On Sun, 29 May 2011 22:23:05 -0400, Sam Takoy
wrote:

Hi,

Am I right that (at least in the HVAC context), a relay switch is a
switch that turned on by a passing current.

Yes. An electrically operated (electric) switch. In addition to
what others have said, a relay switch is usually just called a relay.
Electrical relay is implied because they are by far the most commmon.

For example, in the compressor, there is a line voltage cable that's
always on, but the unit does not turn on until the thermostat completes
a circuit thereby flipping the relay switch on.

It completes the circuit to the relay coil, and the current in the
coil makes it an electromagnet, which closes a mechanical switch for
the usually higher voltage current. (In some relays there can be 8
or more DPDT switches that get closed and opened at the same time.
when the relay is energized. My employer sold me an early Xerox
machine for a dollar. It had about 30 relays, two of which locked
("latching relays", that is, they remained closed after the current
was no longer applied. To open, some other circuit had to send a
current through the coil in the opposite direction.)

The thermostat for the fuser (the heater that melts the "ink dust"
into ink was broken, and when the copies came out, all the text was
there, but you could just put the paper near your lips and blow off
the toner, and the paper would be blank again.

Is that correct?

Yes. But I rephrased it a little.

Also, if a contact in the thermostat is labeled "Fan Relay", does the
word "Relay" refer to the relay switch in the fan? (In that case, why
not just say "Fan"?)

Because it doesn't go straight to the fan, it seems. It goes to a
relay, to the coil of a relay, and the fan is powered when the coil is
energized and the switch in the relay (the secondary) closes and
completes a separate circuit that includes a power supply and the fan
motor.

All of this enables the switches and wires and coils, the control
circuit, to be comparatively thin and run on low voltage AC or DC,
which is safer, and the switched circuit to be AC or DC and anywhere
from low to high voltage, but usually higher than the control circuit,

A(n electric) relay is an elecrically-controlled switch. A solenoid
is an electrical controlled device, possibly a electric switch or
maybe just mechanical, in which a metal rod goes through the center of
the electric coil and moves when the coil is energized.

Thanks in advance,

Sam