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Why You Should Not Mix Full-Wave and Half-Wave Powered Devices

Many devices in the controls and HVAC industry are powered by 24VAC. A transformer is used to convert the higher line voltage of 120 or 240VAC into the lower 24V to power the devices. The total power capacity of the devices is checked to size a proper transformer for the job. But another important detail is mostly missed.

The 24V AC supply is further converted into the DC voltage required by the circuits of the device. Each device has an internal power circuit that does this AC voltage to DC voltage conversion and all devices are not created equal. Some devices use a full-wave rectifier bridge (four diodes) and others use a half-wave rectifier (one diode). It is because of this difference that special care must be taken when connecting AC powered devices together to the same transformer. A full-wave rectifier device converts both the AC sine waves into DC while a half-wave rectifier device only converts one. The full-wave devices are good for high-current devices while half-wave rectifier devices are good for low current applications and for sharing the same transformer.

When full-wave and half-wave rectifier devices are powered by the same transformer and their DC commons are tied together, it creates a short for one half of the AC cycle in the bridge rectifier. This can result in either a blown fuse on the device (if there is one), a blown diode in the full-wave rectifier circuit or a blown transformer.

There are also special considerations when just powering full-wave rectifier devices depending on transformer secondary grounding. A full-wave device may either have the transformer secondary at the input grounded or the output on the DC load grounded. Grounding on both the input and output will result in a short for half AC wave causing damage to the device and/or the transformer. Depending on other circumstances, even full-wave rectifier devices may each need their own separate transformers.

Here is a simple rule. If using a full-wave rectified device, use a dedicated transformer and do not ground the transformer secondary. If using half-wave rectified devices, sharing of transformers is possible but observe high-side and low-side (grounded side) polarities when connecting to the transformer.

It is a good practice to follow the device manufacturers recommendation for powering their devices. Contemporary Controls provides installation guides for its products which specify the device power requirements along with the proper wiring instructions. When in doubt about connecting devices together which will be powered by the same transformer, please contact the device vendor. A little planning and forethought will make for an easier installation and save a lot of aggravation that comes from having an unusable blown up equipment.


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