Meausing voltage across a shunt resistor with an oscilloscope 2

[davidd31415]

Hi,

I am trying to measure the voltage across a shunt resistor with an oscilloscope (actually, one of Yokogawa's 'ScopeCorders'). Since a DC current flows through this resistor in two directions, neither end is at ground potential.

Can this voltage be measured with a single voltage probe?

If not, would this be an application for a diff probe?

Thanks,

David

 

[Skogsgurra]

It depends a lot on what bandwidth you need. And also if your shunt is in a system that is connected to the mains or not.

If you only want to measure low frequencies, below - say - a few hundred Hz, and your circuit is isolated from the mains (battery powered, for instance), then you can use a standard scope input.

In all other cases, you need either a differential input or an external differential probe.

As I said - it depends. If mains connected, do not do it.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[waross]

I believe that your zero reference connection must be to one end of the resistor. As Gunnar says, in many instances this may not be done safely and a differential probe is needed.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[IRstuff]

Unless you know a LOT about the circuit, you should use a differential probe, or take the difference of two separate scope inputs, which most scopes can do.

Even when a resistor is tied to "circuit ground," the circuit ground itself may actually be incidentally coupled to the AC mains. Since scope grounds tend to tied to actual and real AC grounds, spectacular results may ensue.

TTFN

FAQ731-376

 

[davidd31415]

IRstuff, when you say "incidentally coupled to the AC mains," do you mean that the circuit may be coupled to the hot or neutral ?

I'm unclear of the difference between the "actual and real" AC grounds. Would you mind elaboarting?

Thanks,

David

 

[IRstuff]

In two-wire AC-connected devices, the designer might have connected, directly or indirectly, the DC ground to the AC neutral, on the anticipation that the AC neutral is indeed "neutral." However, either errors in design or in fabrication might leave you with the other connection, or, your local AC might have a residual voltage relative to earth ground.

TTFN

FAQ731-376

 

[ScottyUK]

The positve and negative outputs of line-fed bridges are alternately tied, less a diode V_f drop, to either the AC line or the neutral, or in the case of a three-phase bridge tied to whichever lines are most positive and most negative at that moment in time. If you study the waveforms you will see why this is the case.


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If we learn from our mistakes I'm getting a great education!

 

[OperaHouse]

A Hall Effect DC current sensing transformer may be the way to go. This requires no direct electrical connection and even inexpensive ones are good to more than 10KC.

 

[ScottyUK]

"10KC"

This is the 21st Century you know!


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If we learn from our mistakes I'm getting a great education!

 

[Skogsgurra]

Right! It should be kcps!

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[roydm]

I have a table of kcps to Hz if you need it.

 

[Skogsgurra]

Wouldn't a kcps to kHz table be more versatile?

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[roydm]

Yes, you'r right, I have a graph as well

 

[Skogsgurra]

Graphs are much better, but tend to be somewhat expensive. I use a slide rule if I need real good precision.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[jimgineer]

Please fill me in, this is the first I've seen the acronem 'kcps'.

Thanks.

 

[geekEE]

It's an old frequency measurement term. Here's the handy graph for converting between the two units that others have referred to.

Glenn

 

[MacGyverS2000]

geek, thanks for posting the graph. I'll have to super-size it and print it out to hang on the cube wall for reference. ;-)

Dan - Owner

http://www.Hi-TecDesigns.com

 

[zeusfaber]

Going back to the original question, you could see if you could find a scope which wasn't tied to ground itself (I use one of Fluke's Industrial Scopemeters).

A.

 

[ScottyUK]

Jim,

KCPS: kilo-cycles per second. Directly equivalent to kHz. Used by oldtimers who learned their craft in The Good Old Days.


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If we learn from our mistakes I'm getting a great education!