How to mesure 240V AC with an oscilloscope

[kiwiiscraig]

Hi All

I have some thing that I want to work on but the very fist thing that I need to do is use an oscilloscope to view 240 Volts AC at 50 Hz. I have a hitachi oscilloscope v-422 but I need to do this with out killing myself and blowing up the scope.

Any suggestions on how to do this safely would be very much appreciated.

Thanks for the time and effort
Craig
 

[tautech]

Recommend you acquire a 100:1 probe for any mains work.

[David Hess]

When I have to measure the AC main voltage directly with an oscilloscope, my first choice is to use a normal power transformer and measure the output at its secondary.

[edy]

Assuming the OP is working on a mains-powered appliance, the scope usually has 400 V max rating, so using a 10:1 should show the AC fine but RMS is 240 V so peak will be 240 V x root 2 (1.414) so closer to 320-340 I believe (which on 10:1 probe will be -34 V to +34 V showing up on the scope)? Wouldn't the more dangerous part of all this is to ensure proper grounding and measurement technique? I am not so worried about the probe as I would be about how you plan to place your ground lead and especially if using both channels... since they share the ground back to common earth as would your appliance earth. You would want to ground both probes on the same ground as your appliance ground...can check if there is continuity to your mains plug ground pin... if not, the appliance may be using an isolating transformer but you need to ensure that you are not creating a grounding path for mains current. If isolated, I believe you would just be setting that portion of the circuit at grounds reference level and everything else is relative to it.

[David Hess]

The ground connection is a pretty big deal and safety hazard.

Neutral is connected to ground at the distribution point so if there is an AC load between hot and neutral on the wires which the probe is connected to, the voltage drop across neutral may allow considerable ground current through the oscilloscope probe ground.

[mij59]

Hi,

The safes way is to use a high voltage differential probe.
You could use a (medical) isolation transformer.

Take a look at Dave's video "how not to blow up your scope".

[G7PSK]

I do it frequently, I have a 3 pin plug wired with BNC socket fitted and a probe to bnc adapter and use a 100 X probe of good quality (RS). I use this rig to check alternators so in most cases the high voltage is a different source than that powering the scope although I have run the scope from the alternator under test. This would not be a good idea if the plug is reversible in its socket, the UK and CEE form plugs I use are not by design so as long as the sockets are all wired correctly all is safe and to date in nigh on 40 years I have never come across an incorrectly wired socket. 

[Psi]

I've viewed 240VAC on rigol 1052 with 10x probe quite often with no problems.

I even set the probe to x1 by accident once and the scope just clipped the waveform. I disconnected after a few seconds when i realized what i had done. The scope was undamaged.

[tautech]

I've viewed 240VAC on rigol 1052 with 10x probe quite often with no problems.

I even set the probe to x1 by accident once and the scope just clipped the waveform. I disconnected after a few seconds when i realized what i had done. The scope was undamaged.

This is exactly the reason why it is necessary to have at least one 100:1 probe.
They are not switchable and this simple mistake is not possible.

Also the reason why many years ago that I chose to market only those scopes with 400 V inputs.

[Psi]

Yeah, a fixed x10 or x100 probe is definitly the way to go when working with 230V

[kripton2035]

you will blow up the mains differential if earth and ground are connected
so either use a 10x or 100x probe and cut the earth ( use a 2 prongs wire or an adaptor)
or buy a differential probe (around 100€ you can find on the bay)
see dave's video as said earlier in this thread is a good start anyway.

[tautech]

I have been looking at a Siglent accessory ISFE (isolated front end)
http://www.siglent.com/en/product/detail5.aspx?id=100000013462552&nodecode=119008006

Think I will get one with my next shipment to see how user friendly they are.
They do not have a very high frequency spec, but I think they might be a good option for added safety and negate the need for differential probes for simple applications.

Anybody had a play with anything similar?

[kripton2035]

it really looks like a differential probe ... if it's powered (and there seems to be a little plug at the side) it IS a differential probe ...

[tautech]

it really looks like a differential probe ... if it's powered (and there seems to be a little plug at the side) it IS a differential probe ...

These are differential probes:
http://www.siglent.com/en/product/detail5.aspx?id=100000008535191&nodecode=119008006
http://www.siglent.com/en/product/detail5.aspx?id=100000009274253&nodecode=119008006

[Riotpack]

Maybe use a 240-12V TX as someone has mentioned. Load the secondary with a highish value resistor (calculate to flow 100ma or so through it) to lower the impedence and help stop induced noise.

If you need to get the exact voltage, you can get a ELV AC source (like another 240-12 TX), then connect it to the transformer you will be using and measure the voltage on the primary and secondary of your TX with a true RMS meter to get the transformers ratio.

[David Hess]

Maybe use a 240-12V transformer as someone has mentioned. Load the secondary with a highish value resistor (calculate to flow 100ma or so through it) to lower the impedence and help stop induced noise.

If you need to get the exact voltage, you can get a ELV AC source and measure it, then connect it to the transformer you will be using and measure the voltage with a true RMS meter to get the transformers ratio.

I do not think the load resistance will matter.  The output impedance of the transformer is already low.

It may be calibrated in situ by measuring the input side with an AC voltmeter.

If higher bandwidth measurements are desired which a standard power transformer will not pass effectively, then an audio transformer designed for a tube output stage could be used with the primary and secondary swapped.

For really high bandwidth measurements, 10s of kHz to 10s of MHz, I would use an oscilloscope current clamp and small resistive load like an incandescent light bulb.  Multiple turns can be run through the clamp to increase its sensitivity.

[David Hess]

I have been looking at a Siglent accessory ISFE (isolated front end)
http://www.siglent.com/en/product/detail5.aspx?id=100000013462552&nodecode=119008006

Think I will get one with my next shipment to see how user friendly they are.
They do not have a very high frequency spec, but I think they might be a good option for added safety and negate the need for differential probes for simple applications.

Anybody had a play with anything similar?

Some oscilloscopes provide the same function but at full bandwidth.  Each vertical input is galvanically isolated so each single ended probe may be used to make an independent floating measurement simultaneously including direct off-line AC measurements.

it really looks like a differential probe ... if it's powered (and there seems to be a little plug at the side) it IS a differential probe ...

It is not a differential probe.  It is an isolation amplifier of some type similar to but much lower performance than a Tektronix A6902B.  This sort of thing has been replaced by oscilloscopes which include galvanically isolated vertical inputs like the Tektronix THS700 and TPS2000 series.

These do not have symmetrical loading like a differential probe so one side of the circuit to be measured needs to be low impedance but that is not normally a problem with off-line circuits.  The probe ground may be connected to neutral or hot without problems.  With two channels of isolation, both probe grounds are isolated from each other as well and do not even need to connect to the same point.

[geek]

you will blow up the mains differential if earth and ground are connected
so either use a 10x or 100x probe and cut the earth ( use a 2 prongs wire or an adaptor)
or buy a differential probe (around 100€ you can find on the bay)
see dave's video as said earlier in this thread is a good start anyway.

cut the earth from scope or DUT?
is it safe?

[Richard Crowley]

Can anyone explain what is the great fascination people around here have with looking at the mains power on their oscilloscopes?
I mean, you can safely measure the voltage, with most any decent DMM. And you can look at the waveform through any old wall-wart transformer.

[kripton2035]

you will blow up the mains differential if earth and ground are connected
so either use a 10x or 100x probe and cut the earth ( use a 2 prongs wire or an adaptor)
or buy a differential probe (around 100€ you can find on the bay)
see dave's video as said earlier in this thread is a good start anyway.

cut the earth from scope or DUT?
is it safe?

it is safe as soon as you don't touch the wires ...
the earth is here to cut the mains power if someone touches the mains wires
no earth = no automatic cut off
the differential probe is far more safer, if you touch the wires the main differential will cut the power

[Bored@Work]

cut the earth from scope or DUT?
is it safe?

No, it it not. But unfortunately, not enough idiots die doing it, so it gets suggested again and again.

[tautech]

you will blow up the mains differential if earth and ground are connected
so either use a 10x or 100x probe and cut the earth ( use a 2 prongs wire or an adaptor)
or buy a differential probe (around 100€ you can find on the bay)
see dave's video as said earlier in this thread is a good start anyway.

cut the earth from scope or DUT?
is it safe?

It is not considered safe.
Read the attached document ex Tektronix.

[DoDaMaffs]

I am going to put my two bobs worth in and concur
with bored@wok & richard c.

I am suprised that nobody asked kiwicraig what he is trying
to acheive and what safer methods may be employed if
he is not versed in this type of measurment.

You all got side tracked as to whether the DSO or CRO
can, can't or should.

Pete

[vk6zgo]

Can anyone explain what is the great fascination people around here have with looking at the mains power on their oscilloscopes?
I mean, you can safely measure the voltage, with most any decent DMM. And you can look at the waveform through any old wall-wart transformer.

I wonder about that,too,Richard.

On many occasions,I have used an Oscilloscope probe to look at Mains voltages,basically just to see if they are there.
In this case,the probe earth clip is not connected to anything (& is probably best removed ) with the return path to Neutral made via the Earth connection included in the 'scope's power cord,&,in turn, the building Earth/Neutral connection.

Obviously,this convoluted return path will affect accuracy of any high frequency transients on the Mains,but it works OK fot troubleshooting purposes.

[Psi]

Can anyone explain what is the great fascination people around here have with looking at the mains power on their oscilloscopes?
I mean, you can safely measure the voltage, with most any decent DMM. And you can look at the waveform through any old wall-wart transformer.

I wonder about that,too,Richard.

When i was probing the mains it was because i was working on an implementation of UPB (universal powerline bus).
I needed to look at the data pulses i was injected into the mains waveform.