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I am doing some study on the frequency of electrical supply. I need to do the analysis on MATLAB. I want to record the electrical signal as an audio file in my laptop.

As I am a beginner working with 220V mains, I have a fear of busting my soundcard . I need expert advice on how I can develop my circuit and what type of protective measure should be taken.

So far my idea is to use a transformer of 220/12 volts then using a voltage divider circuit. But I actually don't know what is the limit of Line-In voltage for PC soundcard. Up to what extent should I attenuate my signal?

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    \$\begingroup\$ Use the transformer and wire a pot across it then put a 1k resistor from the wiper to the input and do an experiment. \$\endgroup\$ – Andy aka Nov 30 '15 at 10:01
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    \$\begingroup\$ Line-in voltage is about 1V. What sort of frequency analysis are you doing? \$\endgroup\$ – pjc50 Nov 30 '15 at 10:09
  • \$\begingroup\$ @pjc50 I am doing frequency analysis for IEEE Signal Processing Cup. \$\endgroup\$ – Tanzil Bin Hassan Nov 30 '15 at 10:25
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    \$\begingroup\$ If you are interested in the spectrum analysis, then transformer may not be a good idea. As far as I know, it acts as a filter to some extent and may attenuate some frequencies. Is sound card the only way you can capture the signal? You sound card is good for sampling audio bandwidth, but how about the rest of the frequencies? \$\endgroup\$ – Nazar Nov 30 '15 at 13:12
  • \$\begingroup\$ Think about buying a cheap usb soundrecording device... Ebay lists them starting on 1€. maybe throw in an (more expensive) Hub as well, which hopefully would protect your laptop from any over voltage... \$\endgroup\$ – DThought Nov 30 '15 at 13:28
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xoscope, an open source oscilloscope software which is able to use the sound card as input, has a schematic for a probe buffer. I'm not sure if you need it though, it really depends on the analysis you plan to perform. For a simple frequency measurement it would be definitely an overkill, but if you're going to do spectrum analysis, I'd take a look on it.

Note that this probe buffer shares earth with the circuit under test, so you will still need a a galvanic isolation of some sort between mains and your probe. Transformer is one option. If you want to avoid transformers, go for a USB sound card + isolated hub.

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Attenuate Transformer output 12V to say 1mV through resister voltage divider and see. I think it will not damage your sound card. Increase or decrease 1 millivolt output as desired.

Caution: Use good transformer with isolation more than 1KV to 2 KV otherwise your computer / laptop is at risk.........

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    \$\begingroup\$ Line in on a sound card is at linelevel (en.wikipedia.org/wiki/Line_level) so, about 1Vpp for typical consumer equipment. Going down to 1mV would be a strange thing to do. \$\endgroup\$ – JRE Nov 30 '15 at 12:52
  • \$\begingroup\$ Sound cards may have both mic and line inputs, and 1mV might be reasonable for the mic input. \$\endgroup\$ – Spehro Pefhany Nov 30 '15 at 15:43

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