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I have this 230 to 6V rms step-down transformer. I use it to observe the mains voltage.

But recently I needed to create some interference or spike in the mains. So I first tried the following:

I used a filterless power strip. I coupled the transformer's primary side to one of the sockets of the power strip. And at the adjacent socket I plugged a heat gun. I sometimes very randomly saw some tiny fluctuation and that disappeared.

My approach was to see the interference from the nearby AC socket. What type of load would create a visible interference or spike? Or is there any other practical way to achieve this. I want to create a spike or interference by a load and observe that from a nearby AC socket via a scope and a transformer.

More over is there a safe way to observe the Line to earth or neutral to earth voltage on scope? I mean if I use the transformer secondary for the line can I connect the scope ground to the mains earth safely?

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    \$\begingroup\$ What are you hoping to achieve? \$\endgroup\$ – Andy aka Nov 7 '18 at 19:54
  • \$\begingroup\$ Thats all nothing more. \$\endgroup\$ – user16307 Nov 7 '18 at 19:59
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    \$\begingroup\$ You really need to define what you are trying to model on the mains. I'll prove this by providing you with a way to do it which I'm pretty sure isn't what you mean, but certainly would fit some interpretation of what you wrote. Take an old dental x-ray machine and hook it up to the mains and use it. While it is running, have a look at your mains. You will see plenty of spikes and interference. Of course, you'll say, "not that kind." But then, this only begs the question, "what kind, then?" Or, maybe, that dental x-ray machine is the right answer? \$\endgroup\$ – jonk Nov 7 '18 at 20:06
  • \$\begingroup\$ @jonk Any kind of visible interference. Maybe I should go to a dentist and bring the subject there in a way when faking my tooth pain. \$\endgroup\$ – user16307 Nov 7 '18 at 20:35
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    \$\begingroup\$ @user16307 Hopefully you find a dentist with older equipment. Say, 30 years or older? I think the newer equipment is much, much better about injection. Back then, the interference was enough to cause equipment in other rooms to reset themselves! Today? Not so sure. Still, you'll probably see something quite visible, anyway. \$\endgroup\$ – jonk Nov 7 '18 at 20:46
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For EMC testing to meet standards, a so-called CDN (coupling-decoupling network) or LISN (Line Impedance Stabilization Network) is used. You may be able to find schematics and instructions for making such a network. I have built LISNs (for measuring noise created by circuits) from such information and they worked fine for pre-compliance testing.

Here is the internal schematic of a commercial device. As you can see it's a decoupling network on the mains input and a coupling network on the other side to allow injection of signals.

enter image description here

As far as creating nasty noise without any fancy equipment, I've found a classic 100/140W Weller soldering gun to be a good informal source of noise (photo from Amazon.com). Certainly they are better used for creating noise than for soldering for 99% of electronics purposes.

enter image description here

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Any powerline communication device does exactly that: inject a signal on top of the power signal, and extract that at the other end. It works.

The fact that you don't see much with an oscilloscope just means you're using the wrong instrument.

Try the following:

  1. use a dimmer for resistive loads (for incandescent lamps, i.e. the cheapest dimmers you find).
  2. set it to a medium dim
  3. attach a large load, e.g. a hot plate, toaster, large lamp, whatever, close to the maximum rating of your dimmer
  4. use the transformer, plugged into the outlet you want to observe, only as a source for the trigger of your oscilloscope
  5. use that trigger signal to trigger the oscilloscope, observing a capacitive divider: two (high-voltage, medium to small value!!) capacitors of two different sizes in series between live and neutral of the power outlet you want to test.

What does a cheap dimmer do? Typically, it's a phase-fired controller, which means it always turns on the load (like you, flipping a switch) at the same phase (i.e. angle) of the grid voltage.

That's awesome, because a powerline system has different properties, depending on which phase you look at it.

Now, your transformer might not even have the bandwidth to notice the high-frequency noise caused by switching on, but your capacitive divider definitely has – in fact, it should act as kind of a high-pass filter.

By triggering the oscilloscope with your transformer, you make the grid phases take a "fixed position" on your oscilloscope screen. That's immensely helpful, since now your switching noise will also always get a fixed position (since the switching always happens at the same phase), and be much easier to spot.

Compare load with and without dimmer, and with no load and dimmer at all. Then start playing around with the dimming ratio.

schematic

simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ For increased effect add long extension cord ("resistor") to increase resistance from the source. \$\endgroup\$ – Jeroen3 Nov 7 '18 at 20:13
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    \$\begingroup\$ @Jeroen3 but unroll that cable, unless you want to improvise an inductive low-pass filter \$\endgroup\$ – Marcus Müller Nov 7 '18 at 20:22
  • \$\begingroup\$ @MarcusMüller Would you mind to draw what you have written? Otherwise I can draw and ask you whether is correct. This is high current stuff thats why, to be sure. \$\endgroup\$ – user16307 Nov 7 '18 at 20:23
  • \$\begingroup\$ it should be extremely low-current (but relatively high-voltage), but sure, I'll add a drawing \$\endgroup\$ – Marcus Müller Nov 7 '18 at 20:24
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    \$\begingroup\$ One note -- C1-C3 should be Y1 type safety rated capacitors (to protect both you and the scope, lest a non-rated cap short out) \$\endgroup\$ – ThreePhaseEel Nov 8 '18 at 1:33
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You could use a 230 v relay (SPDT or greater) wired as a buzzer. One coil lead is wired through the NC and Common, the other lead straight to mains. The inductive kick-back from the coil should give you some good rough spikes. You can adjust the contact clearance and the spring tension to minimize excess noise.

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  • \$\begingroup\$ Unfortunately you'd also get a lot of broadband RF noise emissions when the contacts arc. Make sure that isn't picked up by your test leads. \$\endgroup\$ – Wossname Nov 8 '18 at 7:50

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