# Ri measuring

#### kuji5

Joined Dec 11, 2022
30
Hello there, I have a question. If I want to measure the resistance (Ri) of my power supply with a value of 0.5 Ohm, should I use the method where an ammeter is connected first and then a voltmeter? Additionally, can someone explain what causes deviation in this measurement method?

#### Ya’akov

Joined Jan 27, 2019
8,146
Hello there, I have a question.
Hello.

As this is course work, your thread has been moved to Homework Help. The rules of AAC limit the sort of help members can provide for course assignments to clarifying your own understanding. For this reason, all course work related questions must be posted to the Homework Help forum in order to make the nature of the question clear to other members. [Moderation]

#### MrChips

Joined Oct 2, 2009
29,194
Real meters will differ from ideal meters.
You need to take into account the properties of real meters.

#### kuji5

Joined Dec 11, 2022
30
Real meters will differ from ideal meters.
You need to take into account the properties of real meters.
Yes, I know that but I cant invent which methode should I use and why.

#### MrChips

Joined Oct 2, 2009
29,194
Think about the internal resistance of each meter.
Which one is significant enough to make a difference?
What should you measure in order to achieve your objectives?

#### kuji5

Joined Dec 11, 2022
30
Think about the internal resistance of each meter.
Which one is significant enough to make a difference?
What should you measure in order to achieve your objectives?
I know that in this method, the V meter also measures the Ri of the Ammeter, which is very small. However, I'm unsure which method is better for measuring small Ri, and I'm also wondering how Ri affects the measurement.

#### MrChips

Joined Oct 2, 2009
29,194
I know that in this method, the V meter also measures the Ri of the Ammeter, which is very small. However, I'm unsure which method is better for measuring small Ri, and I'm also wondering how Ri affects the measurement.
There are not two methods. There is only one method.
I will give you one hint.
The voltmeter has high internal resistance. It does not interfere with the measurement. Connect it and leave it connected. It measures voltage. It does not measure Ri.

#### crutschow

Joined Mar 14, 2008
32,841
There are not two methods. There is only one method.
That's not always true, although it may be in this case.

The best method depends upon the impedances of the meters.
You would need to calculate the power supply impedance with both configurations to determine which gives the least error.

The voltmeter first will cause the supply current to be slightly more than the measured current.
With the voltmeter second, the measured voltage will be slightly less than the power supply due to the ammeter internal resistance.