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|Why You Should Not Mix Full-Wave and Half-Wave Powered Devices
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.
Senior Product Manager
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.
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
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.
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
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.
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 easier installation and save a lot of
aggravation that comes from having an unusable blown up equipment.
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