Let's break down the situation and address your questions

Understanding the Phenomenon: Back EMF (Flyback Voltage)

When you de-energize an inductive load like a contactor coil, the collapsing magnetic field generates a voltage spike known as back EMF or flyback voltage. This voltage spike is intended to discharge the inductive energy within the coil to protect the device from over voltage damage. However, it can damage sensitive electronics if not properly managed. The magnitude of this voltage is influenced by: Inductance of the Coil

Higher inductance means a larger collapsing magnetic field and a higher voltage spike.

Rate of Current Change

Faster shutoff of current produces a larger voltage spike.

Circuit Impedance

Higher impedance in the discharge path will limit current, but increase voltage.

Analyzing Your Measurements



18V Pull-in and Hold

This is expected. The contactor is designed to operate over a voltage range. 18V is within the specified range.

32V Spike on 100ms Scale

This is the back EMF voltage. It's reasonable and within a safe range if the contactor is designed to handle it. This voltage appears to be clamped by an internal diode.

>4000V Spike on 1ms Scale

This is the significant result. Your Fluke 87 is indicating a very short-duration voltage spike that exceeds its measurement range. The actual voltage could be much higher.

Answering Your Questions


"Can I assume that the 100mv peak will go up proportionately with the applied voltage? 24 VDC applied would result in about 43?" No, you cannot assume a direct proportional relationship. The back EMF voltage is more complex than a simple ratio. It's driven by the change in magnetic field strength, and factors like coil saturation and internal components will affect it. The observed 32V on the 100ms scale likely indicates that the coil is in a saturated state and/or the contactor has an internal diode that begins to clamp the voltage at that level. If an overvoltage is present, the diode's clamping effect is limited. Therefore, increasing the supply voltage won't necessarily increase the back EMF voltage by the same proportion. The internal components are already having difficulty controlling the voltage at 18V. Calculating the potential peak

In a perfect scenario, the back EMF would be proportional and could be estimated. But in reality, it's not so simple. Don't rely on a simple ratio calculation.2.

"Do I need to worry at all about the 1ms reading?" Yes, you absolutely need to worry about the 1ms reading! A voltage spike exceeding 4000V, even for a very short duration, is dangerous. It can: Damage the Contactor

Breakdown insulation within the coil, shortening its lifespan or causing immediate failure.

Damage Other Electronics

If the contactor is connected to a PLC, sensor, or other control circuitry, this spike can easily destroy those devices. It can propagate through wiring and damage components even if they are physically distant.

Create Safety Hazards

In extreme cases, a large enough voltage spike can arc and create a fire hazard or shock hazard.

Recommendations


Use a Proper Power Supply

Using repurposed 9V batteries is not ideal for testing inductive loads. Use a regulated 24VDC power supply that is designed to handle the inrush current of the contactor coil. This will provide a stable and consistent voltage.2.

Install a Flyback Diode (if not present)

The most common and effective solution is to add a flyback diode (also called a snubber diode or freewheeling diode) across the contactor coil. This diode is connected in reverse bias with respect to the DC supply. When the contactor is de-energized, the diode provides a path for the collapsing magnetic field to circulate current, clamping the voltage spike to the diode's forward voltage drop (typically 0.7V).

Diode Selection

Choose a fast-recovery diode rated for at least twice the coil's operating voltage and capable of handling the coil's inrush current. A common choice is a 1N4007 (but double-check current requirements).3.

Consider an RC Snubber

In some cases, a resistor-capacitor (RC) snubber can be more effective than a diode alone. The RC snubber dampens the voltage spike and reduces EMI.4.

Check Contactor Specifications

Review the contactor's datasheet. It may already have an internal suppression device (like a diode or MOV). If it does, confirm that it's functioning correctly. If the internal suppression is insufficient or has failed, you definitely need to add external suppression.5.

Test with the Intended Load

If the contactor is driving another load, test the circuit with that load connected. The load's characteristics can affect the back EMF voltage.6.

Use an Oscilloscope

A digital storage oscilloscope (DSO) is the best* tool for analyzing back EMF voltage spikes. It will show you the voltage waveform, its duration, and any ringing or oscillations.7.

Verify Isolation

Ensure that the input and output circuits are isolated. An isolation error will cause the voltage backsurge to bleed into the controlling circuit and cause irreparable damage.

In Summary

That 1ms >4000V spike is a red flag. Do not ignore it. Adding a flyback diode is a crucial step to protect your equipment from damage. Use a proper power supply and a flyback diode, and then re-evaluate your measurements. If the problem persists, consider an RC snubber or consult with an electrical engineer.

Important Safety Note: Working with electrical circuits can be dangerous. If you are not comfortable working with electricity, consult a qualified electrician. Flag for review