Doubt about voltage drop measurement

Thread Starter

viraj.dali

Joined Oct 20, 2016
36
Hallo

I have came across the problem of voltage drop on my PCB.

Please refer the attached image.

There is 24V signal. To this 24V signal I have connected 0.1 Ohm resistance.
I want to measure voltage drop across this 0.1Ohm resistance.
If I connect my voltmeter across resistance directly as shown in image with red and green probe, I receive the correct voltage drop.

But I connect the probe of voltage to connector with red and blue probe, the output voltage drop is increased with 200mV.
Can somebody please let me where does voltage gets drop ??

Is there possibility that voltage drops due to long copper tracks on PCB. ???
Or Can voltage get dropped due to adjustsent tracks on PCB??

Circuit (1)_viraj.dali.png
 

MrChips

Joined Oct 2, 2009
26,077
You have about 5A flowing through the 0.1Ω resistor. The current flowing through the DMM is about 2μA.

No, the difference is not caused by current flowing through the DMM leads nor the PCB tracks, assuming that you are measuring the voltage drop at the same points across the 0.1Ω resistor.

If you are not measuring voltage at the same nodes then all bets are off.
100mΩ is very small. Any additional path that the 5A has to flow will make a difference in your voltage reading.
 

ScottWang

Joined Aug 23, 2012
7,201
The picture of the pcb that you uploaded just a simulation pcb not a real one then how did you measured the voltage?

I can't see the wire connection between the positive of 300mV and the positive 500mV From the picture, did you measure that two points?
 

Thread Starter

viraj.dali

Joined Oct 20, 2016
36
The picture of the pcb that you uploaded just a simulation pcb not a real one then how did you measured the voltage?

I can't see the wire connection between the positive of 300mV and the positive 500mV From the picture, did you measure that two points?

From the picture red and blue are the wire I am connecting
 

RamaD

Joined Dec 4, 2009
328
1. Measure the voltage between the 24V point on the resistor and the connector point where it is supposedly connected.
2. Measure the voltage between the 23.7V point on the resistor and the connector point where it is supposedly connected.

Both should be zero.
 

MisterBill2

Joined Jan 23, 2018
11,875
Hallo

I have came across the problem of voltage drop on my PCB.

Please refer the attached image.

There is 24V signal. To this 24V signal I have connected 0.1 Ohm resistance.
I want to measure voltage drop across this 0.1Ohm resistance.
If I connect my voltmeter across resistance directly as shown in image with red and green probe, I receive the correct voltage drop.

But I connect the probe of voltage to connector with red and blue probe, the output voltage drop is increased with 200mV.
Can somebody please let me where does voltage gets drop ??

Is there possibility that voltage drops due to long copper tracks on PCB. ???
Or Can voltage get dropped due to adjustsent tracks on PCB??

View attachment 143411
Based on years of experience as well as instrumentation best practices, the ONLY way to read voltage drop across a current shunt, which is what your 0.1 ohm resistor is, is to connect correctly across that shunt resistance. Connections at any other point will show the voltage drop across the added resistances as well, and will give the wrong results.
 
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