Oscilloscope problematic.

Hello Ole, people,

I have problems with my new scope.

I wanted to measure the current at a current shunt 50mV per ampere, so I connected the probe to both contacts and wanted to measure the differential voltage. Unfortunately I have earth connection now. The energy flows through the scope. It is not measurable. With the Velleman in battery operation I could look at the sine nicely. What am I doing wrong?

I wanted to measure the voltage curve and current curve simultaneously in the reactive circuit.

regards

Sven

Hi Sven,

Scroll down to digital oscilloscope on this page: https://meettechniek.info/measuring/power.html https://meettechniek.info/measuring/power.html
 Fig. 10: Power measurement with an oscilloscope and shunt resistor. A separation transformer has to be used if measuring on the mains as oscilloscopes generally haven't got isolated grounds. Else a differential probe or a current probe has to be used. The scope itself can be used in differential mode which requires two channels like channel A minus B or A plus inverted B. This will require three channels for a power measurement though.

Measuring low voltage across a current shunt resistor may require a X1 probe or just a 50 Ohm coaxial cable. To avoid measuring too much noise the scope input impedance also must be 50 Ohm like the cable. If the scope doesn't have a switch to change the input impedance to 50 Ohm form the normal 1 MOhm impedance a feed through 50 Ohm BNC adapter is also required. Without the low impedance the signal is reflected when meeting the high impedance of the scope input. Besides that a lot of noise is generated in the high resistance of the usually 1 MOhm input resistor which isn't damped when measuring across a much lesser resistance (that of the current shunt).

Regards
Ole

 

---In [email protected], <smokyatgroups@...> wrote :

 Hi Sven, Try to have only one single earth point in any circuit.
 

 If you are using a 2 channel scope the earth clips on both the the probes are connected together. inside the scope so we are unknowingly shorting the 2 earth clip points together.
 Also often our scopes are earthed back to the ac mains supply, so where we place our earth clips creates an actual earth at that point.
  
 So have the current shunt in the low earthy side of generator source and clip the scope probe earth clip there at that single earth point.
 Leave the earth clip off the voltage measuring probe altogether, just leave it free ...unconnected, or connect it back to same point as the current probes earth point.
 

 When doing your Volts and Amps calculations just remember to subtract the current shunt voltage from the applied voltage measurement.
 As this voltage drop across the shunt is lost to the circuit under test so the circuit "sees" the generator applied voltage minus the current shunt voltage.
 

 If you really need to make a differential measurement between 2 points with an analog 2 channel scope, you can hook both probes to either side of say a shunt resistor.
 Select Channels A+B on the scope and invert one Channel, but to get accurate readings both channel gains need to be identical and both Y Volts/div settings have to be the same on both channels.
 

 Or can use battery powered scope where whole scope and earth clip can float above earth potential.
 Just be aware of Danger from body of scope at high potential if applicable and stray effects at higher frequencies & impedances.
 

 Gerry
 

 

 
 



 
 On Sun, Dec 17, 2017 at 2:57 AM, s.friedrich@... mailto:s.friedrich@... [EVGRAY] <[email protected] mailto:[email protected]> wrote:
 

 
Hello Ole, people,

I have problems with my new scope.

I wanted to measure the current at a current shunt 50mV per ampere, so I connected the probe to both contacts and wanted to measure the differential voltage. Unfortunately I have earth connection now. The energy flows through the scope. It is not measurable. With the Velleman in battery operation I could look at the sine nicely. What am I doing wrong?

I wanted to measure the voltage curve and current curve simultaneously in the reactive circuit.

regards

Sven

Hi Sven,

Doesn't it say the resistance on the shunt resistor. If it is 50 mV out when 200 A going through it its resistance is (50 mV) / (200 A) = 0.25 mOhm or 250 µOhm.

If it's 50 mV per A the resistance is (50 mV) / (1 A) = 50 mOhm.

In the first example power dissipation at 200 A is (200 A)2 X 0.25 mOhm = 10W. In the second case the power dissipation will be (200 A)2 X 50 mOhm = 2 kW.

Unless it's big as a room heater it must be only 250 µOhm resistance. That gives 0.25 mV per A through it.

Regards
Ole




---In [email protected], <s.friedrich@...> wrote :

 Hi Ole,
 
 I connected the scope exactly as in the drawing. Use an X1 probe and set the lowest voltage. There is only noise. With the Vellemann I go on the pictures you can see on the weak display the current flow.
 
 I have the feeling that the earth connection is a problem. When the scope is connected I have ground potential via the phase tester.
 
 I always use a isolation transformer.
 
 Maybe the shunt is too big. I upgraded it from an old 30kw inverter. There is 50mV and 200A on it. I go from 50mV per ampere or am I mistaken!?!
 
 regards
 
 Sven   Gesendet: Samstag, 16. Dezember 2017 um 18:54 Uhr
 Von: "onielsen@... [EVGRAY]" <[email protected]>
 An: [email protected]
 Betreff: Re: [EVGRAY] Oscilloscope problematic.
   Hi Sven,
 
 Scroll down to digital oscilloscope on this page: https://meettechniek.info/measuring/power.html https://meettechniek.info/measuring/power.html
 Fig. 10: Power measurement with an oscilloscope and shunt resistor. A separation transformer has to be used if measuring on the mains as oscilloscopes generally haven't got isolated grounds. Else a differential probe or a current probe has to be used. The scope itself can be used in differential mode which requires two channels like channel A minus B or A plus inverted B. This will require three channels for a power measurement though.
 
 Measuring low voltage across a current shunt resistor may require a X1 probe or just a 50 Ohm coaxial cable. To avoid measuring too much noise the scope input impedance also must be 50 Ohm like the cable. If the scope doesn't have a switch to change the input impedance to 50 Ohm form the normal 1 MOhm impedance a feed through 50 Ohm BNC adapter is also required. Without the low impedance the signal is reflected when meeting the high impedance of the scope input. Besides that a lot of noise is generated in the high resistance of the usually 1 MOhm input resistor which isn't damped when measuring across a much lesser resistance (that of the current shunt).
 
 Regards
 Ole
  
 
 
 ---In [email protected], <smokyatgroups@...> wrote :
   Hi Sven, Try to have only one single earth point in any circuit.
  
 If you are using a 2 channel scope the earth clips on both the the probes are connected together. inside the scope so we are unknowingly shorting the 2 earth clip points together.
 Also often our scopes are earthed back to the ac mains supply, so where we place our earth clips creates an actual earth at that point.
  
 So have the current shunt in the low earthy side of generator source and clip the scope probe earth clip there at that single earth point.
 Leave the earth clip off the voltage measuring probe altogether, just leave it free ...unconnected, or connect it back to same point as the current probes earth point.
  
 When doing your Volts and Amps calculations just remember to subtract the current shunt voltage from the applied voltage measurement.
 As this voltage drop across the shunt is lost to the circuit under test so the circuit "sees" the generator applied voltage minus the current shunt voltage.
  
 If you really need to make a differential measurement between 2 points with an analog 2 channel scope, you can hook both probes to either side of say a shunt resistor.
 Select Channels A+B on the scope and invert one Channel, but to get accurate readings both channel gains need to be identical and both Y Volts/div settings have to be the same on both channels.
  
 Or can use battery powered scope where whole scope and earth clip can float above earth potential.
 Just be aware of Danger from body of scope at high potential if applicable and stray effects at higher frequencies & impedances.
  
 Gerry
  
  
    
 
 
   On Sun, Dec 17, 2017 at 2:57 AM, s.friedrich@... mailto:s.friedrich@... [EVGRAY] <[email protected] mailto:[email protected]> wrote:   
 Hello Ole, people,
 
 I have problems with my new scope.
 
 I wanted to measure the current at a current shunt 50mV per ampere, so I connected the probe to both contacts and wanted to measure the differential voltage. Unfortunately I have earth connection now. The energy flows through the scope. It is not measurable. With the Velleman in battery operation I could look at the sine nicely. What am I doing wrong?
 
 I wanted to measure the voltage curve and current curve simultaneously in the reactive circuit.
 
 regards
 
 Sven

Empty body

Empty body

Empty body

Empty body

Hi Sven,

Nice shot. The power is reactive as the curves are 90 degrees out of phase. The core is being saturated shown by the narrow peaks of the current.

I was looking in the manual for math functions to calculate the power waveform. This scope can only add and subtract the channels just like the old analog oscilloscopes. I wonder why they call it mathematical functions with only those two possibilities which is standard on analog oscilloscopes without calling them math functions! To calculate the power waveform the two channels need to be multiplied.

The waveforms can be transferred to an external USB storage. I wonder in what format. If the data points can be transferred to a spread sheet the calculations can be done externally. The scope is running open sourced Linux software.

Found a YouTube video showing the file transfer with another model (and brand). "Hantek DSO5062B Oscilloscope to PC Interface Review" https://www.youtube.com/watch?time_continue=55&v=GCp0uf_-iZY https://www.youtube.com/watch?time_continue=55&v=GCp0uf_-iZY

Look if there's software for data transfer to a computer or a spread sheet for further calculations if you want to measure power.

Regards
Ole

---In [email protected], <s.friedrich@...> wrote :

 I got it in the triggering you could still set values and a little patience. Thank you
 
 regards
 
 Sven
...

Hi Sven,

Nice shot. The power is reactive as the curves are 90 degrees out of phase. The core is being saturated shown by the narrow peaks of the current.

I was looking in the manual for math functions to calculate the power waveform. This scope can only add and subtract the channels just like the old analog oscilloscopes. I wonder why they call it mathematical functions with only those two possibilities which is standard on analog oscilloscopes without calling them math functions! To calculate the power waveform the two channels need to be multiplied.

The waveforms can be transferred to an external USB storage. I wonder in what format. If the data points can be transferred to a spread sheet the calculations can be done externally. The scope is running open sourced Linux software.

Found a YouTube video showing the file transfer with another model (and brand). "Hantek DSO5062B Oscilloscope to PC Interface Review" https://www.youtube.com/watch?time_continue=55&v=GCp0uf_-iZY https://www.youtube.com/watch?time_continue=55&v=GCp0uf_-iZY

Look if there's software for data transfer to a computer or a spread sheet for further calculations if you want to measure power.

Regards
Ole

---In [email protected], <s.friedrich@...> wrote :

 I got it in the triggering you could still set values and a little patience. Thank you
 
 regards
 
 Sven   ...

Hi Sven, Sometimes the high current shunts are 4 terminal resistors.
 Usually the two larger terminals are for the current in and out connection.
 The actual nominal value of resistance is only correct between the voltage terminals.
 Which aren't designed to carry large currents.
 

 They are also calibrated in so many nominal mV per Amp rather than Ohms.
  Gerry

Hi Sven,
Try to have only one single earth point in any circuit.

If you are using a 2 channel scope the earth clips on both the the probes
are connected together. inside the scope so we are unknowingly shorting the
2 earth clip points together.
Also often our scopes are earthed back to the ac mains supply, so where we
place our earth clips creates an actual earth at that point.

So have the current shunt in the low earthy side of generator source and
clip the scope probe earth clip there at that single earth point.
Leave the earth clip off the voltage measuring probe altogether, just leave
it free ...unconnected, or connect it back to same point as the current
probes earth point.

When doing your Volts and Amps calculations just remember to subtract the
current shunt voltage from the applied voltage measurement.
As this voltage drop across the shunt is lost to the circuit under test so
the circuit "sees" the generator applied voltage minus the current shunt
voltage.

If you really need to make a differential measurement between 2 points with
an analog 2 channel scope, you can hook both probes to either side of say a
shunt resistor.
Select Channels A+B on the scope and invert one Channel, but to get
accurate readings both channel gains need to be identical and both Y
Volts/div settings have to be the same on both channels.

Or can use battery powered scope where whole scope and earth clip can float
above earth potential.
Just be aware of Danger from body of scope at high potential if applicable
and stray effects at higher frequencies & impedances.

Gerry





On Sun, Dec 17, 2017 at 2:57 AM, [email protected] [EVGRAY] <
[email protected]> wrote:

>
>
>
> Hello Ole, people,
>
> I have problems with my new scope.
>
> I wanted to measure the current at a current shunt 50mV per ampere, so I
> connected the probe to both contacts and wanted to measure the differential
> voltage. Unfortunately I have earth connection now. The energy flows
> through the scope. It is not measurable. With the Velleman in battery
> operation I could look at the sine nicely. What am I doing wrong?
>
> I wanted to measure the voltage curve and current curve simultaneously in
> the reactive circuit.
>
> regards
>
> Sven
>
>
>
>
> 
>

Empty body

Wow Sven,
That's a heavy duty shunt you have there!
The big bolts are to carry the large current under test in and out of the
resistor via brass blocks on either end.

The two smaller connections are the voltage measuring points, the
resistance is correct between these two points.
It is correct to place your measuring device here, but like me & Ole
said.... have to pay attention to your earthing points.

I can see a high current connection to the left end but not the right hand
end of the shunt?
Where does this current come out?

I don't exactly understand your words in your email above "I still need
capacitors for the sheath current filter because I lie partially over 600V,
or in series...."

Do you mean the whole shunt is floating at 600V above/below  ground
potential?

Sorry... me easily confused

Since shunt is intended for up to 200A and you are only using it at 35A the
voltage per amp is going to be quite small.
So  noise and contact potential errors may creep in through dissimilar
types of metal at junctions.

I do recommend to try a clip on type ammeter or Hall effect probe for these
kinds of measurements.
Gerry.

Here is a link to some other shunt resistors also
https://www.jaycar.com.au/search?text=current+shunt&CSRFToken=0cf93697-a46e-4fc8-ada7-969a3dc631c3








On Sun, Dec 17, 2017 at 10:53 PM, 'Sven Friedrich' [email protected]
[EVGRAY] <[email protected]> wrote:

>
>
> Hi Gerry, I measure at this shunt. The small lines that are led out, I
> connect to my scope.
> I think 200A max. rich for my max. 35 amps in transverter reactive.
> I still need capacitors for the sheath current filter because I lie
> partially over 600V, or in series.
>
> regards
>
> Sven
>
> *Gesendet:* Sonntag, 17. Dezember 2017 um 02:31 Uhr
> *Von:* "[email protected] [EVGRAY]" <[email protected]>
> *An:* [email protected]
> *Betreff:* Re: Aw: Re: [EVGRAY] Oscilloscope problematic.
>
>
> Hi Sven,
> Sometimes the high current shunts are 4 terminal resistors.
> Usually the two larger terminals are for the current in and out connection.
> The actual nominal value of resistance is only correct between the voltage
> terminals.
> Which aren't designed to carry large currents.
>
> They are also calibrated in so many nominal mV per Amp rather than Ohms.
>  Gerry
>
>
>
>
>
> 
>