Re: Aw: Re: [EVGRAY] Oscilloscope problematic.

Body

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