over from constant voltage to constant current op-
eration, or the reverse, in response to an increase
(over the preset limit) in the output current or volt-
age, respectively. Although the preset limit may
be set higher than the average output current or
voltage, high peak currents or voltages (as occur
in pulse loading) may exceed the preset limit and
cause crossover to occur. If this crossover limit-
ing is not desired, set the preset limit for the peak
requirement and not the average.
3-62 OUTPUT CAPACITANCE
3-63 An internal capacitor (A3C3) 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 con-
stant current circuit. A high-current pulse may
damage load components before the average output
current is large enough to cause the constant cur-
rent circuit to operate.
3-64 The effects of the output capacitor during
constant current operation 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.
TM 11-6625-2958-14&P
c. A large surge current causing a high pow-
er dissipation in the load occurs when the load re-
sistance is reduced rapidly.
3-65 REVERSE VOLTAGE LOADING
3-66 A diode (A4CR106) is connected across the
output terminals. Under normal operation condi-
tions, the diode is reverse biased (anode connect-
ed to the negative terminal). If a reverse voltage
is applied to the output terminals (
POSitive voltage
applied to the negative terminal), the diode will
conduct, shunting current across the output termi-
nals and limiting the voltage across the output
terminals to the forward voltage drop of the diode.
This diode protects the series transistors and the
output electrolytic capacitors.
3-67 REVERSE CURRENT LOADING
3-68 Active loads 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 cannot be allowed to pump current
into the supply without loss of regulation and pos-
sible damage to the output capacitor. 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 operation cycle
of the load device.
3-9
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