JANUARY 31 2003
In high voltage or high current units, silicon diodes should be used. Selenium units
have a higher voltage drop (less efficient) and are less affected by voltage surges, and
they "age" over time and must be eventually replaced. Silicon diode units are
susceptible to damage by voltage or current spikes and must be protected from these
protection provided all units.
Rectifiers are either automatically controlled or manually controlled. On
manually controlled units, the transformer tap connections normally encircle the "center
tap" connections. The center tap connections are the input to the stacks, and the
negative and positive lugs are the output from the stacks.
Automatic rectifiers are sometimes used when the structure size (elevated
water tanks which vary in water levels) is constantly changing, or when the cathodic
protection circuit resistance (anode-to-earth resistance due to extreme dry-to-wet
conditions) changes significantly at a given location. These units can be potential-
controlled, current-controlled, or voltage-controlled. Potential-controlled units are
normally used in elevated water tanks to maintain a set potential on a permanent
reference cell located inside the tank. Since in this condition the amount of structure to
be protected is constantly changing as the water level changes, the amount of current
required is constantly changing, so a current-controlled or voltage-controlled unit must
not be used. When the cathodic protection circuit resistance changes significantly at a
given location due to the anode-to-earth resistance being affected by extreme dry-to-
wet conditions, current-controlled units are used to maintain the correct current applied
to the structure being protected. Voltage-controlled units must not be used in this case,
but potential-controlled units could be used, with an added permanent reference cell
(which adds another component which could possibly be damaged or fail).
There are normally meters and/or a shunt inline between the stacks and the
output lug, lightning protection on the AC input and the DC output, output filters and/or
capacitors to increase efficiency, and circuit breakers to turn the unit on and off.
Quick disconnects, usually near the rectifier, are normally fused and serve to
turn power off to the rectifier. Details of the construction, operation, maintenance and
repair of cathodic protection rectifiers are given in portions of Chapters 4, 5 and 7 of this