Electronics > Beginners

How to measure voltage drop through resistor with oscilloscope?

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Chriss:
Hi!
I put a 1R8 resistor in serie with positive leg of a 2200uF electrolit cap.
Than I set my acope probe positive clip to one side of the resistor and the negative clip to the other side of the resistor.
I feed the circuit with 12VDC.
Put +12v to the resistor and gnd to the minus leg to the cap.
The sine wave on the scope was very interesting for me.
I didn't expected such of diagram.
The voltage started to rise up and suddenly fall back when it reached aroun 5.6v to around 3v and then it started to rise slow up to almost 12v.

Can somebody help to figure out what I did wrong?
Or is that measuring ok?
I also built up in LTS. and MultiSim apps but the result was diferent than my real measuring.

Thanks for any help and suggestion.

ArthurDent:
You are shorting out the cap from what you described. Watch video.

Chriss:
So, what would be the correct way to check the voltage drop trough a resistor in the circuit?
Let's say in a circuit where with the cap is a res in series?

Thanks.

rstofer:

--- Quote from: Chriss on February 22, 2020, 10:22:28 pm ---So, what would be the correct way to check the voltage drop trough a resistor in the circuit?
Let's say in a circuit where with the cap is a res in series?

Thanks.

--- End quote ---

If you insist on doing it with a single channel of a scope, one side of which is grounded to earth through the BNC connector, you need a differential probe.  Good ones are VERY expensive.

Or, you can use two channels (with no ground clips) and the MATH function to get X-Y.  Sometimes you only have X+Y and you have to invert Y but these choices are fairly clear on the menu.  Check the user manual for math functions.

ETA

All of the above assumes your project has an earth reference somewhere.  Without a schematic or a picture of the scope image, that's about all we can say.  If you don't have an earth reference then everything above is nonsense.  But we need pictures.

The X-Y thing will work whether you have an earth reference or not so give it a try.

Chriss:
Here is my circuit and waveform.

My scope and testing circuit are complete isolated from the main power source.
I'm trying to achieve to measure the voltage drop through a shunt res to calc the inrush current.
It is hard to measure with average dmm but I got an idea somehow to determine the voltage drop with
my scope and the make the math.

If that is actually possible to do with a scope.

btw. when I try to simulate this circuit and measure it with LTS and MultiSim I got no reading on the scope and dmm also in the app.
Only 0v.

And here is what LTS simulate when I put the resistor to the positive side.
The blue line is the I(R3)...

I can do whatever I wish with my real circuit I can't get that value like LTS.

Thanks for any help.

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