here arguing that its OK to unearth a scope to do a measurement instead of buying a proper set of differential probes (mine were over £600) and the required safety equipment I mean does this sound professional to you?

To think a n00b comes in here and you don't even mention safety but go on to recommend using a probe to attenuate mains its ridiculous (check a standard probes data sheet, standard probes are not designed to do this) and you are showing a massive lack of understanding and respect for safety

If you remove an earth from a scope (many times in your words) to do a measurement instead of getting the proper equipment then you are a cowboy Mike a massive cowboy and you wouldn't last five minutes working for me

Now leave your pathetic insults and arguments at the door and debate, tell me why you think I am wrong

Resinator - It is a waste of time arguing with you as you do not appear to be a logical person. I will not degenerate into the personal abuse you seem to think is relevant argument.

I have seen your style Mike constantly berating n00bs

Then you have imagined it, along with other things you seem to be imagining.

Mike I have given no abuse, just observations after a few years reading these boards you are very obnoxious and have a very high opinion of yourself but from time to time you let it slip just how little you do know.

You know nothing about me I am a very logical person and I treat mains electricity with the respect it deserves

Your last post confirms it to me that you haven't got a clue what you are talking about, you cant recommend using a probes attenuation to drop mains its just so bad I can't find the words

It seems you have read the links I provided and realised you are wrong so rather than admit you were wrong you will flee the thread trying to say I have imagined things

I imagined nothing mains is dangerous and for posterity anyone wanting to measure mains needs an isolation transformer and differential probes which is two barriers of safety even then measuring mains is not to be taken lightly

Tell me why I am wrong Mike and debate, it seems you might learn something

Heres an app note from a scope manufacturer, shock horror they recommend isolation!!!

http://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=5&cad=rja&uact=8&ved=0CD4QFjAE&url=http%3A%2F%2Fwww.tek.com%2Fdl%2F51W_10640_1.pdf&ei=X7V1VP-1IYfiauuMgrAH&usg=AFQjCNGRpMcmocglLXM8LQQQydiI5CKiPA&bvm=bv.80642063,d.cWc

As instrument tech apprentices, we were taught, over 50 years ago, how to safely monitor mains voltages using a bog-standard oscilloscope and that involved lifting the earth terminal out of the supply line. Yes the scope had to be placed on a fully insulated work-bench and we had to use the "one-hand" technique during all measurements. OK, there were risks, but we were taught what they were, we operated within the risk envelope, we applied our brains and we lived.

Perhaps one of the major problems we have today is that the "Health & Safety" culture removes the need for brain-power and expects everyone to be capable of doing a task from a standard text-sheet without having to apply either thought or common sense.

Resinator:
Mike I have given no abuse

vs.

you are very obnoxious

Keep it civil

I too remember being taught to take out the earth and stand the 'scope on a plywood "elephant box", at BT over thirty years ago.

AWOL:
I too remember being taught to take out the earth and stand the 'scope on a plywood “elephant box”, at BT over thirty years ago.

H&S can seem a bit OTT to us old(er) timers but the sue culture helped bring this about. :sob:

Yes we were shown how to measure mains voltages (the old way) but Mike did not mention anything about insulation safety in the posts.

Do the math and work out how much impedance it takes in parallel with 1M to give 30mA leakage @325V

Well first of all no times 10 probe is only going to be 1M, but lets say it is.
325V / 1M = 0.325mA, or is that not what you mean?

Heres an app note from a scope manufacturer, shock horror they recommend isolation!

Not a surprise as they are in the same mind set as you. I would recommended the same in their position. There are lots of things you recommend as a manufacturer for reasons of avoiding being sued.

I treat mains electricity with the respect it deserves

And so do I, it is just that I seem to understand as well.
Note the comments from others about the no earth on the scope, it has been standard practice and not the cowboy practice you claim it to be.

you cant recommend using a probes attenuation to drop mains its just so bad I can't find the words

Try and find the words please.

It seems you have read the links I provided and realised you are wrong so rather than admit you were wrong you will flee the thread trying to say I have imagined things

You are wrong on all counts here.

Mike you clearly do not have any experience in this field as you seem to be doubting the fact that an RCD trips when using a earth referenced scope to measure mains

Take it from me thats exactly what happens if you don't want to take it from me then try it, this isn't something I am pulling from the air its based on real world experience, I have been there and done it

removing the earth is the cowboys way, it always has been as well I mean removing the earth instead of buying an isolation transformer theres no excuse

The manufacturers recommend it, EEVBlog says the same thing as me as well as countless others, I have highlighted the dangers but all you are bringing is that its OK because Mikle does it in effect Mike trumps safety

To be clear this isn't a silly health and safety rule, what mike proposes is dangerous and the chances of blowing up the scope increase dramatically as a power electronics engineer I just wanted to input the way a professional engineer measures these things

Take it from me thats exactly what happens if you don't want to take it from me then try it, this isn't something I am pulling from the air its based on real world experience, I have been there and done it

So explain to me how it does this.

removing the earth is the cowboys way, it always has been as well I mean removing the earth instead of buying an isolation transformer theres no excuse

Well that is your opinion and I don't expect to shake you from that it is rather like trying to argue evolution to a creationist.

have highlighted the dangers but all you are bringing is that its OK because Mikle does it in effect Mike trumps safety

Now who is moving the goal posts.

as a power electronics engineer

Oh are you? I thought you said:-

I am also a PhD student in power electronics,

I have supervised many PhD students, it is not a passport to experience and knowledge.

I just wanted to input the way a professional engineer measures these things

Thank you for that input.

So explain to me how it does this.

An RCD trips on imbalance

Connect the probe clip (which is earthed) to neutral and you now have a bypass for the currents, what goes out the live isn't returning on the neutral and it trips, based n real world experience which you clearly don't have as you would of never doubted this

The same thing can happen if the loads impedance you are measuring is low enough, think about it see if you can understand it, its not something I am making up

Oh are you

Yes I am, I work for one of the biggest engineering companies in the world and I develop commercial electronics and scientific instruments, not all PhD students are full time at a Uni many are funded and work in the real world me being one of them

I have supervised many PhD students, it is not a passport to experience and knowledge.

I am sure you have, its not in power electronics is it because you wouldn't be saying the crap you have said in here, its a passport to nothing but it should imply the student at least has the basics of his subject, what we are talking about here is basic electrical safety that even a bog standard electrician will know about

keep on denying it Mike and trying to convolute it, the facts are you posted some really bad advice in here that is dangerous and you are wrong, anyone who knows their beans can see this

Hi, not everybody has RCD fitted to their power circuit.
Those that do, do not usually have them tested using a proper testing unit, every 12months.
I don't mean the TEST button.

Tom...... :slight_smile: