Can this scoping harm the oscilloscope or the user?

[Chriss]

Hi!
I have to look the shape of a sine wave on a triak which is switching the main voltage.

If I hook up a 220/12V transformer to the triac and connect my scope to the secondary winding on the transformer, is that setup safe enough just not to blow my scope?
I don't have to care about the voltage hight but of the shape.
I mean, I can't figure out what could go wrong with my scope if I make this setup.

The transformer will separate the scope from the main power source but the shape of the main sine wave can be observed.
I can connect both way my probe to the secondary winding of the transformer and it should still stay safe and not blowing up.

Is there anything else what could go wrong until I stay on the secondary side of the transformer?

The unit under testing will be powered on through a isolation transformer.

Thanks for any advice.

[ataradov]

If done correctly, this setup should be safe. This is no different than probing any other circuit with an isolated mains power supply.

[CaptDon]

I suspect you will see all kinds of horrible overshoots and undershoots and ringing and if the triac is switching in an unsymmetrical form the transformer may saturate and get hot or burn out. Time to use a differential probe with proper voltage rating!!!
 

[RoGeorge]

Proper approach would be to buy an isolated differential probe that can withstand high voltage.

If you only want to see the switching time varying in relation with the mains, so no precise measurements, a simple loop of wires placed nearby the tested circuit might be enough (no direct contact and no transformer, just a loop of wire.  At first, you can simply clip the GND wire to the tip, and try to see the switching moment (synchronize the oscilloscope internally, from Line/Mains).  Something like this:



Might not show the proper waveform, but it should reveal at least the switching moment relative to mains zero crossing.


Speaking for myself only, if no proper differential probe available yet the precise voltage waveform is needed, I would try to float the oscilloscope entirely, by powering the oscilloscope from batteries, or from an isolation transformer.  If you do so, don't connect to the oscilloscope any other cables than the probes (I mean no USB, or LAN or GPIB or serial, or grounding, etc.), and avoid touching any metallic parts of the oscilloscope.

There are also hand-held oscilloscopes powered from rechargeable batteries by design, if you can borrow one.

Beware that the GND of the probes and the grounding wire from the mains power cable are usually connected together internally, inside the oscilloscope, so beware of unintended short circuits made through the GND of the probes and the rest of the metallic parts of the oscilloscope or the mains grounding wire.

Again, floating the oscilloscope might be dangerous if you don't know very well the possible dangers, whatever you do, the outcome is entirely your own responsibility.

[tggzzz]

Proper approach would be to buy an isolated differential probe that can withstand high voltage.

If you only want to see the switching time varying in relation with the mains, so no precise measurements, a simple loop of wires placed nearby the tested circuit might be enough (no direct contact and no transformer, just a loop of wire.  At first, you can simply clip the GND wire to the tip, and try to see the switching moment (synchronize the oscilloscope internally, from Line/Mains).  Something like this:



Might not show the proper waveform, but it should reveal at least the switching moment relative to mains zero crossing.


Speaking for myself only, if no proper differential probe available yet the precise voltage waveform is needed, I would try to float the oscilloscope entirely, by powering the oscilloscope from batteries, or from an isolation transformer.  If you do so, don't connect to the oscilloscope any other cables than the probes (I mean no USB, or LAN or GPIB or serial, or grounding, etc.), and avoid touching any metallic parts of the oscilloscope.

There are also hand-held oscilloscopes powered from rechargeable batteries by design, if you can borrow one.

Beware that the GND of the probes and the grounding wire from the mains power cable are usually connected together internally, inside the oscilloscope, so beware of unintended short circuits made through the GND of the probes and the rest of the metallic parts of the oscilloscope or the mains grounding wire.

Again, floating the oscilloscope might be dangerous if you don't know very well the possible dangers, whatever you do, the outcome is entirely your own responsibility.

Sigh. Again.

Not only is floating the scope highly dangerous, even for experienced people let alone a beginner, too much "unexpected" capacitance on the "wrong" node can destroy the UUT.

I'm getting tired of pointing that out.

Use the right class of probe for the job. Yes, they are expensive - but which is worth more, you life/health, the UUT, or a probe.

[Chriss]

Thnx guy's to pointing me out with nice tips.
I have also a handheld battery powered two channel 70MHz scope too.

Probably I will use this handheld scope for this measuring.

Im not new in the feel of scoping, I use it every day for small DC project and repair, but I have not so much experience with main AC
measuring. I'm also familiar with the risk of scoping the main voltage.
I never bought a differential probe cos I deal very less with high voltage, and when I have to then I use two probes with the MATH function to do the job.

But now I saw laying around an old Nokia 3310 phone charger, that one with a transformer in there.
The coils are completely separated from each other, practically galvanic separation.

Before I dropped in recycling bin I come a cross in mind to make a sort of tool for me.
To measure such of signals, even they are not precise, they are probably usable for rough measuring in some situation.

Here is a picture how I see the setup.
I can't figure out a situation how could I touch with my probe any live point to short out it.

The only common point is the alligator clip and earth/neutral.
I put an Ix variable on the picture, where could go possible some current in some case, but hey!
only if the primary side is shorten out, and than should blow the main fuse and the shorten is cos due some other problems before the scope is connected.

Probably it would trip the main fuse at the moment the primary side of the T1 is connected to the main.

[David Hess]

If I hook up a 220/12V transformer to the triac and connect my scope to the secondary winding on the transformer, is that setup safe enough just not to blow my scope?

That will work fine.  TRIAC switching can produce some nasty waveforms, but the oscilloscope can handled that level of overload if necessary.

[tggzzz]

Im not new in the feel of scoping, I use it every day for small DC project and repair, but I have not so much experience with main AC
measuring. I'm also familiar with the risk of scoping the main voltage.
I never bought a differential probe cos I deal very less with high voltage, and when I have to then I use two probes with the MATH function to do the job.

Thanks for telling us how experienced you are.

I would argue that the "high voltage" isn't the only consideration; "high energy" can be damaging too. Consider how an arc or spot welder works: low voltage but high current "damages" metal too

Can I suggest that FFI on the many classes of probes, why they exist, and where they should/shouldn't be used, you have a look at the references at https://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/ Such reference material is considered, accurate, and well written - unlike anything people might throw at a forum.

[Chriss]

Thanks for the link. I found it very useful, it should be put somewhere in the forum so people can easily access. Even if we experienced user, it is worth to read time by time, just to refresh your memory before it get corrupt. 

[tggzzz]

Thanks for the link. I found it very useful, it should be put somewhere in the forum so people can easily access. Even if we experienced user, it is worth to read time by time, just to refresh your memory before it get corrupt. 

It is widely scattered throughout the forum, in the many places where people ask about probing - typically where mains is involved.