is it safe to probe directly to ac mains with a analog scope?

[little_carlos]

hi, i made a triac dimmer for lights and i want to see how the waveform behaves on the scope, but, is it safe to probe it directly? the scope says 400vpk (not Vpp) max. my home runs on 127V and i dont have neutral conected to mains earth on the dimmer

[Rerouter]

It likely means 400V peak to peak, and 400V max in reference to earth, now providing your houses ground feed is reliable all the way to the socket you will be plugging the scope into.(you may want to test this as i do not know what country your from),

you should be OK to probe to the active, however 2 good points to follow:
- use a 10X probe if you can, to reduce the signal level.

- switch off the breaker or pull the fuse for that circuit first and set up something secure and reliable to hold your probe in place so you don't have the risk of something moving, you grabbing it, and there being one less enthusiast on the forum, also double check its not powered before you start work, safety first,

[grouchobyte]

The short answer to your question is a resounding NO.  Unless you fully understand how power mains are wired vis a vis ground/earth, do not  attach test equipment, especially ground clips/connections to the mains and especially expensive test gear like scopes without a very clear picture of where dangerous voltages and current paths are. You are posting in the beginners forum so I recommend you heed this warning. Most of us "wiser" ones have some smoked test leads and charred crock clips to prove that we once were not paying attention

[Richard Crowley]

I have to agree with @grouchobyte. 
This is one of those questions where: "if you have to ask, you probably shouldn't do it."

Are there safe ways of doing this?  Yes. But you haven't provided enough information to make any specific suggestions.

You could even build-in an attenuated "test point" that would be safe to meter or probe, assuming the ground reference is connected properly.

[tggzzz]

Have a look at the "choosing a probe" and "high voltages" sections in https://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/

You can learn a lot from the other references as well.

[Zero999]

I have to agree with those above who say don't do it.

However, if the oscilloscope is earthed, the x10 probe is suitably CAT rated, the ground clip is not connected to anything and you know what you're doing, it's possible to safely probe the live conductor on a phase controlled dimmer to see the waveform.

But, going from the first post and the fact that this is in the beginner's section, it seems like the original poster doesn't meed all the criteria listed above, so don't do it.

[mauroh]

First of all I agree with all the above: STAY AWAY FROM MAIN!!

In this days I played a bit with a TRIAC dimmer controlled by a ESP8266 and I made exactly what you requested.
To be safe I connected the dimmer and the load after a small isolation transformer made with 2 identical 220/12V transformers, connected "back to back".


I had to reduce the load because my transformers are not able to provide so much current, but to test the circuit a 10W light bulb was more than enough and everything is safer with less current  .
If you connect the scope across the load you will be safe and will get all the nice trace you are looking for.

Safe doesn't mean you can put your finger across the load or wherever you want on the circuit, since even though the current is small, the voltage is there, so pay attention and keep all the high voltage points properly isolated.

See attached what I got:
Pink     = voltage across the load (actually a partition but that doesn't matter)
Blu      = zero cross detection output to the microcontroller
Yellow = trigger from the microcontroller

Mauro

[danadak]

Use two channels, and setup for differential mode, 10x probes. The scope
specs should be able to handle this.


Use the psuedo differential technique discussed here - https://www.google.com/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=tektronix+scope+differential+mode

Regards, Dana.

[exe]

Don't float the scope or just wedge your probes on the mains - bad things can happen.

Could you please explain why floating will not help in this case?

[AndyC_772]

The danger of floating the scope, is that its chassis (including all the BNC connector shells and probe grounding leads) can become live.

My scope says "CAT I, 300V rms" on the front, which might perhaps suggest it was just about capable of probing a live 240V mains circuit without damage or hazard, but I wouldn't even consider actually doing so. Nothing that can resolve mV with a 1GHz bandwidth is likely to react well to being connected to the mains, IMHO.

A proper active differential probe isn't that expensive... certainly not compared to blowing up a scope channel.

[Zero999]

Don't float the scope or just wedge your probes on the mains - bad things can happen.

Could you please explain why floating will not help in this case?

It will help and will work but will probably kill you.

If you connect your scope to an isolation transformer, as soon as you connect your probe ground, your entire scope's chassis is at the potential of the probe ground. So if you then connect your ground to live or neutral, then touch the chassis, bzzzt.

Edit: just to add, I questioned one of our lecturers at university who said we should do this and was shouted down and sent to my tutor for a disciplinary for daring to question someone with "40 years of experience" (teaching theory I will add, nothing practical). The next lab someone did it and got a bad shock. I never got an apology, perhaps because that would admit liability, despite asking for one and at that point. Then I became aware of university politics as I got no help from that lecturer or my tutor again an self taught myself the entire course. Not bitter or anything

Also life lesson: never study EE at a B-rate university.

Firstly, no one at university, college, school etc. should face disciplinary action because they questioned something their tutor did/said, as long as they did so in a respectful manner. If you called your tutor a fucking moron, (even though this may have been understandable, given the circumstances) then you should have been disciplined for that but if you just pointed out that they were wrong, it should have been fine.

Secondly, if someone received a bad shock because they followed the tutor's dangerous instructions, especially after you warned them, you should have reported it to the health and safety department, followed by the authorities if they didn't handle the matter in a satisfactory manner.

[grouchobyte]

Again, Dave explains the "trap for young players" which I beleive is a trap for all players. Personally I really hate exposing 10's of thousands of dollars of test gear and my sanity to stupidity. Watch this video and pay attention.

 I recently mentored some senior university EE students on their sr project which was a mains based device and even with my cautionary advice, I received phone calls and messages asking me why sparks and university lab circuit breakers were popping and TAs were asking questions about non-functional arbs and scopes that once worked but now have that magic smell.

I would even venture to say that there is probably no EE on planet earth that has not learned this lesson.

[RGB255_0_0]

But the biggest shock you get from that video is how the DC chargers are often mains-referenced. I've smoked a few bits because of that. Batteries FTW.