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
- Picotest M3500A with a multipoint card was used; front panel and card can disagree. [Elektroda, faramir1, post #16888145]
- Autorange can inject current pulses that upset inductive measurements. [Elektroda, Adamcyn, post #16888276]
- Front panel read 40 Ω; card channels read ~35 Ω, 8 Ω, and 5 Ω. [Elektroda, faramir1, post #16890398]
- Increasing integration time improved resolution to 1 Ω; target asked was 0.1 Ω. [Elektroda, faramir1, post #16947407]
- For AC, use the term reactance rather than resistance. [Elektroda, Adamcyn, post #16887994]
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?
- Fix range and set longer integration time on the meter.
- Route signal leads away from power and display cables; close unused channels.
- 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]
