On 2/11/2013 10:37 AM, Jeff Liebermann wrote:
On Mon, 11 Feb 2013 08:34:04 -0800, Jeff
wrote:

On Mon, 11 Feb 2013 14:55:42 -0000,
wrote:

Without differential scope amplifier/probe,

Then get a differential scope. You have one in your computah in the
form of the stereo input of your sound card. Just install an
oscilloscope emulator program. Put some large value resistors in
series with the inputs so you don't blow anything up. Favorite
softwa
http://www.sillanumsoft.org
You'll also find it handy when troubleshooting constant voltage
speaker system.

I guess I should offer a few warnings to prevent the usual disasters.
I sometimes use a sound card scope when sniffing around switching
power supplies, where the input section is usually ungrounded. It's
kinda difficult to see the waveforms due to the limited bandwidth of
the sound card, but I have a good imagination. However, there are a
few precautions.

1. Use a USB sound card dongle and not the device on the computah
motherboard. If you do something wrong, and burn up the motherboard
sound chip, you will not be very happy. A $3 dongle is essentially a
throw away device. It's also nice to have a long extension on the USB
cable, instead of a long cable on the probe.

2. Use a protective resistor in series with the input cable. It
protects against overcurrent, but also protects against idiot errors,
such as unplugging the 3.5mm jack while the probes are connected to
the AC mains. The jack will be shorted on its way out, and will make
a nifty spark.

3. Build an attenuator if you're playing with high voltages. My
guess(tm) is that the sound card input can handle about 5V peak to
peak. 10:1 for audio is fine. 100:1 for high voltage. Also, don't
count on the input capacitor, which is probably only good for about
15VDC.


Can you be more descriptive about what you're doing?
If you're connecting a sound card to the line side of a switcher,
I can't imagine anything more dangerous...as in smoke/fire/death.