b)
If performing adjustment in step (a) above does not remove oscillation,
disconnect output capacitor C2(L1791-FP-CNT-1203 PCB) and connect a
capacitor having similar characteristics ( approximately the same
capacitance, the same voltage rating or greater, and having good high
frequency characteristics) directly across load using short leads. Readjust
R145 as in step (a) above after making this change.
4.7
4.8
4.9
OUTPUT CAPACITANCE :
Internal capacitor C2 (L1791-FP-CNT-1203 PCB) 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 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.
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.
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 CR1 is connected across the output terminals. Under normal operation, 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 to the
forward voltage drop of the diode. This diode protects the series transistor and the
output electrolytic capacitors.
REVERSE CURRENT LOADING :
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.
4.10 EXTERNAL PROGRAMMING :
Remote external programming can be achieved changing the external program link
on rear panel and connecting a variable resistor of 10K across the terminals as
shown on the rear panel.
7