[jalessi]

Crevice Reamer,

Often, a DC relay will have a 'flywheel rectifier' [FR] placed reverse-biased across the input terminals, so as to prevent a high back-EMF as the magnetic field collapses upon de-energizing.

Plugging a DC relay with a FR into an AC circuit may cause an overload as half the AC cycle could be passing through the FR as a short-circuit. Incorrectly connecting the DC supply will cause a similar effect. Sometimes a second rectifier is installed in the casing of the relay so as to overcome the problem of reverse polarity.

An AC relay will be wound with different wire as the inductance of the wire will offer some limitation to the current that the relay draws when energized. Instead of an FR it may be possible to place a small capacitor or a neon indicator across the terminals of the relay to minimize any back-EMF effects as the magnetic field collapses, should the need arise, if indeed the relay does not already have these components fitted inside its casing.

So, in summary, they are two different animals. While one may get away with interchanging DC and AC relays on occasions, it is a practice that involves risk to the relays themselves and to the equipment being controlled by their contacts, so it is a practice best avoided.

Jeff...