"Dave Plowman (News)" wrote in message
...
In article .com,
Steve wrote:
How does an AC relay work? I understand that the magnetic field of a
DC relay coil attracts the contact arm and I assumed an AC relay just
had a diode to convert coil current to DC - however when I tried to
find an AC relay fault there was no diode. I thought 50 or 60 Hz
alternating magnetic field cannot produce such a corresponding
movement in a mechanical contact arm so I would have thought the net
magnetic effect would be zero (no overall attraction or repulsion).
Given the relay obviously operated before, I just can't see how. I
did think the 'AC' rating just meant the contacts but I don't see why
this wouldn't just be a current rating.

AC coil relays are common. A magnet of any type will attract a magnetic
substance regardless of polarity - it's just if the second substance is -
or gets - magnetized that they can repel. So you use a low permeability
substance for the armature. Something like soft iron.

--
*Gaffer tape - The Force, light and dark sides - holds the universe together*

Dave Plowman London SW
To e-mail, change noise into sound.


I haven't seen anybody describe the real difference between AC and DC relays.
A relay has a coil and the pole piece, or armature. Voltage applied across the
coil causes current to flow in the coil, creating a magnetic field, which causes
the armature to be pulled into the center of the coil, thus energizing the
relay's contacts. Both types of relays operate on the same principle of
electromagnetism.
The difference between the DC and AC relay is that the AC relay has a shading
pole, or a heavy shorted turn imbedded into one end of the armature. Its
purpose is to maintain a high flux level in the armature when the current in the
main coil goes through zero. This acts to eliminate buzzing or chattering that
is evident when you drive a DC relay with an AC voltage.
There is no diode in an AC relay. A diode is commonly used across the coil of a
DC relay to eliminate the high reverse EMF caused by the collapse of the coil's
magnetic field. If a diode were used on an AC relay, it would create a short
circuit every half cycle, something you want to avoid.

An AC relay can be used in a DC circuit, but not vice versa. If you drive a DC
relay from an AC source, the lack of a shading pole in the relay will cause
buzzing, possibly allowing the contacts to bounce during zero crossings of the
power source.
If you drive an AC relay from a DC power source, the shading pole on the AC
relay will cause the relay to be slow to release when power is removed.

Cheers!!!
--
Dave M
MasonDG44 at comcast dot net (Just substitute the appropriate characters in the
address)

"In theory, there isn't any difference between theory and practice. In
practice, there is." - Yogi Berra