24VAC tutorial and frequently asked questions about irrigation, safety, HVAC and control systems (2023)

24 VAC-Tutorial

Why 24VAC?

24 VAC voltage is mainly used for two purposes: The first is to control doorbells, control panels and thermostats. The other is to provide operational power such as bells, irrigation valves, rotary encoders, low voltage lighting systems, and CCTV cameras. In both cases, low voltage is used because it is safe and does not require an electrician to wire it, which is not the case with 110 VAC or 220 VAC circuits. Also, voltages below 24 would require more amps to deliver the same power, resulting in a larger voltage drop across the resistance wires.

But why AC instead of DC? The advantages of AC are inexpensive solenoids and motors, less erosion of relay contacts, and better arc suppression. AC Solenoids are cheaper than DC Solenoids. This is because DC solenoids rely on coil wire resistance to limit their current. AC magnets can limit their current by their AC impedance. Solenoids and 24 VAC relays do not have the back EMF problems found in DC powered devices, so there is less EMI noise.

24 VAC can also travel longer distances than 12 VDC when using the same wire gauge and cable length. It is not for AC but for 24V. To supply 24 watts you need 1 amp at 24 VAC and 2 amps at 12V. The voltage drop across a wire is only proportional to the amps flowing through it. The 1 volt loss due to ohmic losses in the cable is also more pronounced at 12 V than at 24 V (8% vs. 4%).

CA/CA-- The most common 24VAC source you will find is the wall mounted AC/AC power adapter. This is a low voltage step-down transformer with AC line voltage input and 24VAC output. It's cheap because there are no other active or passive components (except maybe a fuse). However, this means that the output voltage can only be regulated to a limited extent. The transformer is designed to deliver a specific voltage at a specific current; therefore, it often has to be designed specifically for the application. AC/AC adapters don't have onelineregulation at all, so that its output voltage is proportional to the input voltage. Wall transformers have a small amount ofloadregulation as the transformer windings are an important part of the load.

24VAC tutorial and frequently asked questions about irrigation, safety, HVAC and controlsystems (1)

Because the AC line voltage can vary depending on the distance to the transformer and the load the transformer receives from you and your neighbors, the rating of a 24 VAC transformer will vary accordingly. Typical AC line specifications are ±10%, so the 24 VAC transformer output will range from 21.6 VAC to 26.4 VAC by just that factor since the transformer is unregulated.

24VAC basics

24VAC tutorial and frequently asked questions about irrigation, safety, HVAC and controlsystems (2)half-wave rectification
24VAC tutorial and frequently asked questions about irrigation, safety, HVAC and controlsystems (3)full wave rectification

A 24V RMS sine wave has a maximum voltage of 33.9V. When rectified, the pattern shown in the diagram results with peaks at 33.9V and valleys at 0V. The filter of this pulsed waveform matches this out, but there will always be a wave. The output of a 24 VAC transformer is not actively regulated. The transformer is rated to provide 24 VAC at the amperage shown on the label. If you use more current, the voltage will drop, if you use less, the voltage will go up. Also, line voltage can typically vary by ±10% depending on how far you are from the substation, so even the 24VAC transformer rating can vary by ±2.4V. The transformer designed for 24 VAC must be relatively flexible in its acceptable operating voltage. Solenoids and relays that operate on 24 VAC 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 power supply. The advantages are that the speed is independent of the load, they can be used as a timer, they have less EMI than brushed DC motors and they use the available energy efficiently.

Single Diode Half Wave Rectifying DC Adapter is used for battery chargers, toy motors, battery adapters and other cheaper places where 100% ripple and 50% duty cycle is not a concern. Adding a capacitor can smooth out the ripple, but since you need a larger capacitor than a full-wave bridge rectifier, the compensation cost tends toward the full-wave rectifier typically.

An AC/AC transformer has the advantage over DC switching power supplies that it can deliver peak current. For example, a sprinkler valve might have a holding power of 5 watts but an inrush current of 8 watts. The voltage of the transformer drops during the excitation pulse, but it does not have to be rated for the highest power.

Operation of 24 VAC relays and 24 VDC solenoids

The magnetic reactance of the relay or solenoid controls the current drawn to open or hold open the magnetic circuit. 24V DC would only depend on the resistance of the wires. Therefore, a 24VAC relay would overheat when operating on 12VDC.

Capacitively coupled noise

Running a 24 VAC twisted pair across the signal lines can cause problems. AC power can be capacitively coupled to the signal wires to add 50 Hz or 60 Hz noise to the signal. Routing the signal wires away from the twisted pair or shielding the twisted pair solves the noise problem.

Thermostats and HVAC

24 VAC can be used in HVAC systems to send a signal from the thermostat to turn on the heating or cooling. It is also sometimes used to synchronize the thermostat's internal clock and to power the electronics and indicator lights. The thermostat turns the device on by shorting two wires together, known by the colloquial "call". Wiring does not typically provide both 24VAC power lines to the thermostat, but uses one line (red) connected to the correct "call" wire to turn on the heating or cooling functions. For thermostats that require power to operate, you can power the other leg of the 24V AC supply, which is usually blue. This is sometimes referred to as the "C" wire, short for "common," meaning "ground." Some thermostats will also attempt to steal some energy from the "call" wire, but if they steal too much the heater will turn on.

A typical multistage heat pump system has the following wiring and connections:

description Kor Terminalcode
24VAC return Rot Rf
Pide Kalor Branco W1
Turn on blower Green GT
Compressor stage 1 Gelb A1
Stage 2 compressor not by default Y2
To change orange or brown C/O, O or B


Another benefit of using 24VAC in your power distribution is to power "resolvers" which look like a motor but use phase to return rotor rotation angle to a controller. Analog resolvers have a high angular resolution and are robust and reliable axis encoders. The windings are designed to produce a sine wave output and a cosine wave output. The comparison of these two signals results in an absolute speed value. The zero value is typically when the sine wave is at its maximum and the cosine is at its minimum. The position is found by taking the ratio of the sine and cosine winding voltages and the tangent of the arc. The ratio makes the reading inherently immune to power line noise and compensates for temperature variations.

irrigation systems

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

lighting systems

Various low-voltage incandescent lamp systems have been installed in buildings. They are usually powered by large toroidal transformers that have multiple input taps to compensate for the voltage drop across the lamp. A large building might have a closet full of toroids.

solar systems

A solar panel and battery combination is inherently DC. But what if you need AC? You need to find a way to reverse this.PowerStream has a range of 24V AC drives with 12V, 24V and 48V inputs to run security cameras, irrigation systems, etc. on solar power source.

emergency battery

It is often difficult to find a good solution to provide battery backup for a low voltage AC system. PowerStream has also developed a unique 24VAC battery backup. It can be inserted anywhere in a 24 VAC system to charge a battery and provide uninterrupted 24 VAC to downstream equipment.

24 VAC to DC converter

Several converters are available to convert 24 VAC to usable DC power. 24VDC address conversion is available for driving PLCs, 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 range of fluorescent lamp ballasts that operate on 24 VAC.
voltage conversion Products
24VAC to 24VDC converter
24 V CA und 5 V CC
24 VAC bis 1-18 VDC variabel
24 V CA und 12 V CC
24VAC 40W ballast for fluorescent lamp
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