End Rush Current Explained: Turn-Off Transients vs. Inrush Surges

End rush current refers to the transient electrical spike—technically known as interrupting current, turn-off transient, or inductive kickback—that surges through a circuit the exact moment an inductive load is switched off. While startup surges get all the attention in boardroom presentations, the 'end rush' at power-down is the silent killer that destroys mechanical switch contacts and fries microcontroller driver transistors on the workbench. In this guide, we will break down the physics of this turn-off surge, run a hard numeric example using real component values, and clarify the persistent confusion between end-of-cycle transients and startup inrush.

The Physics of the Turn-Off Transient

To understand what happens at the end of a switching cycle, we have to look at the fundamental law of inductors: they resist changes in current. The governing equation is V = L(di/dt), where V is the induced voltage, L is inductance, and di/dt is the rate of change of current over time.

When you open a switch or turn off a transistor feeding an inductive load (like a motor, relay coil, or solenoid), you are forcing the current to drop from its steady-state value to zero almost instantly. Because the time change (dt) approaches zero, the induced voltage (V) theoretically approaches infinity. The collapsing magnetic field desperately tries to keep the electrons moving in the same direction.

What It Changes in a Real Circuit

This transient spike fundamentally alters the lifespan and reliability of your installation. In mechanical switches and relays, the high voltage ionizes the air gap as the contacts separate, creating an arc. This arc melts microscopic craters into the metal—a process called contact pitting—which can reduce a relay's mechanical lifespan from 100,000 cycles to under 5,000. In solid-state circuits, the voltage spike easily exceeds the breakdown voltage (V_CEO or V_DS) of your driving transistor, causing avalanche breakdown and immediate, catastrophic silicon failure. Furthermore, the rapid dV/dt generates massive electromagnetic interference (EMI) that can reset nearby logic boards or corrupt ADC readings.

Worked Numeric Example: Calculating the Inductive Spike

Let us move away from abstract theory and calculate the exact end rush transient using a common bench component: the Omron G5V-2 12V DC signal relay.

• Coil Resistance (R): 288 Ω
• Coil Inductance (L): ~1.2 H (Henries)
• Supply Voltage: 12V DC

First, we find the steady-state current using Ohm's Law:

I = V / R = 12V / 288Ω = 41.6 mA (0.0416 A)

Next, we calculate the magnetic energy stored in the coil at steady state:

E = 0.5 × L × I² = 0.5 × 1.2 × (0.0416)² = 1.04 millijoules.

Now, assume you are driving this relay with a standard 2N2222 NPN transistor, and the microcontroller pulls the base low to turn it off. The transistor stops conducting, and the current drops from 41.6 mA to 0 A in roughly 1 microsecond (1 µs) because there is no freewheeling path.

Applying the inductor formula:

V_spike = L × (di / dt) = 1.2 × (0.0416 / 0.000001) = 49,920 Volts

End Rush vs. Inrush: Clearing Up the Confusion

The most common mistake hobbyists and junior technicians make is confusing end rush current with inrush current. Because both involve 'surges' and 'spikes', the terms are frequently swapped in forum posts and casual bench talk. Here is exactly how they differ.

Where You Meet This in Practice

You will encounter turn-off transients anywhere an inductive load is switched by a semiconductor or mechanical contact. Here is how to handle them based on your specific hardware topology, referencing the standard flyback diode topology and modern TVS alternatives.

1. Slow-Switching DC Relays and Solenoids

For standard 12V or 24V DC relays switching a few times a second, a standard rectifier diode is perfect. Place a 1N4007 in reverse bias across the coil (cathode to the positive supply, anode to the transistor collector). When the transistor turns off, the end rush current circulates safely through the diode and the coil until the magnetic field decays. The clamp voltage is only ~0.7V above the supply rail.

2. High-Speed PWM Motor Drives

A standard 1N4007 has a slow reverse recovery time (trr ~30µs). If you are driving a DC motor with 20 kHz PWM, the diode will not turn off fast enough, causing shoot-through and massive heat. Here, you must use a Schottky diode like the 1N5819 or SS34, which has virtually zero reverse recovery time, keeping your MOSFETs safe at high switching frequencies.

3. Fast Decay Requirements (Active Clamping)

The problem with a standard flyback diode is that it keeps the coil voltage very low, meaning the current decays slowly (di/dt is small). If you need a relay to drop out instantly, use a Transient Voltage Suppressor (TVS) diode like the SMBJ15A. This clamps the end rush spike to a safe 15V, which is high enough to force the magnetic field to collapse rapidly, but low enough to protect your driving transistor.

4. AC Inductive Loads (Contactors and AC Motors)

Diodes do not work on AC. For AC contactors, you must use an RC snubber network (typically a 100Ω resistor in series with a 0.1µF X2-rated capacitor) placed directly across the switch contacts or the coil. This limits the rate of voltage rise (dV/dt) across the opening contacts, preventing the air from ionizing and extinguishing the arc instantly.

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