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this is a noob question but here it is:
How can i connect an osciloscope to the 230V mains power line to see the voltage sine wave ?
- what schematic do i need
- how safe is it ?

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    \$\begingroup\$ it will also depend on the type of oscilloscope you have, could you please give us info on the one you are using? \$\endgroup\$ – jsolarski Jan 3 '11 at 12:09
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    \$\begingroup\$ it's an old Russian scope, it has 50Vpp and a high input impedance, i know the switches to change the time range and it's ok, but how do i connect it phisicaly to the 230V mains... \$\endgroup\$ – s.mihai Jan 3 '11 at 12:23
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    \$\begingroup\$ The way I actually do it is by not touching mains at all! I just stick the probe into my thumb and walk around 'til I get ~1Vpp noisy signal 60Hz signal. :) \$\endgroup\$ – tyblu Jan 4 '11 at 13:55
  • \$\begingroup\$ 230V is 50Hz back here in Europe \$\endgroup\$ – s.mihai Jan 4 '11 at 15:46
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Use a transformer (12V, say), it will be much safer even if your scope will handle the mains voltage.

That particular scope will probably be damaged, DO NOT connect it to the mains! 230 V is the rms voltage, the peak voltage is 230 * 1.412 V.

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The ground clips on your probes are connected to Earth ground by means of the oscilloscope power plug, in a roundabout fashion. This means you can't connect it anywhere but Earth ground (green wire in North America), and even then the path between your circuit and your oscilloscope plug may be so long that there is a voltage differential and capacitance, making for a rough transition back to 0V.

To get around this, isolate one of your 'scope or circuit with an isolation transformer. This gives galvanic isolation, allowing you to use a circuit ground at a different potential than your oscilloscope ground. Not all transformers isolate, but many common ones do. You can use 2 like step transformers from wall warts, if you want; for example: two 120V-to-12V step down transformers with the secondaries (12V-side) wired back to back; 1:1 isolation transformers are also available. It is good practice to measure the potential between system grounds while taking other measurements. Keep a voltmeter hooked up, in high voltage mode, between your oscilloscope ground and circuit ground, one of which should be the Earth ground. (A banana plug fits pretty well into the ground hole in North American sockets.) This will give you an indication as to how dangerous completing the circuit, be it with your hands, a spanner, or unisolated grounds, might be!

Another option, if you only want to measure AC waveforms, is to use a small 1:1 transformer for the probe alone. The probe ground reference clips to primary #1 (X1), the probe itself clips to primary #2 (X2), then the secondary taps are used as your probing tools. This is not as safe as isolating the 'scope or circuit.

Note that isolating your oscilloscope is also dangerous, as it can lead to a hot chassis. This means that the 'scope case can be charged to dangerous levels. It is better to isolate the DUT itself, and treat it with care.

Another way to isolate a system is to put it on battery power. If not practical for the DUT, you can do it to the 'scope, though you may have to build your own unit if the "old Russian scope" didn't come with one. Note that ground potential safety with the multimeter applies here, too. It is possible to equalize grounds using a current limiting resistor in between. Once the voltage is within ~100V you can use a 100kohm pot and turn it from 100kohm to 0ohm over a few seconds.

You will be measuring +/-325V (230Vac). Ensure you have the proper probes for this and your 'scope.

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    \$\begingroup\$ +1 I have experience with NOT following your very first advice. The explosion is quite beautiful and probe didn't even melt all the way to liquid :D \$\endgroup\$ – Vorac Jul 6 '12 at 9:27
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Use step down transformer and reduce the amplitude depends upon the oscilloscope specification say 12 v. And then you can measure and analyse those input. The values you can manipulate from transformation ratio of transformer.

i.e. transformer ratio K = voltage at secondary / voltage at primary.

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