Need help identifying resistor.

[Kim Christensen]

And like I said I had a feeling the multimeter was influencing this discrepancy.  I guess the part that I am having a hard time understanding is why when you switch from the course current setting to the mA setting why would there be a different voltage drop.  I guess the explanation is the course and fine current settings differ internally by that difference.

That's exactly the case. Multimeters set to "current" are actually millivoltmeters reading the voltage drop across various shunt resistors internal to the meter. The lower current ranges have higher resistance shunts than the high current ranges.
ie: When you change current ranges on your meter, you're inserting different value shunt resistors in series with the load.

I think I understand your 2nd suggestion.  The idea is you would be creating a known fixed current to take the measurements off of.  That is a pretty clever way to accomplish that by utilizing the power supplies current limiting capability ( although I did notice when it was happening in the beginning the current limiting would drift).  Thanks for the knowledge.  I think the multimeter and the resistor would fail at 40 A.

Most lab power supplies have adjustable current limiting. You would normally set this to a sane level such as 100mA or so.
In the absence of a power supply with built in current limiting, you could put a resistor in series with the output to limit the short circuit current to a sane level.
ie: If you had a known 100 ohm 2W resistor in the junkbox, you could put that in series and set the supply to 10V. Then the current would never go above 100mA.

[JJ_023]

And like I said I had a feeling the multimeter was influencing this discrepancy.  I guess the part that I am having a hard time understanding is why when you switch from the course current setting to the mA setting why would there be a different voltage drop.  I guess the explanation is the course and fine current settings differ internally by that difference.

That's exactly the case. Multimeters set to "current" are actually millivoltmeters reading the voltage drop across various shunt resistors internal to the meter. The lower current ranges have higher resistance shunts than the high current ranges.
ie: When you change current ranges on your meter, you're inserting different value shunt resistors in series with the load.

I think I understand your 2nd suggestion.  The idea is you would be creating a known fixed current to take the measurements off of.  That is a pretty clever way to accomplish that by utilizing the power supplies current limiting capability ( although I did notice when it was happening in the beginning the current limiting would drift).  Thanks for the knowledge.  I think the multimeter and the resistor would fail at 40 A.

Most lab power supplies have adjustable current limiting. You would normally set this to a sane level such as 100mA or so.
In the absence of a power supply with built in current limiting, you could put a resistor in series with the output to limit the short circuit current to a sane level.
ie: If you had a known 100 ohm 2W resistor in the junkbox, you could put that in series and set the supply to 10V. Then the current would never go above 100mA.

So the way you explain the current settings makes sense to me now.  I was aware of how multimeters function when measuring current.  My understanding is through a shunt resistor which is basically a piece  of thick wire of a known resistance (looks like a giant staple on the inside of a multimeter).

My problem is I am accumulating knowledge about these topics but I still lack the experience to string that knowledge together to get a higher understanding of the subject matter.  So it's like I am aware of and know all of the pieces individually but sometimes have a hard time fitting those pieces together.  So your explanation ( as well as others on this forum) help my understanding of the subject matter and how all of these things come together. 

I appreciate your suggestion on the 100 ohm resistor.  My gut feeling tells me it probably would not be as accurate as the 4 wire method that I implemented previously that was suggested to me by somebody on this post.  Is my assumption correct?

Again thank you for helping out.

[Kim Christensen]

My understanding is through a shunt resistor which is basically a piece  of thick wire of a known resistance (looks like a giant staple on the inside of a multimeter).

For the high current range, it's sometimes just a piece of wire. But for the lower ranges they'll be precision resistors.
Set one of your meters to measure resistance. Set the other one to measure current on the mA range. Connect the red lead of one meter to the red of the other. Do the same for black. Now change current ranges and watch the ohms reading on the other meter change.

I appreciate your suggestion on the 100 ohm resistor.  My gut feeling tells me it probably would not be as accurate as the 4 wire method that I implemented previously that was suggested to me by somebody on this post.  Is my assumption correct?

The 100 ohm resistor (For example) will not effect the accuracy of the measurement but does limit the resolution. It simply sets the max current through the entire circuit. If you changed it to 220 ohms, you'd still get the same result after working out ohms law on the DUT:

[Vovk_Z]

  I obtained this particular resistor from a donor board. 
According to the way that I interpret the color bands it should be .25 Ohm +/- 5%.
Is my interpretation correct?
I also measured with a multimeter and got .252 Ohm. 

1. It looks like a resistor. It is identified by you as 0.25 R resistor. It is measured by you as 0.25 R resistor.
2. If something looks like a duck, swims like a duck, and quacks like a duck, then it probably is a duck.

[JJ_023]

My understanding is through a shunt resistor which is basically a piece  of thick wire of a known resistance (looks like a giant staple on the inside of a multimeter).

For the high current range, it's sometimes just a piece of wire. But for the lower ranges they'll be precision resistors.
Set one of your meters to measure resistance. Set the other one to measure current on the mA range. Connect the red lead of one meter to the red of the other. Do the same for black. Now change current ranges and watch the ohms reading on the other meter change.

I appreciate your suggestion on the 100 ohm resistor.  My gut feeling tells me it probably would not be as accurate as the 4 wire method that I implemented previously that was suggested to me by somebody on this post.  Is my assumption correct?

The 100 ohm resistor (For example) will not effect the accuracy of the measurement but does limit the resolution. It simply sets the max current through the entire circuit. If you changed it to 220 ohms, you'd still get the same result after working out ohms law on the DUT:

Thanks for the explanation as well as the experiment suggestion.

Here it is what I got doing the range experiment.

On the A setting I could barely read any resistance it was flickering between .1 and 0 Ohms.

On the mA setting it showed 2 Ohms.

On the uA setting it showed 101.2 Ohms.

Now moving onto the resistor experiment again.  I set up the experiment and started cranking the voltage until I was slightly above 100mA.

Here are the results.

15.5 mv  (using a cheaper meter)

101.15 mA ( using the best meter)

This calculates to the resistor value of  .15324 Ohms

This is in line with the previous experiment, as well as confirming my colorblindness.

Thank you so much for walking me through this.  Like I said I knew all of this knowledge previously in terms of the different bits and pieces I just never thought of it at the time to put it together. 

My follow-up question is how accurate do you think that reading is?  If you had to guess a percentage.

The meter is capable of .1 % accuracy on DC current.  The meter that was used for the voltage is accurate to .8%

My guess would be around 1%.

[JJ_023]

  I obtained this particular resistor from a donor board. 
According to the way that I interpret the color bands it should be .25 Ohm +/- 5%.
Is my interpretation correct?
I also measured with a multimeter and got .252 Ohm. 

1. It looks like a resistor. It is identified by you as 0.25 R resistor. It is measured by you as 0.25 R resistor.
2. If something looks like a duck, swims like a duck, and quacks like a duck, then it probably is a duck.

In this particular case the duck turned out to be a duckling that measured .15 Ohm thus confirming my and everyone else's colorblindness.   

[Kim Christensen]

My follow-up question is how accurate do you think that reading is?  If you had to guess a percentage.
The meter is capable of .1 % accuracy on DC current.  The meter that was used for the voltage is accurate to .8%
My guess would be around 1%.

That sounds about right... Apx 0.9% if I calculated that correctly. (Ignoring the tiny voltmeter circuit loading error)

[wraper]

The meter is capable of .1 % accuracy on DC current.  The meter that was used for the voltage is accurate to .8%

My guess would be around 1%.

You need to look into full spec for the range you are using. Usually it comes as percentage + number of counts of least significant digit. So if you're measuring at the bottom of the range, it can be way worse than percentage spec. Also you need to consider resistor temperature (temperature coefficient of resistance), as your measurement can be right for the conditions but not exactly what you think it is.

[JJ_023]

The meter is capable of .1 % accuracy on DC current.  The meter that was used for the voltage is accurate to .8%

My guess would be around 1%.

You need to look into full spec for the range you are using. Usually it comes as percentage + number of counts of least significant digit. So if you're measuring at the bottom of the range, it can be way worse than percentage spec. Also you need to consider resistor temperature (temperature coefficient of resistance), as your measurement can be right for the conditions but not exactly what you think it is.

That is a valid point.  The good multimeter is .10% +2 on current.  The so-so multimeter is .8% +/-1.

I am well aware of temperature drift associated with resistors.  I do appreciate your statement and it would be valid for a particular instance where you are designing a circuit that would operate over different temperature ranges.  I am wondering how that statement is valid when I am trying to ascertain the value of it at a particular moment in time.  Wouldn't temperature already be considered in that equation?

Thank you for your input and time.   

[wraper]

Well, you measured the current and voltage drop across the resistor. Say you should be within 0.2% according to specs of your meters. The problem is that resistor could be at 120^oF at the time, for example. So while you measured with 0.2% tolerance, it's only true at this certain resistor temperature and can be quite a bit more off from resistance at room temperature. In this particular case, ~1.5mW of power dissipation won't meaningfully rise the temperature but you need to be aware of this in general. The same goes for meter specs, usually tolerance spec is given for a quite narrow temperature range. And that's considered it had been timely calibrated.
Although in general you get significantly more accurate measurements than the meter specs but you cannot be 100% sure without doing all proper procedures.

[JJ_023]

Well, you measured the current and voltage drop across the resistor. Say you should be within 0.2% according to specs of your meters. The problem is that resistor could be at 120^oF at the time, for example. So while you measured with 0.2% tolerance, it's only true at this certain resistor temperature and can be quite a bit more off from resistance at room temperature. In this particular case, ~1.5mW of power dissipation won't meaningfully rise the temperature but you need to be aware of this in general. The same goes for meter specs, usually tolerance spec is given for a quite narrow temperature range. And that's considered it had been timely calibrated.
Although in general you get significantly more accurate measurements than the meter specs but you cannot be 100% sure without doing all proper procedures.

Thank you for such an elaborate explanation. 

One of the things I do enjoy is reading data sheets, especially ones from Texas Instruments and Analog.  Those always have temperature charts indicating what you are talking about in a graphic.  I also know that whatever the rating is typically you try to discount 1/3 from the value.  So if a resistor might dissipate 3 W I would try to keep it around 2 W or less at whatever temperature that corresponds to.

I was wondering where you got the .2% from the specification of the meter.  I've never once heard or read such a statement so it is a little bit confusing.

[wraper]

I wrote 0.2% just as example, it's not about this particular case, the same as temperature having nothing to do with your measurement.