Phone Magnetometer: Coil Activity Is Not Voltage Proof

After the fix, the relay or solenoid responds correctly and a proper electrical test confirms the circuit state. Before the fix, the panel symptom is usually simple: an output appears commanded, but the contactor, relay, or valve does not move. A phone magnetometer may show whether the local magnetic field changed, but it cannot prove that voltage is present, identify the voltage, establish current continuity, or declare the circuit safe.

Reject the wrong fixes first

Start here. Remove these shortcuts from the decision path before interpreting the phone trace.

• Using the phone as a multimeter: The magnetometer measures magnetic flux density at its sensor. It does not measure terminal voltage, insulation integrity, contact resistance, or protective-earth continuity.
• Treating any magnetic reading as proof of coil pickup: Permanent magnets, adjacent conductors, motors, steel structures, and nearby energized coils can change the reading. The phone may detect a field even when the device under test has not operated.
• Moving the phone into an energized cabinet for a stronger signal: A consumer phone has no stated electrical measurement category in the supplied information. Do not place it inside an exposed energized enclosure or use it to establish absence of voltage.
• Testing a fuse on a capacitive touchscreen: Some intact fuses may affect a touchscreen, but some blown fuses may also appear to pass this test. That is not a continuity measurement and cannot clear a fuse for service.
• Burnishing contacts before diagnosing the circuit: Cleaning may temporarily change contact resistance while leaving a failed coil, mechanical bind, incorrect command, or upstream fault unresolved.
• Replacing fuses repeatedly: A replacement fuse that opens again is a symptom. Find the overload or ground fault before energizing the circuit again.

A noncontact voltage indicator is also not a substitute for a meter when you must prove absence of voltage. Use each instrument only for the quantity it is designed to indicate.

Understand what the phone actually detects

A current-carrying conductor produces a magnetic field. A relay or solenoid coil produces a stronger, localized field when energized, and its magnetic circuit uses that field to move an armature or plunger. The phone magnetometer samples the field near the handset's sensor and reports changes along one or more axes.

The measurement is indirect. The displayed value depends on coil geometry, current, distance, phone orientation, sensor location, nearby ferrous material, background field, and magnetic interference. Two phones can produce different values at the same point. A case containing a magnet can overwhelm the change you want to observe.

A detected field answers one narrow question: did the magnetic environment at this sensor position change when the command changed? It does not prove that the armature moved or that relay contacts transferred. A mechanically stuck solenoid can generate a field without completing its stroke. A relay can pull in while its power contacts remain open or resistive.

Separate symptoms from causes

Set up a repeatable magnetic check

Use a raw-sensor application such as phyphox or a purpose-oriented application such as Danfoss Magnetic Tool. The application name does not make the phone a rated electrical tester; it only exposes or processes magnetometer data.

1. Confirm that the test point is external to exposed energized conductors. If reaching the coil requires entering an energized enclosure, stop and use the approved work method and instruments.
2. Remove magnet-equipped accessories where practical. Check whether the idle reading changes sharply as the case or accessory moves.
3. Locate the magnetometer within the handset. Move the phone around a known low-risk coil to identify which corner or edge gives the clearest response.
4. Place the phone against a fixed, nonmoving reference surface near the relay or solenoid. Keep the same distance and orientation for every comparison.
5. Record a command-off baseline. Watch all available axes or the calculated field magnitude rather than relying on one instantaneous sample.
6. Command the device once while leaving the phone stationary. Note whether the trace changes at the same time as the command.
7. Remove the command and confirm that the trace returns toward its baseline. Repeat the off-on-off sequence to reject a coincidental disturbance.

For comparative vibration work, mount the phone securely and place it at the same point and orientation on each machine. A 15-30 second recording was used for pump comparisons in the supplied case, but that duration is a collection habit, not a universal vibration-analysis requirement. Do not attach a phone to moving machinery unless the mounting method and machine rules permit it.

Use the result to choose the next instrument

If the trace does not respond, check the electrical path. Confirm the control command, protective device, interlock state, output device, wiring, and coil terminals. Select a meter and probes rated for the circuit and the environment. Measure the quantity that separates the remaining causes: voltage across the coil during the command, resistance or continuity with the circuit safely isolated, or current using an appropriate method.

If the trace responds but no motion occurs, stop chasing upstream logic first. Check whether the rated coil voltage is actually present under load, then investigate a seized armature, obstructed valve, damaged linkage, contamination, or a coil that generates flux without enough force to complete travel.

If the relay clicks and the load remains off, the magnetic check has finished its job. Test the contact side. Verify the incoming supply, transferred contact state, downstream conductor, and load. A field at the coil says nothing about the condition of those contacts.

When troubleshooting a conductor rather than a coil, expect weaker and less localized readings. Outgoing and return currents can produce opposing fields, cable construction changes the external field, and adjacent loads add interference. A missing phone indication cannot prove that the conductor is de-energized.

Verify the repair electrically and mechanically

1. Operate the command and verify the intended coil voltage at the device with the correct instrument.
2. Confirm that the relay armature, solenoid plunger, valve, or contactor travels fully without hesitation.
3. Verify that the switched contacts change state and that the downstream load receives its intended supply.
4. Remove the command and confirm complete release. Check that the load returns to its required safe state.
5. Repeat several operating cycles while watching for intermittent wiring, delayed motion, chatter, or a field that remains after the command is removed.
6. Restore guards and covers, then perform the machine-level functional test required by the maintenance procedure.

The phone trace can be retained as a relative comparison only when the phone, application, position, orientation, machine state, and nearby equipment states remain controlled. Do not assign pass/fail limits from readings taken with a different handset or geometry unless your maintenance program has established that correlation.

Control the hazards and false positives

Keep the phone out of locations where it can fall into machinery, bridge conductors, distract the technician, or enter a restricted electrical work area. Metal phone parts and conductive debris increase the consequence of poor placement. Follow the site's energized-work, arc-flash, lockout, and equipment-access rules; the magnetometer adds no protection.

Control magnetic interference before diagnosing the machine. Check for magnetic phone cases, tool magnets, speakers, motors, neighboring contactors, and steel surfaces. Establish the baseline at the final measurement position. If the result changes when you move or rotate the handset rather than when the command changes, that is not the fault.

Do not use a magnetic response to bypass an interlock, clear a safety function, approve a fuse, or release equipment to production. The final decision must come from the electrical and functional checks required for that circuit.

FAQ