PF1350 TECHNICAL MANUAL REV 1 2/06/14
3.3
OPERATING CONSIDERATIONS (continued)
The second current limit circuit responds to long term overloads. Once the current is
increased beyond the output rating, the unit will enter a constant current mode. In this
mode, the output voltage will reduce to limit the current if the load resistance is reduced
further. The voltage will recover automatically once the overload is removed. The front
panel Constant Current LED may light or flash under these conditions depending on the
nature of the overload.
The internal temperature of the unit is monitored. If the temperature rises above a safe
value, the outputs will be disabled and the front panel fault LED will turn on. Normal
operation will resume once the unit has cooled to a safe level.
3.4
LIMITING LOAD IN RUSH CURRENTS
Loads that present high inrush currents may be started by limiting the current at start-
up. This can be done externally with a simple series resistor and relay. The resistor is
placed in series with the load for the estimated or measured inrush period. Once the
inrush current has stabilized or tapered off, the relay is used to switch the resistor out of
the circuit. NTC (Negative Temperature Coefficient) “thermistors” may be used. These
devices start off with a nominal resistance value when cold (off). Once power is applied
the current flow causes the temperature of the resistor to increase which in turn reduces
the value of the resistance. The “hot” value may be several times lower than the cold
value. These devices are available for currents exceeding 15 Amps.
3.5
OPERATION INTO LINEAR LOADS
The model P1350 will provide the best overall performance into a linear load. A linear
load is characterized by that fact that its current wave shape is sinusoidal. The phase
relationship between the voltage and current may be anything between zero and 90
degrees (leading or lagging). Some examples of linear loads are as follows: Most AC
Motors, Power Transformers, Heating Elements, Resistors, Capacitors, Most Inductors,
Incandescent Lighting ( without dimmers ), and most Solenoids
3.6
OPERATION INTO LINEAR LOADS
Operation into these types of loads usually causes little interaction with the AC power
supply. The main concern with a linear load is the inrush current associated with it. Most
heating elements and resistors have little or no inrush concerns and usually do not
present any problem for the power source. Inductive and capacitive loads may present
a special problem based on their construction and the way in which they are energized.
Motors and tungsten filament lamps also present some special
A
start-up
@
concerns. The
following is intended to give the end user some insight into applying the AC source to
these types of loads.
