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Measuring the resistance of a hot resistor

J

Jon Danniken

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?

Thanks for any insight into this,

Jon
 
J

Jonathan Kirwan

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?

Thanks for any insight into this,

Jon

Try doing the measurements, in circuit? By this, I mean measuring
both the voltage across it, in parallel, when in-circuit and measuring
the current through it (meter set to measure at least 7A, which is
what 100W suggests) using a series arrangement. The two will tell you
what you need to know: R=V/I.

Jon
 
J

Jon Danniken

Jan 1, 1970
0
Jonathan Kirwan said:
Try doing the measurements, in circuit? By this, I mean measuring
both the voltage across it, in parallel, when in-circuit and measuring
the current through it (meter set to measure at least 7A, which is
what 100W suggests) using a series arrangement. The two will tell you
what you need to know: R=V/I.

The measurement I am taking requires a value for the resistor itself,
independant of reactive elements in the circuit. As such, I need to measure
the resistance out of circuit.

Jon
 
E

Eeyore

Jan 1, 1970
0
Jon said:
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?

I suspect your circuit is powered up when you're making the hot measurement and
not when you're making the cold measurement.

Graham
 
H

Homer J Simpson

Jan 1, 1970
0
The measurement I am taking requires a value for the resistor itself,
independant of reactive elements in the circuit. As such, I need to
measure the resistance out of circuit.

The resistance will not vary much with temperature. For accurate
measurements under load, measure the voltage and current at the same time.

--
..

--
..
..
..
..
..
..
..
..

--
 
M

me

Jan 1, 1970
0
Try doing the measurements, in circuit? By this, I mean measuring
both the voltage across it, in parallel, when in-circuit and measuring
the current through it (meter set to measure at least 7A, which is
what 100W suggests) using a series arrangement. The two will tell you
what you need to know: R=V/I.

Jon


(meter set to measure at least 7A, which is what 100W suggests)

or 1 mA or 100 Amps or.....
 
M

me

Jan 1, 1970
0
The measurement I am taking requires a value for the resistor itself,
independant of reactive elements in the circuit. As such, I need to
measure the resistance out of circuit.

Jon

Use a DC circuit to indirectly measure the reistance as described (roughly)
above.
 
P

Phil Allison

Jan 1, 1970
0
"Jon Danniken"
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?

** It will have the same resistance cold or hot, +/- 5%.

You have some silly bad contact issue with your probes.



........ Phil
 
J

john jardine

Jan 1, 1970
0
Jon Danniken said:
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?

Thanks for any insight into this,

Jon

Like a thermocouple, it's generating a few DC mV when hot. This upsets the
ohmmeter readings.
john
 
P

Phil Allison

Jan 1, 1970
0
"john jardine"

Like a thermocouple, it's generating a few DC mV when hot. This upsets the
ohmmeter readings.


** The resistance wire in the power resistor cannot do that - but maybe
there is a thermocouple created by the terminals or when the DMM's probes
are used to make connection - as little as 0.5mV would explain the OP's
dilemma.

DMMs are poor at reading low ohms.

If the OP simply connects a resistor of known value in series with the power
resistor and applies say 12 volts DC - the voltage will split in the
same ratio as the two values.

Should put his false anxiety to rest.



....... Phil
 
J

Jon Danniken

Jan 1, 1970
0
john jardine said:
Like a thermocouple, it's generating a few DC mV when hot. This upsets the
ohmmeter readings.

Aha, thanks, John. I measured the resistor again while it was hot and
you're right, it develops 0.4mV across itself when it is hot.

Jon
 
J

Jon Danniken

Jan 1, 1970
0
me" said:
Use a DC circuit to indirectly measure the reistance as described
(roughly)
above.

Excellent, thanks for that, Me. I'll cook up a little DC circuit to toss it
into for the measurement.

Thanks,

Jon
 
J

Jon Danniken

Jan 1, 1970
0
"Phil Allison"
Should put his false anxiety to rest.

Hey Dr. Phil, maybe you should go piss off to a psych group and let the rest
of us discuss electronics.

Jon
 
P

Phil Allison

Jan 1, 1970
0
"Jon Danniken CUNTHEAD "
"Phil Allison"

Hey Dr. Phil, maybe you should go piss off to a psych group and let the
rest of us discuss electronics.


** Huh - what the **** is your problem ??

Why so keen to prove to the whole world what a pig ignorant & asinine
CUNTHEAD you are.

When it was obvious all along anyhow.




....... Phil
 
R

Ross Herbert

Jan 1, 1970
0
Excellent, thanks for that, Me. I'll cook up a little DC circuit to toss it
into for the measurement.

Thanks,

Jon

You could also do a direct measurement with the resistor heated by a
hot plate or a hot air gun...
 
M

MassiveProng

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?

Thanks for any insight into this,

Put a good ammeter in series with it, and place a volt meter across
it. Read both at run time, extrapolate, and you'll get very accurate
results.
 
M

MassiveProng

Jan 1, 1970
0
As such, I need to measure
the resistance out of circuit.


No. With the ammeter right next to it, and the voltage reading has
ZERO effect, you WILL get the exact resistance at the moment you take
the readings. Your imagined parasitics won't exist nor matter.

The value extrapolated WILL be exact.
 
M

MassiveProng

Jan 1, 1970
0
Excellent, thanks for that, Me. I'll cook up a little DC circuit to toss it
into for the measurement.

Yes. A very precise solution. Rent a thermal imager, and get some
real repeatable results!
 
M

MassiveProng

Jan 1, 1970
0
You could also do a direct measurement with the resistor heated by a
hot plate or a hot air gun...


Do you not think that a known voltage and a good current reading
would not be more accurate than an unknown calibration level ohm
meter?
 
R

Ross Herbert

Jan 1, 1970
0
Do you not think that a known voltage and a good current reading
would not be more accurate than an unknown calibration level ohm
meter?

I am fully aware of the method of performing a DC current and voltage
measurement and applying Ohm's Law. I was simply suggesting an
alternative which would avoid the need to lash up a circuit which
requires both an ammeter and a voltmeter simultaneously in circuit in
order to achieve best accuracy. Two measurements with meters each
having accuracy and tolerance inaccuracy will also magnify any
calculation error.

A single meter measuring resistance directly might be more accurate
depending on the meter used.
 
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