Measuring Constant Voltage with an Oscilloscope

[JohnKElectroTech]

I'm a newbie of course, and I'm using a PC-based oscilloscope to measure the voltage on a new 1.5V AA battery. I applied a probe to the positive terminal of the battery with the negative untouched. I get a repeating, vaguely sinusoidal pattern with a frequency of about 20 ms and amplitude ranging from about +2V to -2V. Can you tell me why I don't get a constant voltage of about 1.5V? Other settings are trigger = auto, time/division = 10 ms, Volts/div = 1 V, DC, ground = 0, YT (instead of XY).

Thanks in advance for your help.

 

[MikeMl]

You need to hook the shield of the probe (or coax cable) to the minus side of the battery. Without the shield hooked up, the entire battery is picking up AC hum (50/60Hz depending on your country) which is driving the scope input. If you hook up the shield to negative, that should disappear. The same thing will happen if you just touch the probe tip (center conductor of the coax) with your finger...

 

[Roff]

What type of PC-based scope are you using?

 

[bountyhunter]

I'm a newbie of course, and I'm using a PC-based oscilloscope to measure the voltage on a new 1.5V AA battery. I applied a probe to the positive terminal of the battery with the negative untouched. I get a repeating, vaguely sinusoidal pattern with a frequency of about 20 ms and amplitude ranging from about +2V to -2V. Can you tell me why I don't get a constant voltage of about 1.5V?

Because you need a real oscilloscope.

 

[Dx3]

All voltages are volts compared to someplace else. If you don't connect the ground wire of the probe to the battery, you are measuring a AA size antenna compared to earth ground. If you connect the scope ground to the battery negative, you are measuring the + voltage of the battery compared to the - end of the battery.

 

[JohnKElectroTech]

Thanks, Mike and DX3. Your suggestions fixed the problem. Roff - the scope is a DS1M12 by usb instruments.

 

[audioguru]

Will a pc based 'scope measure a DC voltage? It uses a sound card which is AC-coupled, not DC coupled.

 

[Roff]

Will a pc based 'scope measure a DC voltage? It uses a sound card which is AC-coupled, not DC coupled.

That's why I asked what type of scope it is. The DS1M12 apparently does not use the sound card, as it is DC coupled.

 

[kjennejohn]

In this case, PC based means it is a small box that uses the PC's USB port for data streaming (maybe even power) and the OS graphical capability to plot the signal(s). This as opposed to a full standalone o-scope with its own display screen.
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