24 VAC tutorial
The 24VAC is used primarily for two purposes. The first is
control, such as in door bells, and thermostats. The other is to supply
operating power, such as in irrigation valves, rotation encoders and CCTV
cameras. In all cases the low voltage is used because it is safe and doesn't
need an electrician to wire it, which is not the case for 110VAC or 220VAC
But why AC instead of DC? The advantages of AC are lower
cost solenoids and motors, less relay contact erosion and better arc
suppression. Solenoids are cheaper because they don't need current limiting
solely on wire resistance because inductance limits the current. 24VAC
solenoids and relays do not have the back-EMF problems that DC operated devices
have, so there is less EMI noise.
24VAC can run longer distances than 12VDC. It isn't
because of the alternating current, but because of the 24V. To supply 24 Watts
will require 1 amp at 24VAC and 2 amps at 12V. The voltage drop in a wire is
proportional the amps only. Losing 1 volt due to ohmic losses in the wire is
more significant for 12V than for 24V (8% versus 4%).
AC/AC -- The most common source of 24VAC is the
AC/AC mount power supply. This is a simple step-down low voltage transformer,
with mains voltage AC in and 24V AC out. There are no other active or passive
components. The transformer is designed to give a certain voltage at a certain
current. AC/AC adapters have no line regulation at all, so their output voltage
is proportional to the input voltage. Wall transformers have a small amount of
load regulation, since the transformer windings are a significant part of the
Since the mains voltage can vary, depending on the
distance to the transformer, and the load the transformer sees from you and
your neighbors, the output of a 24VAC transformer will also vary accordingly.
Typical specifications for the mains are ±10%, so the output of the 24
VAC transformer will vary from 21.6VAC to 26.4VAC from this factor alone.
A 24V RMS sine wave has a peak voltage of 33.9 V. When
rectified this results in the pattern shown in the diagram, with peaks of 33.9V
and valleys of 0V. Filtering this pulsed waveform will even this out, but there
will always be some ripple. The output of a 24VAC transformer is not actively
regulated. The transformer is designed to source 24VAC at the nominal current
listed on the label. If you draw more current the voltage will sag, if you draw
less then the voltage will increase. Furthermore, the mains voltage can
typically vary by ±10% depending how far you are from the mains
substation, so even the nominal voltage of the transformer can vary
±2.4V. Thus, equipment designed for 24VAC must be flexible in its
acceptable voltage. Solenoids and relays actuated by the 24VAC power are very
flexible in their input voltage, as are synchronous motors. A synchronous motor
is designed to run at the AC frequency of the source power. The advantages are
that the speed is independent of the load, can be used as timers, have less EMI
than brushed DC motors and make efficient use of available power.
The single diode, half wave rectification DC adapter is
used for the cheapest of battery chargers, toy motors, battery adapters and
other places that the 100% ripple and 50% duty cycle won't matter. Adding a
capacitor can smooth out the ripple, but since it needs a larger capacitor than
full wave bridge rectification, the cost trade off usually leans toward the
full wave rectifier.
An AC/AC transformer has the advantage over switchmode DC
power supplies in that it can supply surge current. For example a sprinkler
valve can have a holding power of 5 watts, but an inrush current of 8 watts.
The transformer will sag in voltage during the inrush pulse, but will not have
to be rated at the higher power.
Capacitively coupled noise
Running a twisted pair of 24VAC next to signal wires can
cause trouble. The AC can capacitively couple to the signal wires to add 50Hz
or 60Hz noise to the signal. Moving the signal wires away from the twisted
pair, or shielding the twisted pair will solve the noise problem
Thermostats and HVAC
The 24VAC can be used in HVAC systems to send a signal
from the thermostat to turn on the heat or cooling. It is also sometimes used
to synchronize the thermostat's internal clock and supply power for electronics
and pilot lights. The thermostat turns the unit on by shorting two wires
together, know by the jargon "call." The wiring doesn't usually supply both
lines of the 24VAC power up to the thermostat, but uses one line (Red) which
connects to the correct "call" wire to turn on the heat or cool functions. For
thermostats that require power to operate the other leg of the 24VAC supply can
be run, which is often blue. This is sometimes called the "C" wire, short for
"common," meaning "ground." Some thermostats also try to steal some power from
the "call" wire, but if they steal too much the heater will turn on.
A common multi-stage heat pump system has the following
wiring & connections:
|Call for heat
|Force fan on
|Compressor stage 1
|Compressor stage 2
||Orange or brown
||C/O, O, or B
None of the DC wall warts mentioned so far have any line
regulation. It is possible to add a three terminal voltage regulator to the
adapter to perform volt line and load regulation, but because of the limited
heat sinking capability these are limited to low power.
ResolversAnother advantage of using 24VAC in your
power distribution is to power "resolvers" which look like a motor, but use the
phase to send back the angle of rotation of the rotor to a controller. Analog
resolvers have high angular resolution, and are robust and dependable shaft
encoders. The windings are designed to create a sine wave output and a cosine
wave output. Comparing these two signals gives an absolute rotation reading.
The zero reading is typically when the sine wave is maximum and the cosine is
minimum. The position is found by taking the ratio of the sine and cosine
winding voltages and taking the arctangent. The ratio makes the reading
inherently immune to noise on the power line, and compensates for variations in
Many sprinkler systems use 24VAC as a safe way to get AC
power to timers, motors, valves, and solenoids.
A solar panel and battery combination are intrinsically
DC. But what if you need AC you have to find a way to invert it. PowerStream has a variety of 24VAC
inverters with 12V, 24V and 48V inputs to allow security cameras, irrigation
systems, etc. to operate off the solar power supply.
PowerStream also has developed a
unique 24VAC battery backup. It can be inserted anywhere in a 24VAC system to
charge a battery and supply an uninterruptible 24VAC to devices downstream.
24VAC to DC converters