Are there any (maybe PC based)that are under $100 that will give me sign waves of AC? I want to see how dirty my power is in Virginia and then check what Trip Lite Isobar and Panamax does to it?

TrueRTA (www.trueaudio.com) has a spectrum analyzer that has a ocilloscope function, but you will have to reduce the AC line voltage down to less than a volt or else you'll blow your laptop (and possibly yourself) to smitherines. The spectrum analyzer can display the harmonic distortion of the AC line voltage. I would make a resistive voltage devider to reduce the raw AC voltage rather than a transformer or VARIAC since these will introduce their own distortion to the waveform. If you're not comfortable making such a devider, don't attempt it!.

My AC has about 3% total harmonic distortion, and none of this can be eliminated by any power filtering device - these are only made for very high frequency RFI/EMI. The only way to get rid of the distortion (which is essentially harmless by the way) is by power regeneration, which is massive overkill.

Soundhound,
I have that software and mike preamp per your recommendation. I am just remembering from my IC class from a long time ago. I get a 1 ohm and 150 ohm power resistors and solder them on a board in a votage divider circuit with the 150 ohm one on top and the 1 ohm resitor on the bottom, solder on AC plug on the input (to plug into the outlet)and solder on a miniphonoplug at the output. I just plug it right into the sound jack on the PC. Or do I need to go through the preamp. I figure the output is around .8 volt through this voltage divider. What wattage resistors should I use?

It doesn't matter what wattage resistors since HOPEFULLY!! you won't be drawing any current and therefore no heat to dissipate. I would construct the devider with a 150 ohm resistor in each of the AC leads with the 1 ohm across them at the end. Peraonslly, I would probably scale both the 150 and 1 ohm resistors up by a factor of ten, and therefore have 1k5 and 150ohm resistors. This would offer a little bit more protection if the end gets shorted to something - it will limit the current more.

You don't need to go into the mic preamp since you will be at line level - unless your mic preamp has a line input. Using that if it has one would protect your laptop more if things get nasty.

Good luck----and don't try to measure with this thing while in the bath tub......

Through a fuse in it for mir protection.

One does not have voltage divider action between two resistors unless there IS current flow. It is the current flow that creates the voltage divider action, baby!

150 ohm plus 1 ohm (total: 151 ohms) fed by 120vac would dissipate almost 96 watts. Scaling up by a factor of ten would reduce this to 9.6 watts, more do-able in a small package. Further scaling may be of benefit, depending on the line-level input impedance of your sound card. The line level input impedance of the sound card will set an upper bound as to the resistor values of the divider. Power dissipation will set a lower bound.

Perhaps protection such as back to back zener diodes of several watts rating across the sound card inputs in case the low ohm resistor opens should be considered. Paralled LEDs of reversed polarity is another possibility IF the large resistor in the divider is high enought to limit the LED current to a safe value. The light would indicate an overload condition.

Of course, the input impedance of the line level input of the sound card would have to very high compared to the paralell value of the voltage divider resistors or connecting the sound card will significantly affect the voltage divider results. When dealing with measuring the AC line voltage, some failure analysis should be done.

Computer based sound cards (ones that are contained entirely inside the PC, as a plug-in card on the motherboard) have voltage refferences at chassis (case) ground, which is connected to the gound conductor in the three conductor power cord and then to earth. This is not quite the same as the voltage on the neutral wire, which is what the hot AC lead is refferenced to.

To get around this, you can do the dual divider circuit as Soundhound suggested. Hook up one divider between the hot and gound, the other divider between return and gound, with the small (low ohm) resistor at the ground end.

Have the output of the hot divider go to the right line level audio channel and the return divider go to the left line level audio input. Make certain that the gain (volume setting on the sound card) is the same for left and right channels. The best way to do this is to set the gain (volume) to max and change the voltage divider ratio (both voltage dividers must be the same).

Then, have the software subtract the left channel signal from the right channel signal, the resultant waveform will be the hot signal relative to the return signal, what you are interested in. Then you can do FFTs and other analysis on the waveform.

OOH! Pretty pictures.

Paul

Well, yes, duh....I was suffering from brain fade......there would obviously be current draw from the voltage devider. Realistically, I would probably increase the resistor values 100 times to 15K and 1.5K.

I know Ohm's law---I really do...

Thanks gentlemen,

I will scale by 100 and go with 5watt resistors. It's still much cheaper than $250 for any usuable scopes on e-bay.