Powering Multiple Devices from a Common Transformer

After properly sizing a control transformer for your application, there is an additional step in the selection process that should be considered. Even though the necessary steps have been taken to select a properly sized transformer to power multiple 24 VAC devices (such as BAS controllers, transducers, actuators and power supplies) there is a potential, but not so obvious, problem that can exist. If not foreseen and corrected, this problem can cause blown fuses and/or physical damage to devices when the system is energized.

Many electronic HVAC controllers and interface devices are designed to take AC power on their input terminal strips. However, because the electronic components on these devices require DC power, the AC power at the terminal strip is converted to DC by use of an internal (on board) power supply circuit (rectifier/filter/regulator). Two common types of power supply circuits used are called a “Full-wave bridge rectifier” and a “Half-wave rectifier.” The problem with powering multiple devices from one AC power source stems from some devices using the Half-wave circuit and some using the Full-wave circuit.

To better understand why mixing Half- and Full-wave rectifiers on a common AC power source can be a problem, it is important to understand the difference between these two power supply circuits.

NOTE: AC “G” and Control signal “-” are the SAME electrical connection

The Half-wave rectifier is shown in Figure 1. Note that one side of the AC power input is connected directly to the negative side of the DC output. On the transformer’s positive half-cycle, the diode conducts. This allows the transformer current to charge the filter capacitor, as well as supply load current. On the transformer’s negative half-cycle, the diode turns off. The stored charge in the filter capacitor supplies load current until the next positive half-cycle. The Half-wave rectifier only uses transformer current every other half-cycle. It is not very efficient, but it is inexpensive and does a good job for low current power supplies.

NOTE: AC “G” and Control signal “-” are NOT the same electrical connection.

The Full-wave bridge rectifier is shown in Figure 2. Note that neither side of the AC power input is connected directly to the negative side of the DC output. On the transformer’s positive half-cycle, diodes D1 and D3 conduct. This allows the transformer current to charge up the filter capacitor, as well as supply load current. On the transformers negative half-cycle, diodes D2 and D4 conduct, supplying transformer current to the filter capacitor and load. The Full-wave rectifier utilizes the transformer current on both positive and negative half-waves. It is more efficient, and that is why it is often used in higher-current power supplies.

So what is the problem with using a common AC transformer to power a device that has a Full-wave rectifier and a second device that has a Half-wave rectifier? Consider Figure 3 which shows just such a set-up. The control signal “-” terminals on both modules are either directly connected together (as shown) or indirectly connected by a common ground. Looking carefully, you can see that the minus output of the Full-wave rectifier is connected directly back to one of its AC inputs via the “pass thru” common connection inside the Half-wave device. This connection actually places diode D4 in the bridge directly across the AC transformer winding. Every time the AC voltage goes to the polarity that turns on the diode, the diode shorts out the transformer. In other words, the diode shorts the transformer 50% of the time. There are several symptoms that can occur from this situation:

A fuse blows every time the commons of the devices are connected together.
The diode shorting the transformer fails open.
The diode shorting the transformer fails shorted and the transformer may burn up.
The circuit board foil traces connecting to the diode melt open or are blown off the board.

Now that the problem has been pinpointed, some general guidelines can be given for connecting multiple AC powered devices together.

It is okay to connect multiple devices to the same AC transformer and share signal commons if: (a) every device uses a Half-wave rectifier; and (b) the same AC lead on every device is used for a common.
It is okay to connect the signal common of a device that uses a Full-wave rectifier to other signal commons if the Full-wave rectifier device has a dedicated isolated AC power transformer connected only to the device’s power screws and nowhere else.
It is okay to connect the signal common of a device that uses a Full-wave rectifier to other signal commons if the device contains its own internal AC isolation transformer.
It is okay to power a Half-wave rectifier device and a Full-wave rectifier device from the same AC transformer if the signal commons are absolutely, positively isolated from each other.
If it is absolutely necessary to power a Half-wave device and a Full-wave device from the same AC source, and their DC control signals have to interact, a signal isolator such as the DT13 will be necessary.

These guidelines all require that the type of input power circuit (Full- or Half-wave) be known. Since most devices do not come with detailed schematic diagrams, this may be difficult to determine. Following are a few ideas that may help:

If a 24 VAC powered device has three wiring terminals, (power “+”, signal”+” and a shared common for power “-” and signal “-“), then a Half-wave rectifier is being used.
If a 24 VAC powered device has four wiring terminals, (power “+”, power “-“, signal”+”, signal”-“), first check the product’s data sheet for any clues. Look for any notes that might indicate that the power “-” and the signal “-” terminals are electrically the same and hence, a Half-wave device.
If no indication is given on the product’s data sheet, try using an ohmmeter to check the resistance between the signal “-” terminal and both of the power terminals. A reading of a couple of ohms or less between the signal “-” and either power terminal would indicate a common connection between these terminals and a Half-wave circuit.
If no common connection can be found, the device is probably a Full-wave rectifier. However, some devices incorporate a built-in isolation transformer that isolates both power terminals from both signal terminals. The differential pressure model T40 is an example. This type of device can be used with either Full- or Half-wave devices.
Finally, the manufacturer or supplier of the device should be able to provide the required information. Some manufacturers may require that only their products be connected to a common transformer. Others may even require that each of their devices have its own dedicated and isolated transformer.

In summary, many devices (BAS controllers, transducers, actuators and power supplies) used for the control of HVAC systems can be externally powered from 24 VAC. Because the electronic components of these devices require direct current, the AC power is internally converted to DC. When powering multiple devices from a common 24 VAC source, problems can arise due to different techniques that are used to convert the AC to DC. In other words, it is not as simple as plugging various appliances into an AC outlet in your house. Whenever you have different devices from different manufacturers, be careful to separate those devices that utilize a Full-wave rectifier from those using a Half-wave rectifier. When any doubt exists, provide a separate transformer. The small expense of an additional transformer or two will more than make up for all of the time and money spent on troubleshooting.