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[Solved] Measuring Coil Resistance: Constant vs. Jumping Values with Picotest Multimeter & Manual Meter

faramir1 9477 15
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  • #1 16887770
    faramir1
    Level 4  
    Posts: 48
    Rate: 4
    Board Language: polish
    Hello, I am tasked with measuring the coil resistance. By doing this manually with a meter, the resistance is constant and does not jump, while when I use the picotest multimeter, the resistance jumps terribly. I will add that I measured the resistors with a multimeter and it was ok, the resistance slightly changed. Anyone know what this problem may be?
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  • #2 16887781
    jarek_lnx
    Level 43  
    Posts: 22530
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    Board Language: polish
    Different meters have different noise immunity and this may be the reason.

    What is this coil? is this coil shielded? or is it exposed to variable magnetic fields and is there any known source of such a field nearby?
  • #3 16887837
    Anonymous
    Level 1  
  • #4 16887974
    Freddy
    Level 43  
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    faramir1 wrote:
    Hello, I am tasked with measuring the coil resistance.
    Under what conditions? For DC or AC?
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  • #5 16887994
    Anonymous
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  • #6 16888012
    Freddy
    Level 43  
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    Adamcyn wrote:
    In the case of alternating current, the Author would use the term reactance.
    If he knows what it is and if he is a teacher
    faramir1 wrote:
    I have a task
    he didn't do much myku :)
    Picotest - there is more than one. What settings in Picotest during measurements? This is also very important.
  • #7 16888048
    Anonymous
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  • #8 16888145
    faramir1
    Level 4  
    Posts: 48
    Rate: 4
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    He measures using a multi-point card from his own program, this is the pic35 test m3500a. I tried using the card reader program that is on the picotest page and shows the same jumping resistance. In this case, I could only rule out software errors. I don't remember at the moment what settings I will look at when I come to work.
  • #9 16888276
    Anonymous
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  • #10 16888381
    faramir1
    Level 4  
    Posts: 48
    Rate: 4
    Board Language: polish
    I will check tomorrow with this setting autorange may help

    Added after 13 [hours] 10 [minutes]:

    I don't know a bit how to configure it in a multimeter, I can prompt you with what other commands, as long as someone knows.

    Added after 3 [hours] 19 [minutes]:

    However, this is not the range because I set it manually on the front panel, I closed the channels, the manual measurement is like from the card.
  • #12 16889901
    faramir1
    Level 4  
    Posts: 48
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    Board Language: polish
    The best part is that as much as this resistance from the front panel and not using a multi-point card, this resistance is correct and amounts to 40 ohm
  • #13 16890034
    Anonymous
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  • #14 16890398
    faramir1
    Level 4  
    Posts: 48
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    Board Language: polish
    The measurement using the card is 35 ohms 8 and 5 ohms for 1,2 and 3 channel. However, the measurement using the 40 ohm front panel was measured with a meter and also 40 ohms. Tomorrow I will look at it through the crossing power and signal cables to the monitor with my measurement.
  • #15 16947407
    faramir1
    Level 4  
    Posts: 48
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    Hello, I have recently worked on improving results and obtained better results in setting integration time. Now the multimeter shows with accuracy to one. However, this is not enough in my case because I received the instruction to get accuracy to 0.1 ohma. Maybe you write what you set in your multimeters so that it affects the accuracy I manipulated range and resolution.
  • #16 19838961
    faramir1
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    Posts: 48
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    One of the problems that occurs with automated measurements is that the equipment heats up, 100% I'm not sure but after doing about 100 measurements in a cycle something started to go crazy and it did every time. The same if the bench has no cooling and is very hot, which the equipment does not like.
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Topic summary

✨ The discussion revolves around measuring coil resistance using a Picotest multimeter compared to a manual meter. Users report that while the manual meter provides stable resistance readings, the Picotest multimeter shows fluctuating values. Possible causes for this discrepancy include differences in noise immunity between meters, the influence of external magnetic fields, and the measurement method employed by the Picotest, which may involve pulsing current. Suggestions include testing the Picotest on different coils, using capacitors to observe changes, and checking settings such as autorange. Users also mention the importance of integration time settings for achieving accurate measurements, with a goal of reaching precision to 0.1 ohm.
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FAQ

TL;DR: After ~100 automated measurements, "the equipment heats up" and resistance readings can jump; fix by longer integration time, disable autorange, and reduce magnetic/thermal noise. [Elektroda, faramir1, post #19838961]

Why it matters: This FAQ helps engineers and technicians get stable coil resistance numbers with Picotest M3500A or similar meters.

Quick Facts

Why does my coil resistance jump on a Picotest but stay steady on a manual meter?

Different noise immunity and test methods cause this. Autorange and pulsed test currents can energize the coil’s magnetic field. Energy then returns to the meter and disturbs the resistance reading. Shielding and fixed range reduce these effects. [Elektroda, Adamcyn, post #16888048]

What settings should I change first on the Picotest M3500A?

Disable autorange, fix the resistance range, and increase integration time. Longer integration averages noise and improved accuracy to 1 Ω in the thread case. Use front-panel confirmation before switching through the card. [Elektroda, faramir1, post #16947407]

How do autorange pulses affect inductors?

Autorange sends varying current pulses to select a range. In an inductor, that stores magnetic energy. When polarity or level changes, part of that energy returns and skews the ohms result. “Autorange sends various current pulses.” [Elektroda, Adamcyn, post #16888276]

Is this a DC resistance problem or an AC reactance issue?

If you measure coil resistance with DC, it is pure ohms. With AC, the coil presents reactance (frequency-dependent). Use the correct term to avoid confusion during setup and reporting. [Elektroda, Adamcyn, post #16887994]

Why does the front panel show 40 Ω but the multipoint card shows 35/8/5 Ω?

The switching card can add paths, offsets, and noise. Channel wiring and nearby cables can couple interference into inductors. In the reported case, front panel was stable at 40 Ω, while card channels varied widely. [Elektroda, faramir1, post #16890398]

Could thermal drift or equipment heating cause jumping values?

Yes. Long automated runs heat instruments and fixtures. After about 100 measurements, the user saw instability that repeated when the bench was hot. Add cooling or duty-cycling to stabilize readings. [Elektroda, faramir1, post #19838961]

How can I verify if external fields are corrupting my coil measurement?

Move the setup away from power supplies, transformers, or monitors. Try a shielded coil or add capacitors across the coil to change its response and observe differences. Consistent change indicates field coupling. [Elektroda, Adamcyn, post #16887837]

What quick test distinguishes instrument issues from coil issues?

Measure known resistors instead of the coil. In the thread, resistors measured fine, while the coil jumped. That points to measurement method and environment, not the meter’s basic ohms function. [Elektroda, faramir1, post #16887770]

How do I measure coil resistance using a power source instead of the DMM’s ohms mode?

Apply a known DC current and measure the coil’s voltage drop. Use Ohm’s law (R = V/I). This avoids autorange pulse artifacts in inductors and can be more stable with large L. [Elektroda, Adamcyn, post #16889705]

What’s a simple 3‑step procedure to stabilize card-based measurements?

  1. Fix range and set longer integration time on the meter.
  2. Route signal leads away from power and display cables; close unused channels.
  3. Add cooling and pause cycles after ~100 readings. [Elektroda, faramir1, post #19838961]

How do I know if integration time is helping?

Increase integration time and watch the standard deviation of repeated readings. In the thread, tuning it improved resolution from noisy to 1 Ω steps. Keep other settings fixed during tests. [Elektroda, faramir1, post #16947407]

Can an oscilloscope help diagnose the measurement problem?

Yes. Probe the coil terminals during measurement. Look for pulses or ripple synchronous with range changes or card switching. That reveals whether the method injects disturbing waveforms. [Elektroda, Adamcyn, post #16890034]

Does adding a capacitor across the coil ever help?

A small capacitor changes the impedance versus frequency. If readings stabilize with the capacitor, the issue is likely induced AC or pulsed test currents coupling with the coil’s inductance. [Elektroda, Adamcyn, post #16887837]

What’s the failure mode if I ignore thermal management?

Extended runs without cooling can cause drifting offsets and sudden instability. The user reported repeatable misbehavior on a hot bench after long cycles. Add fans or pause schedules. [Elektroda, faramir1, post #19838961]

When should I avoid autorange entirely?

Avoid autorange when measuring inductors or low-ohms parts through switching cards. Fixed range prevents disruptive current changes and improves repeatability. Confirm with a front‑panel spot check. [Elektroda, Adamcyn, post #16888276]

Why is terminology important in my lab notes and reports?

Using resistance for DC and reactance for AC avoids misinterpretation and wrong settings. Clear terms speed troubleshooting and peer review. It also guides correct instrument configuration. [Elektroda, Adamcyn, post #16887994]
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