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Measuring the resistance of a hot resistor

R

Ross Herbert

Jan 1, 1970
0
Yes. A very precise solution. Rent a thermal imager, and get some
real repeatable results!


Now that's what I call a nonsensical suggestion. It would be cheaper
to simply buy a new resistor than to rent a thermal imager.
 
M

MassiveProng

Jan 1, 1970
0
. Two measurements with meters each
having accuracy and tolerance inaccuracy will also magnify any
calculation error.


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.
 
M

MassiveProng

Jan 1, 1970
0
A single meter measuring resistance directly might be more accurate
depending on the meter used.


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.
 
M

MassiveProng

Jan 1, 1970
0
Now that's what I call a nonsensical suggestion. It would be cheaper
to simply buy a new resistor than to rent a thermal imager.


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.
 
R

Ross Herbert

Jan 1, 1970
0
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.


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.
 
R

Rich Grise

Jan 1, 1970
0
I am using a 100W, 2 ohm power resistor (old wound cement unit) in a circuit
to do some basic measurements with. When cold, the resistor measures 2.1
ohms on both of my DMMs.

After use, when the resistor is hot, I attempted to measure the resistance,
but I ended up with some crazy results, which, depending on which way I
connect the leads, tell me it is either between 3 to 4 ohm, or 0 to 1 ohm.

Since the lead placement affected the reading, I assumed that somehow this
resistor was generating a voltage when hot, but I can detect no voltage with
any of my meters.

What is going on here, and why in the heck is lead placement affecting the
results? Additionally, how can I accurately measure the resistance of this
hot resistor?


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

Good Luck!
Rich
 
P

Puckdropper

Jan 1, 1970
0
*snip*
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.

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

Puckdropper
 
R

Ross Herbert

Jan 1, 1970
0
*snip*


The question is, how do you know the new resistor isn't just as 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 http://www.resistor.com.tw/pdf/Dr1-3.pdf 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.
 
R

Ross Herbert

Jan 1, 1970
0
Clip an ohmmeter to it, and warm it up with a heat gun. (the resistor, not
the meter!) :)

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.
 
M

MassiveProng

Jan 1, 1970
0
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.


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

MassiveProng

Jan 1, 1970
0
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.


READ THE POST TITLE, idiot.
 
M

MassiveProng

Jan 1, 1970
0
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"

WHICH ARE NOT CHEAP.
 
M

MassiveProng

Jan 1, 1970
0
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.


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.
 
R

Rich Grise

Jan 1, 1970
0
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.

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

Cheers!
Rich
 
R

Ross Herbert

Jan 1, 1970
0
WHICH ARE NOT CHEAP.


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)
http://www.gossenmetrawatt.com/resources/zz_tam/hit22-26m/db_gb.pdf 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.
 
R

Ross Herbert

Jan 1, 1970
0
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.
 
M

MassiveProng

Jan 1, 1970
0
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.


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!
 
M

MassiveProng

Jan 1, 1970
0
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).


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

MassiveProng

Jan 1, 1970
0
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.


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.
 
M

MassiveProng

Jan 1, 1970
0
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).


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.
 
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