Why the current is tremendous high?

[bittware]

Hello experts,
I use a 0.1ohm(25Watt) resistor in series with DC motor winding in order to sample the current flowing through motor.
I expect to see the voltage about 30~40mV(e.g. 300~400mA), but the voltage displayed in oscilloscope is tremendous high which reach above 40V!!!
I use 1Mohm oscilloscope probe, the pin put on the one end of resistor and the gnd reference put on the other end of resistor.
I am puzzled. Please help me to point out where I did wrong.
Thanks in advance.

 

[whitenoise]

Obviously you measure isn't correct (I hope for your motor safety :wink: )
Check you measure system:
-the resistor is connected with one terminal to ground?
-the gnd of the oscilloscope probe is floating respect the earth?
-are you sure that the resistor is 0.1 ohm?

excuse my curiosity ... why have you used a 25W resistor when you expected a power dissipation of tens of milliwatt?

bye wn.

 

[batdin]

That's right,your oscilloscope ground may be the problem.
In this case you will see AC voltage,not DC.
Try isolating the ground by using a sticky tape to prevent the contact between scope plug groung and AC mains ground.And try not to touch the metal parts afterwards.
That is,if your resistor is really 0.1 ohms as the previous hint was.

 

[bittware]

Obviously you measure isn't correct (I hope for your motor safety :wink: )
Check you measure system:
-the resistor is connected with one terminal to ground?
-the gnd of the oscilloscope probe is floating respect the earth?
-are you sure that the resistor is 0.1 ohm?

excuse my curiosity ... why have you used a 25W resistor when you expected a power dissipation of tens of milliwatt?

bye wn.

Hello whitenoise,
The gnd of the oscilloscope probe is floating. Must gnd probe be connected with earth? Doesnt the display in oscillosocpe represent the voltage difference between the two probes?
As for the power of resistor, I am not so sure if it is 25Watt. I guess it from it surface lable "RHAK25". It is relative big.

 

[bittware]

That's right,your oscilloscope ground may be the problem.
In this case you will see AC voltage,not DC.
Try isolating the ground by using a sticky tape to prevent the contact between scope plug groung and AC mains ground.And try not to touch the metal parts afterwards.
That is,if your resistor is really 0.1 ohms as the previous hint was.

Hello batdin,
I am sorry I didn't understand very well about your explain"Try isolating the ground by using a sticky tape to prevent the contact between scope plug groung and AC mains ground.And try not to touch the metal parts afterwards."
Is there any relationship between gnd probe and AC mains ground?

 

[pisoiu]

...Is there any relationship between gnd probe and AC mains ground?

All lab equipment and other mains powered equipments in metal chasis have their chasis connected to earth, third connection on wall plug. It is a matter of safety. If your DUT have a galvanic connection to mains, when you touch the probe GND to that equipment, actually you connect that point to earth. If you touch the gnd probe to mains null, usually nothing happen, but if you touch the phase, usually something noisy and smokey will happen...

/pisoiu

 

[RoZ]

> Doesnt the display in oscillosocpe represent the voltage difference betwe
en the two probes?

If You have a two channel scope, the safest is two connect both grounds and
simply use both 'hot inputs' to measure the voltage difference (Y1-Y2).

? What kind of probes are You using. Usual probes often have a builtin voltage-divider (10:1) which is in newer (digital) scopes already accounted for and which should be explicitly disabled in the menu.

RoZ

 

[rikie_rizza]

How much is the supply voltage? If you want to sample current flow, than you should have a force before you can sense....If you want to sense current, you should force voltage...or the other wise.