Need help identifying resistor.

[JJ_023]

Attached to this Post is a resistor that is in question.  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.  I do not put a lot of stock into this as I know that low ohm values are not accurately measured with the multimeter and a 4 wire method should be used.

Because of that fact I decided to test it using Ohm's law.

I set a regulated linear lab supply to 12.011 V ( measured).  I then put the multimeter in series with the resistor and the reading I got was .536A.  Using Ohm's law the resistance value is 22.4 Ohm.

I then verified the resistance of the leads by shorting them together and got .080 Ohm.

So I am off by a factor of 100 from what is actually measured through Ohm's law and what I interpret the color codes to be and what the meter measured the resistance at. 

I was wondering if someone would please chime in to help clarify some of this.

I would like to thank everyone that contributes in advance.

[themadhippy]

current limiting by your power supply? is it able to chuck out nearly 50A?

[JJ_023]

current limiting by your power supply? is it able to chuck out nearly 50A?

That would be it.  The power supply is rated for .5A.  You live and you learn.  Thank you so much.

[Jwillis]

The colour code does indicate a value of 0.25Ω. If your cold resistance at 20^oC was measured at 0.25Ω then the active resistance becomes 22Ω then it sure sounds like that's a PTC resistor.
Put together your test rig again and restart at a lower voltage at room temperature and see if the current drops at a steady rate as the resistor warms up. A lower voltage will slow down the temperature increase of the resistor making it easier to monitor.

[JJ_023]

The colour code does indicate a value of 0.25Ω. If your cold resistance at 20^oC was measured at 0.25Ω then the active resistance becomes 22Ω then it sure sounds like that's a PTC resistor.
Put together your test rig again and restart at a lower voltage at room temperature and see if the current drops at a steady rate as the resistor warms up. A lower voltage will slow down the temperature increase of the resistor making it easier to monitor.

Thank you for the explanation as well as the suggestion.  I know what a PTC resistor is I never knew they came in a package that resembled a regular resistor. 

Here is the problem I see with your suggestion.  The lab power supply that I am using tops out at about .5A.  Assuming that the resistor is .25 then the voltage for the experiment would need to be set at .125V for everything to function without the current regulation of the power supply being a factor. 

Would it not be easier to just apply heat from a heat gun to test to see if it is a PTC resistor?

I am still going to try your suggestion but will also try a different experiment that I thought of which would entail starting at around 5 V.  Then a multimeter would be connected in series to measure the current.  I should theoretically still get the .536A because of the current limiting on this power supply.  Then slowly start turning the voltage down and see if I can catch the point at which the current starts going down.  At that point the current limiting on the power supply should not be an issue.

Am I missing anything from my train of thought?

[ebastler]

I don't see a reason to assume you have a PTC resistor. The 0.5A current limit of the power supply used for the test fully explains the observation. Occam's razor etc... Jwillis probably posted his suggestion before he had read the subsequent posts by themadhippy and yourself. 

[Jwillis]

I don't see a reason to assume you have a PTC resistor. The 0.5A current limit of the power supply used for the test fully explains the observation. Occam's razor etc... Jwillis probably posted his suggestion before he had read the subsequent posts by themadhippy and yourself.

I did not notice the .5A limit on current as you said. That would certainly explain the strange behaviour and measurements. It's likely not a PTC given all the information.

[DavidAlfa]

Doesn't seem a PTC, but a shunt (Current sensing resistor).
Though the first band looks brown to me, red perfectly matches 0.25 Ohms, so I'd say it's all right!

If you measured 0.5A, either the DMM ammeter had high internal resistance, other PSU limited the current beyond 0.5A.

0.5A over 0.25 ohms would only drop 125mV and disspiate 62mW, this won't cause any heating nor resistance change in a PTC!

[JJ_023]

Thank you to everyone that has contributed so far.  I agree with the post that was debating between Brown and Red for the color, I personally had a very hard time telling what it is and I have direct access to the item.  It might be one or the other.

Keep in mind that this resistor was pulled off of a board so it might actually be faulty.

So I did the following experiment to further this exploration.

I set a random voltage of 14.2 mV ( it's the best I could do with a fidgety knob).  I verified this voltage with 3 separate meters.  The cheapest being a $40 meter and the more expensive one being a $500 meter.  All 3 were very close in line so let's assume that voltage correct.

I then proceeded to take a current measurement.  On the A scale i got .048 A.  ( the multimeter is capable of 20 A on this range for 30 seconds).  I then went to the mA and got 6.028ma.

That would make the resistor .3087 Ohm on A range and 2.355 Ohm on the mA range.

Then I set the voltage to .0027V got .008A which is .3375 Ohm then I switch to the milliamp range got 1mA which respectively gives a value of 2.7 Ohm.

The meter is somewhat consistent in its discrepancies between A and mA range selection for the 2 voltages

Not sure why there is such a discrepancy. 

Any insight would be greatly appreciated.

[Kim Christensen]

I verified this voltage with 3 separate meters.  The cheapest being a $40 meter and the more expensive one being a $500 meter.  All 3 were very close in line so let's assume that voltage correct.

Since you've got more than 2 meters, put an ammeter and a voltmeter in circuit at the same time. Have 4 attachment points to the resistor like this:

[DavidAlfa]

At such low voltage and resistance, wires will heavily affect the testing, easily adding those 50mOhms.
Also the 14.2mV source voltage is just crazy.

Just set the PSU to 1V, 1A CC limit, measure voltage at the resistor terminals, the reading in volts will be exactly the ohms.

I'd assume the initial 0.25 Ohm reading was ok.

[JJ_023]

I verified this voltage with 3 separate meters.  The cheapest being a $40 meter and the more expensive one being a $500 meter.  All 3 were very close in line so let's assume that voltage correct.

Since you've got more than 2 meters, put an ammeter and a voltmeter in circuit at the same time. Have 4 attachment points to the resistor like this:

Thank you for the suggestion.  Where do you suggest I put the best meter.  My guess would be to measure current?

[JJ_023]

At such low voltage and resistance, wires will heavily affect the testing, easily adding those 50mOhms.
Also the 14.2mV source voltage is just crazy.

Just set the PSU to 1V, 1A CC limit, measure voltage at the resistor terminals, the reading in volts will be exactly the ohms.

I'd assume the initial 0.25 Ohm reading was ok.

I get your suggestion and it makes a lot of sense the issue I am having is that the lab power supply will only putting out .5A before going into current limiting so I have those limitations that I am dealing with. 

I am trying to use a low-voltage to circumvent those issues.  It's not digitally controlled but is a knob which is somewhat finicky to get an exact voltage that's why I am using these crazy figures because basically where it lands is where I measure, because once I start getting into an area where it is not finicky then the voltage is too high and I end up dealing with the current limitation of the power supply.

[JJ_023]

I verified this voltage with 3 separate meters.  The cheapest being a $40 meter and the more expensive one being a $500 meter.  All 3 were very close in line so let's assume that voltage correct.

Since you've got more than 2 meters, put an ammeter and a voltmeter in circuit at the same time. Have 4 attachment points to the resistor like this:

So I followed your suggestion and my hypothesis on putting the more expensive meter to measure current.

so here is what I get.

On the course amperage setting.   I get 22.1mV and .143A as a reading which would equate to .1545 Ohm.

I then repeated everything utilizing the mV range on the meter.  I get 2.8mv and a current of 18.395mA  which would equate to .1522 Ohm.

I don't understand why the meter is dropping the voltage reading by a factor of 10 simply by going to mV from V.  The numbers in the end seem to come out awfully close together so it is seeming that the actual resistance might be 15 ohm.  As someone previously said they could not tell the difference between brown and red (nor could I)  and Brown would make it 15 ohm.

If anybody is willing to explain some of these oddities (to me they are oddities) it would be greatly appreciated.

Thank you to everyone that has contributed so far.

[wraper]

Unless OP and multiple people in comments are color blind, I fail to see how do you all get 0.25 ohm since first band is obviously brown, not red, and it should be 0.15 ohm.

[Kim Christensen]

I see your resistor as possibly being Brown Green Silver Gold which equals 0.15 ohm 5% which matches what your measuring.

Your ammeter & leads in series has resistance. So when you swap out the ammeter with a different one, or change ranges on the one you have, there will be a different voltage drop across it. Thus the remaining voltage across the resistor will be different as will the current through it. But ohms law will always apply as you've seen.
To minimize the differences when swapping ammeters, you can set your power supply open circuit voltage to apx 6V and rely on the fact that the power supply will limit the current to apx 0.5A (As long as the ammeter and resistor can handle 0.5A of current)

[JJ_023]

Unless OP and multiple people in comments are color blind, I fail to see how do you all get 0.25 ohm since first band is obviously brown, not red, and it should be 0.15 ohm.

I guess I am colorblind along with everybody else.  lol

[wraper]

Though the first band looks brown to me, red perfectly matches 0.25 Ohms, so I'd say it's all right!

I would not trust 2 lead measurement, especially without ensuring reliable contact and that the way you attach resistor is the same as when you zero out lead resistance. Just connecting probes closer/further from the handle can affect the reading.

[JJ_023]

I see your resistor as possibly being Brown Green Silver Gold which equals 0.15 ohm 5% which matches what your measuring.

Your ammeter & leads in series has resistance. So when you swap out the ammeter with a different one, or change ranges on the one you have, there will be a different voltage drop across it. Thus the remaining voltage across the resistor will be different as will the current through it. But ohms law will always apply as you've seen.
To minimize the differences when swapping ammeters, you can set your power supply open circuit voltage to apx 6V and rely on the fact that the power supply will limit the current to apx 0.5A (As long as the ammeter and resistor can handle 0.5A of current)

I think it is turning out to be .15 Ohm as well as a pretty effective test for colorblindness. 

I understand everything you are saying in the 1st paragraph.  It's all basic Ohm's law stuff.  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.  For some reason it's not clicking.

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.

[ArdWar]

I also measured with a multimeter and got .252 Ohm.

Measuring low resistance with a "simple" two lead multimeter will not necessarily give accurate reading. The connector, cable and test point resistance will add a non-insignificant amount to the measured resistance.

I set a regulated linear lab supply to 12.011 V ( measured).  I then put the multimeter in series with the resistor and the reading I got was .536A.  Using Ohm's law the resistance value is 22.4 Ohm.

Oh no...
Just curious, what do you think will happen. What's your expected measurement values.

On the A scale i got .048 A.  ( the multimeter is capable of 20 A on this range for 30 seconds).  I then went to the mA and got 6.028ma.
...
That would make the resistor .3087 Ohm on A range and 2.355 Ohm on the mA range.
...
Not sure why there is such a discrepancy.

You're most likely approaching the measurement accuracy and resolution limit for your ammeter. Unless you got a good meter the 20A range is not expected to give accurate reading at low current (or any current at all in my experience)

Unless OP and multiple people in comments are color blind, I fail to see how do you all get 0.25 ohm since first band is obviously brown, not red, and it should be 0.15 ohm.

Don't be so harsh lol. With the proliferation of cheap low CRI LED bulbs you can't ever sure what color in a photo are supposed to be in real life.

[JJ_023]

Though the first band looks brown to me, red perfectly matches 0.25 Ohms, so I'd say it's all right!

I would not trust 2 lead measurement, especially without ensuring reliable contact and that the way you attach resistor is the same as when you zero out lead resistance. Just connecting probes closer/further from the handle can affect the reading.

Thanks for pointing that out, I was actually aware of the added resistance so I would always try to clip onto the resistor as close as possible to the body of the resistor.  Using the body of the resistor as a stop would guarantee pretty decent repeatability.

[ArdWar]

Thanks for pointing that out, I was actually aware of the added resistance so I would always try to clip onto the resistor as close as possible to the body of the resistor.  Using the body of the resistor as a stop would guarantee pretty decent repeatability.

Not really, the "lead" resistance here is mostly the metal-metal contact interface between your test pin and component lead. I don't think anyone can even get consistent resistance value since it's pretty much depends on how much you force them together, and what compound is actually contacting. A thin oxide layer for example will throw off your measurement.

[wraper]

Unless OP and multiple people in comments are color blind, I fail to see how do you all get 0.25 ohm since first band is obviously brown, not red, and it should be 0.15 ohm.

Don't be so harsh lol. With the proliferation of cheap low CRI LED bulbs you can't ever sure what color in a photo are supposed to be in real life.

Well, even with monitor color reproduction variation, there is no way this would show red on a display. it's still one photo and at least OP should have seen if it does not match with real life. To get such a picture with red band you should have lighting with almost no red spectrum in it.

[JJ_023]

I also measured with a multimeter and got .252 Ohm.

Measuring low resistance with a "simple" two lead multimeter will not necessarily give accurate reading. The connector, cable and test point resistance will add a non-insignificant amount to the measured resistance.

I set a regulated linear lab supply to 12.011 V ( measured).  I then put the multimeter in series with the resistor and the reading I got was .536A.  Using Ohm's law the resistance value is 22.4 Ohm.

Oh no...
Just curious, what do you think will happen. What's your expected measurement values.

On the A scale i got .048 A.  ( the multimeter is capable of 20 A on this range for 30 seconds).  I then went to the mA and got 6.028ma.
...
That would make the resistor .3087 Ohm on A range and 2.355 Ohm on the mA range.
...
Not sure why there is such a discrepancy.

You're most likely approaching the measurement accuracy and resolution limit for your ammeter. Unless you got a good meter the 20A range is not expected to give accurate reading at low current (or any current at all in my experience)

Unless OP and multiple people in comments are color blind, I fail to see how do you all get 0.25 ohm since first band is obviously brown, not red, and it should be 0.15 ohm.

Don't be so harsh lol. With the proliferation of cheap low CRI LED bulbs you can't ever sure what color in a photo are supposed to be in real life.

Thank you for your contribution.  I was aware that a 2 lead method of measuring resistance was not anywhere near ideal that is why I tried using Ohm's law to at least kind of guess where I am at on resistance.

If there was no current limiting on the power supply (which actually saved my oversight) and the resistor was actually .25 Ohm the current would've been just over 48 A.  I am aware of those implications.

As far as the accuracy of the amp meter It's a $500 multimeter and supposedly has decent accuracy (+/- .10% +2). .01uA is its capability for resolution.
I still don't quite understand the discrepancy between A readings and mA readings.

I am glad I did make this post because it pointed out some oversights on my behalf as well as imparting some additional knowledge. 

[JJ_023]

Thanks for pointing that out, I was actually aware of the added resistance so I would always try to clip onto the resistor as close as possible to the body of the resistor.  Using the body of the resistor as a stop would guarantee pretty decent repeatability.

Not really, the "lead" resistance here is mostly the metal-metal contact interface between your test pin and component lead. I don't think anyone can even get consistent resistance value since it's pretty much depends on how much you force them together, and what compound is actually contacting. A thin oxide layer for example will throw off your measurement.

Thanks for pointing that out.  I tried to use micro clips because I figured it would be a lot more repeatable.  I have a considerable amount of experience with metrology when it comes to measurements having to do with machining so I am always keeping in mind things like thin oxide Layers, repeatability, temperature drifts, etc. so I tried to carry some of that discipline over to EE.  Even though I try sometimes my inexperience catches the best of me as illustrated in some of the examples in this post.