January 15, 2021

24VAC Tutorial

24VAC inverters, with 12VDC, 24VDC, and 48VDC inputs
12 volt direct current to 24 volt alternating current 60 Watt  pure sine wave inverter
24VAC input and output uninterruptible power supply.
24VAC low voltage battery backup

24 VAC tutorial

Why 24VAC?

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, low voltage lighting systems 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 circuits.

But why AC instead of DC? The advantages of AC are lower cost solenoids and motors, less relay contact erosion and better arc suppression. AC solenoids are cheaper than DC solenoids. This is because DC solenoids depend on the resistance of the coil wire to limit their current. AC solenoids can limit their current by their AC impedance. 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 also more significant for 12V than for 24V (8% versus 4%).

AC/AC -- The most common source of 24VAC is the AC/AC wall 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 (other than a fuse). The transformer is designed to give a certain voltage at a certain current, so often have to be designed specifically for the application. 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 load.

AC transformer wall plug-in power supply, as used in an AC adaptor

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.

24VAC fundamentals

Half wave rectification pulsed sine wave Half wave rectification
full wave rectification pulsed sine wave Full wave rectification

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 24VAC 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:

Description  Color  Terminal Code  
24VAC return Red RF  
Call for heat White W1  
Force fan on Green GT  
Compressor stage 1 Yellow Y1  
Compressor stage 2 Not Standardized Y2  
Changeover Orange or brown C/O, O, or B  


Another 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 temperature.

Irrigation systems

Many sprinkler systems use 24VAC as a safe way to get AC power to timers, motors, valves, and solenoids.

Lighting systems

A number of low voltage incandescent lighting systems have been installed in buildings. They typically run off of large torroidal transformers, which have multiple input taps to account for voltage drop at the bulb. A large building might have a closet full of these torroids.

Solar systems

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.

24VDC to 18VAC 40 Watt  pure sine wave inverter for Bosch MIC550 series cameras

Battery backup

It is often hard to find a good solution to give a low voltage AC system a battery backup. 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 low voltage battery backup

24VAC to DC converters

There are various converters available for converting 24VAC to useful DC power. A direction conversion to 24VDC to run PLCs is available, as are 24VAC to 12VDC converters, 24VAC to 5V converters, and even an adjustable DC output converter for special applications. In addition we have developed a fluorescent light ballast series that runs on 24VAC.
Voltage conversion Product
24VAC to 24VDC converter 24 volt AC to 24 volt DC converter 60 watts
24VAC to 5VDC 24VAC to 1V, 3V, 3.3V, 4V, 4.8V, 5V, 6V 8V, 8.4V, 9V, 10V, 11V, 12V, 13V, 14V, 14.4V, 15V, 16V, 17V, 18V
24VAC to 1-18VDC variable 24VAC to 3.3V, 5V, 6V, 8V, 8.4V, 9V, 10V, 12V, 14V, 15V, 16V, 17V, 18V converter
24VAC to 12VDC Efficient AC/DC power converter 24VAC to 12VDC at 1.6 amps, switchmode
24VAC to 40W ballast for fluorescent bulb 24VAC fluorescent light ballasts for 40 watt tubes

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