Resistance and Inductance in DC Coils: Why Solenoids Burn Out PLCs

Resistance is the opposition to steady current flow that dissipates energy as heat, while inductance is the opposition to changes in current that stores energy in a magnetic field. When you wire a 24V DC solenoid, a contactor coil, or a relay, these two properties are locked in a constant tug-of-war. Resistance dictates your steady-state heat and continuous current draw; inductance dictates how fast the coil energizes and how massive the destructive voltage spike is when you turn it off. If you ignore the interplay of resistance and inductance, you will eventually melt a switching transistor or weld a relay contact shut.

The Core Physics: Steady State vs. Change

To understand what resistance and inductance change in a real circuit, you have to look at the timeline of a switching event. When you apply voltage to a coil, the resistance (measured in Ohms, Ω) immediately tries to limit the current to a steady value based on Ohm’s Law (I = V/R). However, the inductance (measured in Henries, H) fights this change. It generates a back-electromotive force (back-EMF) that opposes the rising current.

Think of it like water flowing through a pipe containing a heavy, rusted water wheel. The pipe’s narrowness is the resistance—it limits the maximum flow rate and creates friction (heat). The heavy water wheel is the inductance. When you open the valve, the wheel takes time to spin up, delaying the water flow. But once it’s spinning at full speed, if you suddenly slam the valve shut, the momentum of the heavy wheel will crush the valve and burst the pipe. In electronics, that "burst pipe" is a massive voltage spike known as inductive flyback.

The speed at which the current rises is defined by the L/R time constant (τ = L / R). It takes exactly five time constants (5τ) for the coil to reach 99.3% of its final steady-state current.

Where You Meet Resistance and Inductance in Practice

You encounter the clash of resistance and inductance anywhere a magnetic field is used to do mechanical work. On the jobsite or at the workbench, this primarily means:

• Pneumatic/Hydraulic Solenoid Valves: Low resistance, high inductance. Prone to destroying solid-state PLC outputs.
• Contactor and Relay Coils: The inductance holds the mechanical armature in place, but causes severe arcing across mechanical switch contacts upon release.
• Stepper and Servo Motor Windings: In 3D printers and CNC routers, the inductance of the motor phases limits the maximum stepping speed (current can't rise fast enough at high RPMs).
• Long Cable Runs: The wire itself has resistance (causing voltage drop) and inductance (causing high-frequency signal degradation in VFD motor leads).

Worked Numeric Example: The 24V DC Solenoid Valve

Let’s put real numbers to the theory. Suppose you are wiring a standard SMC SY5000-series pneumatic solenoid valve to a 24V DC power supply. You measure the coil with your multimeter and find the following specifications:

• Voltage (V): 24V DC
• DC Resistance (R): 48 Ω
• Inductance (L): 1.5 H (Henries)

1. Calculating Steady-State Current:
Using Ohm's law, the continuous current once the magnetic field is fully established is I = 24V / 48Ω = 0.5 Amps. This is what your power supply must be rated to handle continuously.

2. Calculating the Turn-On Delay:
The L/R time constant (τ) = 1.5 H / 48 Ω = 0.03125 seconds (31.25 ms).
To reach full pulling force, the valve needs 5τ: 5 × 31.25 ms = 156.25 ms. If you are trying to pulse this valve at 20 Hz (50 ms per cycle), it will never fully open because the inductance prevents the current from rising fast enough.

3. Calculating the Turn-Off Flyback Voltage:
When you open the switch, the inductor tries to maintain the 0.5A current. The formula for inductive voltage is V = L × (di/dt).
If a solid-state PLC transistor turns off in 1 microsecond (1 µs), the rate of current change (di/dt) is 0.5A / 0.000001s = 500,000 A/s.
Flyback Voltage = 1.5 H × 500,000 A/s = 750,000 Volts.

Obviously, the circuit won't actually reach 750kV. Parasitic capacitance and the breakdown voltage of the switching transistor will clamp the spike. However, the transistor will experience avalanche breakdown—usually around 60V to 100V for standard silicon—which is more than enough to permanently short the silicon die and destroy the output channel.

Real-World Scenario Walkthrough: The Burned-Out PLC Output

Here is a classic failure mode I see when hobbyists and junior technicians transition from wiring simple lights to industrial automation.

The Setup:
A maker is automating a custom liquid filling machine using an Automation Direct BRX PLC. The PLC has sourcing transistor outputs rated for a maximum of 0.5A per channel at 24V DC. They wire a heavy-duty 24V DC hydraulic directional valve directly to Output Y0.

The Numbers:
The hydraulic valve coil draws 0.4A in steady state. Since 0.4A is well below the PLC’s 0.5A maximum rating, the builder assumes the circuit is perfectly safe and omits a flyback diode to save space in the control panel.

The Outcome:
The machine runs flawlessly for the first three days. On day four, the valve chatters violently when turning off, and the PLC output Y0 stops working entirely. The PLC’s diagnostic LED for that channel turns red. Even when the PLC logic commands the output OFF, the valve remains partially energized.

What Went Wrong:
The builder only looked at resistance (steady-state current) and ignored inductance. The hydraulic valve had a massive 2.5H inductance due to its large iron core. Every time the PLC transistor switched off, the collapsing magnetic field generated a 200V+ flyback spike. By day four, the cumulative stress of thousands of voltage spikes caused thermal runaway in the PLC’s internal MOSFET. The transistor shorted out internally, permanently connecting the 24V supply to the valve coil, bypassing the PLC logic entirely.

The Fix:
Replace the damaged PLC output module and install a 1N4007 flyback diode in reverse-parallel across the valve coil (cathode to +24V, anode to the PLC output). The diode provides a safe, low-resistance path for the inductive current to circulate and decay harmlessly as heat, clamping the voltage spike to roughly 0.7V.

1. De-energize the PLC and lock out the 24V power supply.
2. Verify the circuit is dead using a multimeter (measure < 1V DC across the coil).
3. Strip the 1N4007 diode leads and crimp them into the same ferrule terminals as the solenoid coil wires.
4. Ensure the silver stripe (cathode) on the diode points toward the positive (+24V) supply wire.
5. Restore power and test the switching speed; the valve should now turn off cleanly without chatter.

Common Confusions and Mistakes to Avoid

When working with resistance and inductance, a few misconceptions consistently lead to blown components and misdiagnosed circuits.

Confusing DC Resistance with AC Impedance

This is the most common mistake when troubleshooting 120V AC contactor coils. A technician will pull a contactor coil, measure it with a standard DMM, and read 15 Ω. Applying Ohm’s law (120V / 15Ω), they calculate the coil should draw 8 Amps. They conclude the coil is shorted and throw it away.

In reality, a 120V AC coil relies on inductive reactance (X_L = 2πfL) to limit current, not just DC resistance. At 60 Hz, that 15 Ω DC resistance might equate to an AC impedance (Z) of over 1,000 Ω, resulting in a normal operating current of just 0.12A. Always check the manufacturer’s nameplate for the AC VA (Volt-Ampere) rating rather than relying solely on a DMM resistance check. For a deeper dive into measuring motor and coil windings, refer to the testing guidelines published by Fluke.

Assuming Wire Resistance Acts Like Load Resistance

In long cable runs (e.g., a 100-foot run of 18 AWG wire to a 24V solenoid), the wire adds series resistance. While this drops the voltage reaching the coil, it does nothing to mitigate the inductive flyback spike generated at the coil itself. The flyback voltage is generated locally at the inductor and will still destroy the switching transistor at the source end of the cable, regardless of how much wire resistance is in between. You must place the snubber diode or RC network directly across the coil terminals, not at the PLC end of the cable.

FAQ: Resistance and Inductance