Relay

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A relay is an electrically operated switch.

Contents 1 Polarisation 2 AC/DC 3 Power use 4 Common abbreviations 5 Spark suppression 5.1 Metal Oxide Varistor 5.2 Double zener diodes 5.3 Diode 5.4 RC circuit 6 Failure modes

Polarisation

Both the coil and the contacts may be polarized. The contacts may depend on a specific direction of material migration, if the polarity is wrong the contacts may suffer failure after a fraction of the rated number of operations.

AC/DC

The ratings are different for AC and DC currents. DC currents cause more arc erosion on the contacts and generates material migration in a single direction. DC currents will have a lower current/voltage rating.

Power use

P=V²/R

Common abbreviations

• NC - Normally closed
• NO - Normally open
• SP - Single pole
• DP - Double pole
• ST - Single throw
• DT - Double throw
• CO - Change over

Spark suppression

Metal Oxide Varistor

Double zener diodes

Diode

RC circuit

Use in parallel with the contacts at high voltages or the load at low voltages.

Failure modes

The contacts may weld together at high currents.