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Using Step-down or Isolation Transformers
When using a step-down or isolation transformer to supply power to the controller, be
aware of the following potential issues, all of which are drawn from experience:
•Step-down transformers are typically derive 120 VAC from much higher voltage
lines that are also being used to power heavy equipment. Often, when motors are
first energized in heavy equipment, the stall currents that result can cause the voltage
supplied to the step-down transformer to drop by as much as 50%. Because step-
down transformers are ratio-metric devices, the voltage supplied to the controller
drops proportionately, causing a potential system malfunction.
•The relatively light load imposed by the controller on the transformer secondary may
result in excessive voltage being present, representing a potential hazard to the
system.
•Neither type of transformer represents ideal protection against electrical noise.
Inter-winding capacitances can result in noise transference among windings, and
differential noise may still be electromagnetically coupled to the secondary.
•Adequate grounding of the controller is still critical to its proper performance. For
more information, see the section on The Importance of Proper Grounding.
These issues often point to the use of an independent power feed (120 or 240 VAC) for
the controller and associated components. The resulting additional stability and design
integrity are often well worth the slight additional cost.
The Importance of Proper Grounding
The controller is outfitted with an integral line filter to protect against electrical noise
carried on the AC power line. This line filter, along with the shielding provided by the
controller enclosure itself, depend on the presence of a good ground connection for
effectiveness. This is normally accomplished by connecting the controller’s power cord
to a well-grounded outlet.
As with any electronic equipment, the controller’s ground should follow a direct, low-
impedance path to the plant’s power source. If possible, this path should not be shared
by any machinery which injects a large amount of electrical noise into the ground.
In instances where step-down or isolation transformers are being used to power the
controller, the ground connection should still be made directly to the controller.
For further consideration regarding noise protection, refer to the Application Note,
Reducing Noise Susceptibility. Application notes may be obtained at no charge from
your distributor or directly from CTC.
The Controller’s Power System
The controller’s power system provides two operating voltages, electrically isolated
from one another:
•+5 VDC: Powers the controller’s internal circuitry, including the CPU and the
logic-level circuitry on the various modules.
•+24 VDC: Available with a current capacity of up to 1.5 Amp (maxium), to power
external control devices, such as solenoid valves, relays, sensors. You can also use
this power supply with some modules to power circuitry on the outbound side of any
on-board isolation.
The +5 V supply is not usually made available at any of the module connectors.
NOTE: Do not make any external connections to either the +5 V power supply or its common, as an
increased susceptibility to electrical noise may result.
Model 2700 Power Connections
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