Understanding Snubber Circuits for Inductive Loads

A snubber circuit is an electrical circuit used to suppress voltage spikes and transients caused when switching inductive loads. These spikes are dangerous to switching elements like relays, transistors, or triacs and can lead to electrical noise, contact pitting, and premature failure.

Why Snubber Circuits Are Needed

When a switch (mechanical or semiconductor) opens an inductive load, the collapsing magnetic field generates a high voltage spike due to the energy stored in the inductance. This can result in arcing across relay contacts or overvoltage damage to semiconductors.

This phenomenon is well-documented in engineering literature:

1. The Art of Electronics by Paul Horowitz and Winfield Hill (3rd Edition) explains that snubber circuits are used to protect switching devices from inductive kickback. It specifically states that turning off inductive loads causes voltage spikes that must be suppressed by a diode or snubber.

2. Power Electronics: Converters, Applications, and Design by Ned Mohan confirms that snubbers are essential to protect semiconductor devices from the effects of inductive switching.

3. IEEE papers such as "Snubber Circuits: Theory, Design and Application" outline that snubbers suppress voltage transients that occur when current through an inductor is suddenly interrupted.

4. Application notes from STMicroelectronics and Texas Instruments also support this, stating that snubbers are critical for switching inductive loads like motors and transformers. For example, ST’s AN308 and TI’s SLVA255 make direct statements about the role of snubbers in protecting switches from the energy stored in inductive loads.

When Snubbers Are Necessary

You should use a snubber circuit in the following cases:

• Switching inductive loads such as motors, relays, solenoids, and transformers.

• Using mechanical relays or triacs that control inductive AC loads.

• Observing arcing or premature contact wear.

• Experiencing false triggering or noise on nearby circuitry due to switching.

Snubbers are generally not necessary for resistive loads like incandescent bulbs or heating elements. However, capacitive-drop LED bulbs, while not inductive, can benefit from snubbers due to inrush currents and fast rise times, which can also damage mechanical contacts.

Types of Snubber Circuits

1. RC Snubber: Used mainly with AC inductive loads or triacs. Consists of a resistor and capacitor in series connected across the switch.

2. Flyback Diode: Used in DC circuits across inductive loads. This diode allows current to circulate through the load when the switch opens, safely dissipating stored energy.

How to Calculate RC Snubber Values

The goal is to suppress the voltage spike without creating too much power loss or delay in switching. Here’s a basic approach for RC snubbers:

1. Capacitance (C)

Start with a value in the range of 100 nanofarads to 470 nanofarads. For 230V AC use, the capacitor must be rated for X2 class (safety-rated for across-the-line applications) with a voltage rating of 250V AC or higher.

2. Resistance (R)

Calculate the resistance using the formula: