Understanding Relays

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VOLTAGE SPIKES
When the switch is closed (shown left), current flows through the coil from positive to negative as shown in red. This current flow creates a magnetic field around the coil. The top of the coil is positive, and the bottom is negative.

When the switch is opened (shown on right), current stops flowing through the control circuit coil, and the magnetic field surrounding the coil cannot be maintained. As the magnetic field collapses across the coil, it induces a voltage into itself, creating a reverse polarity voltage spike of several hundred volts. Although the top of the coil is still 12 volts positive, the bottom of the coil produces several hundred positive volts (200+ volts or more); 200 is "more positive" and stronger than 12 volts, so current flows from the bottom of the coil up towards the top.