Using the ground lead on the LeCroy LC584AXL.... 1

[melone]

I am trying to measure the voltage across a series resistor (in the power leg of my supply circuit?, and would like to use only 1 scope probe. Is there any easy way to do this without floating the scope? I really don't want to take 2 absolute readings and perform math (can't trigger on math function, and resolution is limited).

Thanks,

 

[IRstuff]

If it's on the hot side, you're pretty much out of luck, without differential probes.

You could breadboard a unity-gain diff amp and glue it to your probe and power it with a couple of 9V batteries.

TTFN

 

[busbar]

If AC, a current transformer might suffice, but ‘scope floating can be a bit Darwinian. Burr-Brown isolation amplifier?

 

[kodabear]

Most scopes tie the probe ground to Earth ground (the third prong on the AC power plug). This means that if you use one of those adapters meant to add a third prong to an outlet that only has hot and neutral, and don't connect the adapters third prong connector to the outlet, you now have a differential probe. This only works if you are using one probe because generally all probe grounds are tied together.

 

[steveowens]

Power the scope with a 12V battery and an inverter. Now you can put scope "gnd" anywere.

Steve Owens
Finish Line Product Development Services
Steve.Owens@FinishLinePDS.com

http://www.FinishLinePDS.com

 

[melone]

KODABEAR, what you are suggesting is floating the scope.

 

[electricuwe]

May be you can supply power to your circuit via an isolation transformer.

 

[melone]

I tried floating the scope and it still didn't work. (This should answer both steveowens, and electricuwe's posts) Therefore, it appears that the negative of the scope probe is connected to neutral on the incoming power. I am still trying to figure out the logic on this one. The only thing that I can come up with is for safety.

 

[Lewish]

melone, you are correct, most O'scopes tie the neutral of the incoming power line to the negative side of the scope's power supply output, which is the negative side of your probe. I let a friend borrow my scope and offered him an isolation transformer to use with it. He felt he didn't need the transformer for what he was doing. Well, a $3500.00 repair bill later, the scope still doesn't work correctly all the time.

Here is something that might help you. I came across it yesterday while looking for something else. - Electronic Design, Ideas for Design, Aug. 19, 2002, page 62. Titled: "High Voltage Monitor Features High Accuracy". It will give better than 20ppm accuracy up to 400Volts. The article can be downloaded in .pdf format.

The only other option is a differential probe, but then you have to decide what to do with the ground terminal on it.

 

[melone]

Doesn't the isolation transformer perform the same function as floating the scope? If the neutral is tied to probe negative inside the scope, how does a transformer help? I am not critisizing the suggestion, I just don't understand it.

Thanks for the help!

 

[busbar]

melone—just for giggles—what is the voltage to ground on the series resistor of interest?

 

[melone]

Anywhere from 1.9 to 2.2V....why?

 

[busbar]

Apparently, there are no imminent safety concerns.

Accessories like

http://home.europa.com/~oatpmk/ampintro.htm

or

http://www.linkinstruments.com/adf25.htm

are probably out of budget.

 

[Lewish]

Yes, an isolation transformer does float the scope. But, floating the scope allows its chassis to be at whatever potential you connect the probe ground lead to. I do this all the time with motor drive electronics.

How did you float your scope? Was it with an isolation transformer? I ask since you say floating your scope didn't work. I can't think of any reason that would not have worked.

 

[melone]

I used an 3 prong to 2 prong adapter that eliminates the earth ground connection at the power outlet. I thought that the earth ground was connected to neutral (inside the scope) and by removing its connection at the wall outlet, the neutral could then "float" to any potential.

 

[nbucska]

What is the bandwidth you are interested in ? <nbucska@pcperipherals.com>

 

[melone]

DC - 200MHz

 

[nbucska]

make a diff--single ended amplifier <nbucska@pcperipherals.com>

 

[Lewish]

Sorry, melone, but that is exactly what my friend did when he blew up my O'scope. That is not floating a scope. That is only removing the Earth connection. You can only float a scope thru an isolation transformer.

The result is, that you can end up with a difference in potential between the ground of the scope and the ground of your circuit and may possibly induce very LARGE ground currents into the input amplifier of your scope.

My friend was trying to do something similar to you. He circuit had about 12 volts across the current sense resistor. The Tektronix repair person estimated that something like 100 Amps was dumped into the vertical amplifier input. It melted a lot of stuff.