Rectifiers and Power Supplies
Power Supply
Capabilities
Most house circuits are fused for 15 or 20 Amps.
At 120 Volts, this can provide 1800 or 2400 Watts
respectively.
This is a tremendous amount of power available from each
fused circuit in a home.
Power supplies convert 120 Volt wall plug voltage down to
a lower voltage and current capability that is safer and
more useful for users.
Power Transformation Capabilities
AC power supplies contain the transformer as a key
element.
The transformer reduces the voltage level to a useful
lower voltage with a current capability determined by the
transformer design.
The addition of:
- fuse protection
- variable output voltage control
- desired connectors to interface with the load
completes a basic AC power supply
Direct Current Power Supplies
DC power supplies contain the AC transformer to reduce
the power supply voltage to a lower level.
However, DC power supplies must also convert (rectify)
the AC voltage and current to DC.
The rectification process is accomplished using diodes
connected to the output secondary winding of the
transformer.
The rectified DC output is then filtered and controlled
to produce the desired effect.
DC Half Wave Rectification
Only the positive half of the AC voltage waveform reaches
the load.
Half-wave rectifiers are used in the low quality power
supplies.
DC Full Wave Rectification
The negative half of the AC voltage waveform is rectified
to positive voltage.
Both halves of the AC waveform reach the load.
Full-wave rectifiers are used in most power supplies.
DC Bridge Rectifiers (Full Wave)
Filtering
Regulation
Regulation is the capability of a power supply to
maintain the desired output voltage when the output
current increases from a low level to a high level.
Model Railroad Power Supplies
Most newer “brand name” MRR train controllers
are well designed, filtered, and will stay in regulation
over the stated power output parameters.
When using DC or AC wall plug power sources you may
encounter units that do not have sufficient filtering
and/or regulation.
If a unit you are using exhibits:
- excess AC hum from the unit or circuits powered by the
unit
- output voltage drops as the load increases
You may be encountering poor filtering or low power
capacity.
Wall Plug-In Power Supplies
These are great power supplies for specific uses - such
as for lighting buildings, powering accessories and
otherwise augmenting you train controller power supply.
These are available in a large choice of output voltages
and output current capabilities.
For MRR use - you would probably cut off the end plug,
split the wires apart, strip the ends and determine
polarity (if DC) using your multi-meter.
Examples
Team Digital note: The above wall transformer table
is current as of 4/08.