Am I right that (at least in the HVAC context), a relay switch is a
switch that turned on by a passing current.
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.
Is that correct?
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"?)
Thanks in advance,
Yes. Similar to the solenoid on a car starter. It enables a low-voltage (and
current) relay to supply higher voltage and current to the load. Imagine the
wiring necessary to snake two 60-amp wires to a thermostat!
Because "Fan Relay" has more letters than simply "Fan."
a relay is an electromagnetic switch. Voltage is applied to a coil which
creates an electromagnet, which pulls in the switch contacts. In the case of
your air-conditioning system, typically 24 volts from the air handler,
switched through the thermostat, send the 24 volts to the condenser relay,
pulling in the contacts, which in turn send 240 volts to the motor. The fan
relay, which is in the air handler, works the same, via the fan terminal of
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.
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.
Yes. But I rephrased it a little.
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
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.
In relay logic, we use terms like contacts, amarture, pole, reed. etc.
Not all relays are switches. As an example some are just indicators.
Any one remembers olden days telephone switching plant? Stroger, EMD
making all those noises like hundreds of crickets. Burnished contacts?
Those are lights, indicator lights**, controlled by relays maybe, but
why aren't the relays that control them still switches?
**As opposed to illumination lights and maybe some other kinds if
there are other kinds. (LED watches?)
They were relays. Sometimes you would find steppers. Another place
you find lots of them is inside old pinballs. They made complex (for
the time) logic via multiple relays latching relays and steppers.
They even have a "clock" made from a motor with wheels and contacts so
that relay logic can be walked through.
Sam, in HVAC speak, many trades have their own dialect to describe the
same common technology, the big high current relay that switches power
on to your AC compressor is called a "contactor". In the electrical
trade, the high current relays will also be called "contactors". Also
in HVAC speak you will see "relay" and "pilot or pilot duty" relay. The
term "relay" will be used to describe relays for turning on air handler
blowers, draft inducer blowers, condenser fans and other low current
line voltage operated devices. A "pilot" relay is often used to describe
relays used for low voltage and low current control related tasks. For
example, if the contactor coil in an HVAC system drew more current than
the small contacts in the thermostat could handle, a pilot relay would
be used between the t-stat and the contactor. Another example would be
a furnace equipped with a humidifier which may be controlled by a pilot
relay interfacing with the thermostat, humidistat, air pressure sensor,
sail switch and/or air handler blower relay.
The word "relay" in the term "relay switch" is descriptive, like
"combination lock". You could argue that the word "switch" is
redundant in any vaguely electrically related context.
Meanwhile, in case no one has mentioned it yet, many of what are
normally referred to as relays or contactors today are actually solid
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