Measuring the resistance of a hot resistor

On Tue, 27 Feb 2007 01:37:49 GMT, Ross Herbert Gave us:

HOWEVER, any chaining errors on those two types of measure add up to far less than the standard error of an ohmmeter.

Voltage accuracy and current accuracy are usually two on meters that are pretty high accuracy.

And it won't MAGNIFY any calculation error, merely chain through. You shouldn't make ANY calculation error with simple ohm's law, and the READING errors are not going to be that great, even chained through two devices.

On Tue, 27 Feb 2007 01:37:49 GMT, Ross Herbert Gave us:

Technically not incorrect, but with ohm meters the inaccuracy is higher than when reading potential or current. In nearly all cases for your everyday handhelds. You have to have a pretty expensive instrument to get truly accurate resistance readings, and a four wire setup typically. Also, in circuit the opportunity to measure right at the operating temperature cannot be beat.

If one truly wants to characterize the resistor in question, a DC Excitation Station is needed.

A DC power source.

A place to mount the resistor being examined

A voltmeter

A current meter

A good chart capable spreadsheet app.

Basic reading from an ohm meter on the resistor.

Ideally: A thermal imager, but a temp probe (J/K?)

Calculate from your ohmmeter reading what to pump your resistor with to begin to bring it up to your operating temperature.

Start lower than that. Characterize the device as it heats up. Your readings will reveal its resistance and temperature curves and at what wattage. The thermal imager would be a plus. You could investigate pulse applications then.

On Tue, 27 Feb 2007 01:39:37 GMT, Ross Herbert Gave us:

We were just talking about how to characterize a resistor.

You could have already had it in your lap, had you not even come in here looking for a refresh primer on Ohm's law.

at

with

what

I agree with everything you say...

However, we are talking about a simple resistor. You don't need all of the fancy tests which cost more than a new resistor in order to determine if it is faulty in some way. It would be far quicker, easier and cheaper to simply chuck out the old resistor and replace it. This would be the most reliable method of fault determination in this case.

Clip an ohmmeter to it, and warm it up with a heat gun. (the resistor, not the meter!) :-)

Good Luck! Rich

Ross Herbert wrote in news: snipped-for-privacy@4ax.com:

*snip*

The question is, how do you know the new resistor isn't just as faulty?

Puckdropper

--
To email me directly, send a message to puckdropper (at) fastmail.fm

of

easier

This

case.

faulty?

Firstly, it would be advantageous to know if the original resistor was new or pulled from the spare parts box.

The OP just said it was a 100W "old wound cement" type so I assume it was something he had on hand. Perhaps he could indicate whether it was new or used and what the construction actually looks like. Cylindrical ceramic types like these

are usually very reliable but if it was an adjustable (slider type) it may have had the resistance wire damaged by overtightening the slider or some other abuse.

a circuit

measures 2.1

resistance,

way I

1 ohm.

somehow this

voltage with

affecting the

of this

resistor, not

I've just been involved in discussion with Massiveprong about using this approach. It seems that in order to fully characterise the unit in question we need to carry out a whole bunch of sophisticated tests with expensive equipment. Sort of like when you find you're up to your arse in alligators you suddenly remember that you only came to drain the swamp.

On Tue, 27 Feb 2007 02:37:09 GMT, Ross Herbert Gave us:

The post title, dingledorf... READ the POST TITLE.

On Tue, 27 Feb 2007 02:37:09 GMT, Ross Herbert Gave us:

READ THE POST TITLE, idiot.

On Tue, 27 Feb 2007 08:56:25 GMT, Ross Herbert Gave us:

WHICH ARE NOT CHEAP.

On Tue, 27 Feb 2007 09:03:22 GMT, Ross Herbert Gave us:

You're an idiot. I gave a basic test scenario that required NO special equipment, THEN I mentioned specialized enhancements that could be performed on any observations made in the normal setup.

LOOK AT THE POST TITLE, dumbass.

The best thing to do with Big Prick is to ignore him.

Cheers! Rich

all

was

The OP was trying to determine why his readings in his unexplained circuit were variable particularly when the resistor was hot. He didn't say what was the function of the circuit in which this resistor was fitted, but since it was a 2 ohm 100W unit one would assume the circuit was for some practical purpose. If you are building a circuit with this sort of component it implies you have some intended use or purpose for it and you wouldn't just pluck an old 2 ohm 100W resistor out of somewhere and use it unless you were anticipating some hefty current. It sounds to me as if the circuit wasn't behaving as expected and he determined that the 100W resistor might be the cause of the problem.

The OP just wanted to know why the variance in his readings when measuring the cold Vs hot resistance and a number of practical suggestions as to how he could do that have been made. I simply suggested an easy method which didn't require anything other than a DMM which would give him a good idea whether he was on the right track or not and you responded in a negative manner which basically pooh-poohed the idea.

I have no argument against using the DC voltage/current measurement principle and it can be extremely accurate. However, in order to achieve good accuracy when measuring a 2 ohm 100W resistor when hot it is necessary to have a stable, low impedance source of current capable of at least 5-6A in order to heat the 100W resistor sufficiently (ie. need to dissipate at least 50W in order to get it hot enough). You also need a meter for measuring current and another for measuring voltage, each with good resolution and low error tolerances in order to produce meaningful results when dealing with measurements on low resistance circuits. These items may or not be in the possession of the OP (he hasn't said so).

However, if he has access to a good quality DMM such as a Gossen MetraHit 25S (now superseded, but it's what I use)

or similar - as long as it has a ZERO ohms cancellation function, you can get very good results by direct reading.

You conveniently snipped my suggested possible cause for variance in his initial readings and made no comment on it. If the said resistor was a slider type (OP doesn't say exactly what type he is using) the wire might have been damaged by over-tightening the slider or some other abuse, thus leading to errors especially when hot.

A little more explanation from the OP might be helpful.

------------------ My you do tend to get your knickers in a knot, don't you?

See my comments further up the thread....

On Wed, 28 Feb 2007 02:58:37 GMT, Ross Herbert Gave us:

One could also assume that it would be EASY to remove it and test it WITH DC on a bench!

That way, any and all confusion about its behavior could be resolved.

So STFU!

On Wed, 28 Feb 2007 02:58:37 GMT, Ross Herbert Gave us:

It is certainly easier to get an accurate current and voltage meter than it is an accurate ohm meter. Sheesh.

On Wed, 28 Feb 2007 02:58:37 GMT, Ross Herbert Gave us:

It seems obvious to me that it is a fixed value resistor, and no "slider" was involved. The "some other abuse" is that which you apply to your head in over-complicating such an easy task.

On Wed, 28 Feb 2007 02:58:37 GMT, Ross Herbert Gave us:

No. It only needs to get as hot as he is seeing it get in actual operation.

In a good design, that will be somewhere below half the rated wattage of this part, but that doesn't mean good design practices were used. They could have overdesigned it to keep it running cool, or under designed it causing hotter than optimal or normal operation.