2
The lower voltage is rectified by the output rectifier „U
2
“ and smoothed by a storage choke „TL“ and
large electrolytic capacitor „C
f2
“. An output filter „F2“ suppresses high frequency interference
products.
Heart of the control circuits is a controller, which comprises PWM modulator, an error amplifier, and
5,1V reference, oscillator and totem pole output stage. Secondary side consists of the voltage and
current regulators, 5V/0,5A reference for powering display unit and precise 5V/5mA reference for
command signal setting. Current regulator output signal passes via an optocoupler to the error
amplifier input.
The settings of output DC voltage and a current limitation can be reached trough the built-in
potentiometers – ROUGHLY in the range 0
÷
95% of U
max
(I
max
) and FINE in the range 0
÷
5% of U
max
(I
max
).
The built-in insulated interface RS 232 and the external analogue one are optionally available to set
the output voltage and current. The external analogue interface should be equipped by two external
DC/DC (5V/5V) insulated converters. One for voltage setting and the second one for current. Under
remote control condition the REMOTE switch must be switched on and the FINE potentiometers
must be turned to the left zero position.
Output voltage and current are picked-up and come both to the control circuit and display unit.
Voltage is indicated on the 3-digit display and the current at the same time on the 3-½-digit display.
Power supply output may be turned on or off by the Disable OUTPUT switch at any time
independently on the load. Turning unit on or off by a mains switch may be also done at any position
of this switch. It is recommended to use Disable OUTPUT switch prior to the mains switch to
disconnect a load. Input circuit devices (NTC, diodes, capacitors) stress will be decreased. Mains
current will be less than 0,1 Amps during STAND-BY state. This makes it possible to leave the input
power on and use disable OUTPUT switch for turning on and off. Turning output off takes app. 0,1s
and turning on of soft start process takes app. 2s until a duty cycle doesn’t reach max. value. Real
output voltage falls to zero depending on the load. There is no output current sink capability. Bleed
resistor connected across a filter circuit discharges capacitors when the unit is turned off. Only a few
mAmps discharge current source improves this feature.
From an auxiliary mains transformer are powered both the primary side control circuits (30V AC)
and secondary one (12V AC) which involves also display unit circuits and fans speed control circuits.
PTC thermistors pick-up a temperature of the both heat sinks.
CONSTRUCTION DESCRIPTION AND COOLING
Power circuits are placed on the main mother PCB. Display unit is soldered to it and control unit is
connected via a connector. Further elements lay-out and cooling system shows following fig.2.
