How to test Line Voltage with PC Scope?

[mikedmonds]

Dear Forum,

I have a line voltage circuit that I need to test using a non-isolated PC Scope (tower, plug-in).

Can I just put an isolation transformer on the circuit input and test away, or should I isolate the scope?

Thanks!
Mike

 

[Sceadwian]

What are the specs on the scope you're using?

 

[mikedmonds]

Hi Sceadwian,

It is a Dell GX280 with a Virtins PC Scope Probe, using a National Instruments based PC Scope program.

VIRTINS Sound Card Oscilloscope Probe Kits- the world's first Download manual

Probe Specification
Maximum Allowable Input Voltage: ±24 V
Built-in input protection circuit ensures its output to be less than 3V at all time, provided the input voltage is within the allowable range
Three switchable attenuation positions: 1, 2, 3, with input impedance equal to 1kΩ+sound card input impedance, 201kΩ+sound card input impedance, 10.001MΩ+sound card input impedance, respectively. Note that the sound card input impedance, either MIC IN or Line In, is card dependent and ranges from 600 Ω to 50 kΩ.
Frequency range: 0~200 kHz
Soft molded strain reliefs at all stress points assure long cable life.
Accessories include sprung hook, 6" ground lead.
Its back end is a RCA plug (male) and can be connected to an ordinary sound card through a stereo-to-2RCA converter or a mono-to-1RCA converter. No converter is required for those sound cards that have RCA jacks for input and output.
Can be used in Signal Generator for signal output. The output impedance is about 1 kΩ, 201 kΩ and 10.001 MΩ for attenuation switch Positions 1, 2, 3, respectively.
Probe kit for VIRTINS Sound Card Multi-Instrument series (1 channel, Part Number P601PC) (US$29.95, Order now)

Contains: 1 x Probe, 1 x 3.5 mm Stereo-to-2RCA converter, 1 x 3.5 mm Mono-to-1RCA converter and accessories.
You may need from 1~4 sets of this probe kit per license of the software, depending on your purpose of test. We recommend the standard configuration be 2 sets per license.

https://www.zeitnitz.de/Christian/scope_en

I would make sure to only measure voltages lower than 24V. I don't want to blow up my computer!

Thanks for your suggestions!

Sincerely,
Mike

 

[Sceadwian]

You can't isolate the scope because it's the computer, computers require true earth grounds do not attempt to use an isolation transformer on it. You would have to isolate the circuit.
Keep in mind the scope probe may be rated for 200khz but you'll never be able to read anything higher than a 22khz signal even if your sound card supports sampling rates higher because there is an extremely sharp cutoff filter at 22khz that can't be bypassed (without severe alterations to the sound card which generally aren't practical or safe)

Isolating the circuit would be highly recommended, it's not required if you make sure that the scopes ground is always true ground in relation to the circuit you're testing, the probe also has an attenuation switch that allow you to switch a resistance inline with the scope tip which can prevent damage from even a horrible mistake.

If you use a proper voltage divider network you could extend the voltage that you can measure by quiet bit, easily into the mains voltage range, but you lose resolution, the nice thing about using a sound card scope though is that even if it's slow the bit depth is usually much higher than conventional DSO's, they're usually only 8 bit, where as even a cheaper sound card will be garunteed to have a 16bit and very likley 24bit depth as well giving you a very good dynamic range.

 

[mikedmonds]

Thanks Sceadwian!
Sounds like the PC scope is a good way to go. I only paid $70 for the computer, and the probe was $30; so for $100 I've got a scope, plus a good computer! I only need to measure to 300Hz.
The sound card is 24 bit, so resolution shouldn't be a problem!
God bless,
Mike