PF1352 USER MANUAL REV A 12/21/15
These AC power supply models are based on electronic circuits that utilize various
power semi-conductors. As such, there is a limit to the amount of current that can be
supplied. Certain loads may draw short duration, very high peak currents that may
activate protection circuits within the AC power supply. The Model PF1352 incorporates
two distinctive over current circuits. The first responds to massive overloads like short
circuits. This circuit can respond in a few tens of microseconds. Once activated, the AC
output of the power supply is disabled and the front panel display will flash “
O/L
”.
The second current limit circuit responds to long term overloads. Once the current is
increased beyond the current limit setting, 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 display may flash “
C/C
” 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 output will be disabled and the front panel display will flash “
O/T
”. Normal
operation can resume once the unit has cooled to a safe level and the
MODE
key or
reset
button is pressed.
3.7
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 also 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.
3.8
OPERATION INTO LINEAR LOADS
These models 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
