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LOAD CONSIDERATIONS
This section provides information on operating your supply with
various types of loads connected to its output.
PULSE LOADING
The power supply will automatically cross over from constant-
voltage to constant current operation in response to an increase
(over the preset limit) in the output current. Although the preset
limit may be set higher than the average output current, high peak
currents (as occur in pulse loading) may exceed the preset cur-
rent limit and cause cross over to occur. If this cross over limiting
is not desired, set the preset limit for the peak requirement and
not the average.
REVERSE CURRENT LOADING
An active load connected to the power supply may actually
deliver a reverse current to the power supply during a portion of
its operating cycle. An external source can not be allowed to
pump current into the supply without loss of regulation and possi-
ble damage to the output capacitor of the power supply. To avoid
these effects, it is necessary to preload the supply with a dummy
load resistor so that the power supply delivers current through the
entire operating cycle of the load devices.
Figure 16. Reverse Current Loading Solution
OUTPUT CAPACITANCE
An internal capacitor, connected across the output terminals of
the power supply, helps to supply high-current pulses of short
duration during constant voltage operation. Any capacitance
added externally will improve the pulse current capability, but will
decrease the safety provided by the current limiting circuit. A
high-current pulse may damage load components before the
average output current is large enough to cause the current limit-
ing circuit to operate.
The effect of the output capacitor during constant current opera-
tion are as follows:
a. The output impedance of the power supply decreases with
increasing frequency.
b. The recovery time of the output voltage is longer for load
resistance changes.
c. A large surge current causing a high power dissipation in the
load occurs when the load resistance is reduced rapidly.
REVERSE VOLTAGE LOADING
A diode is connected across the output terminals with reverse
polarity. This diode protects the output electrolytic capacitors and
the series regulator transistors from the effects of a reverse volt-
age applied across the output terminals. For example, in series
operation of two supplies, if the AC is removed from one supply,
the diode prevents damage to the unenergized supply which
would otherwise result from a reverse polarity voltage.
Since series regulator transistors cannot withstand reverse volt-
age, another diode is connected across the series transistor. This
diode protects the series regulators in parallel or auto-parallel
operation if one supply of the parallel combination is turned on
before the other.
BATTERY CHARGING
The power supply's OVP circuit contains a crowbar SCR, which
effectively shorts the output of the supply whenever the OVP trips. If
an external voltage source such as a battery is connected across the
output, and OVP inadvertently triggered, the SCR will continuously
sink a large current from the source; possibly damaging the supply.
To avoid this a diode must be connected in series with the output as
shown in Figure 17.
Figure 17. Recommended Protection Circuit for
1-15
Battery Charging
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