Measuring the resistance of a hot resistor

Discussion in 'Electronic Basics' started by Jon Danniken, Feb 25, 2007.

1. Jon DannikenGuest

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

2. Jonathan KirwanGuest

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

3. Jon DannikenGuest

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

4. EeyoreGuest

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

Graham

5. Homer J SimpsonGuest

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

--
..

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

--

6. meGuest

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

or 1 mA or 100 Amps or.....

7. meGuest

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

8. Phil AllisonGuest

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

........ Phil

9. john jardineGuest

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

10. Phil AllisonGuest

"john jardine"

** 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

11. Jon DannikenGuest

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

12. Jon DannikenGuest

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

Thanks,

Jon

13. Jon DannikenGuest

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

Jon

14. Phil AllisonGuest

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

Why so keen to prove to the whole world what a pig ignorant & asinine

When it was obvious all along anyhow.

....... Phil

15. Ross HerbertGuest

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

16. MassiveProngGuest

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.

17. MassiveProngGuest

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 value extrapolated WILL be exact.

18. MassiveProngGuest

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

19. MassiveProngGuest

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?

20. Ross HerbertGuest

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.