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 limiting is not desired, set the preset limit for the peak requirement and not the average.
3-62OUTPUT 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 P UlSe current capability, but will decrease the safety provided by the constant current circuit. A high-current pulse may damage load components before the average output current is large enough to cause the constant current 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 power dissipation in the load occurs when the load resistance is reduced rapidly.
3-65REVERSE VOLTAGE LOADING
3-66 A diode (A4CR106) is connected across the output terminals. Under normal operation condi- tions, the diode is reverse biased (anode connected 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 terminals 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.
3-67REVERSE 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 possible 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.