6 | SAMLEX AMERICA INC.
SAMLEX AMERICA INC. | 7
SeCtIOn 3 |
Description & Principle of Operation
PRinciPLe oF oPeRAtion
Please refer to the Layout at fig. 2.1. The unit consists of 2 Sections - Switch Mode Power
Supply (SMPS) Section and Battery Backup and Charging Section. The unit works as a DC
Uninterruptible Power Source (DC UPS) in conjunction with an external 12V, Lead Acid
backup battery. As long as AC input power is available, the unit will put out 13.8 ± 0.2V
at the Load Terminals (3, 4) and provide a maximum continuous load current of 19A to
23A. At the same time, charging current of up to a maximum of 4A (When battery is
completely discharged to Standing Voltage of 11.1V) will be fed through the Battery
Terminals (5, 6) to charge the external battery. The voltage at the Battery Terminals (5,
6) will always be clamped to the actual voltage of the battery corresponding to its State
of Charge. If AC input power fails, the DC load will be instantaneously transferred to
the external 12V backup battery and the battery will start discharging. When AC input
power is restored, the DC load will once again be transferred instantaneously to the
Power Supply Section and the external backup battery will be recharged and kept in
charged condition all the time at float Voltage of 13.8 ± 0.2V (when fully charged).
When operating in DC UPS Battery Backup Mode (external 12V battery is connected),
the unit is capable of providing short-term surge current overload of up to 50A for < 1
sec (external 25A fuse on the load side will not blow as 50A overload occurs for a very
short period of < 1 sec). During this overload of 50A, the Power Supply Section will
provide 25A and the balance 25A will be supplied by the battery and the battery will
discharge at 25A for the period of the surge. During the period of this surge current
overload, the voltage seen by the load will be = (battery voltage – 0.3V± 0.2V).
If battery backup function is not used (external backup battery is not connected), the
unit will work as a normal power supply with ability to supply 23A continuous at 13.8
± 0.2VDC at the Load Terminals (3, 4). The maximum overload current will be limited to
25A. Under overload conditions, the output voltage will not be regulated and will drop.
cooLinG AnD oVeR teMPeRAtURe PRotection
The unit is cooled by convection and in addition, has a temperature-controlled fan
located at the bottom for forced air-cooling. Two Normally Closed Thermal Switches are
mounted on the windings of the Switching Power Transformer – one for fan control and
the other for over temperature shut down. When the temperature of the transformer
windings rises to ≥ 60°C ± 5°C / 140°f ± 9°f, Thermal Switch for fan control will open and
will activate fan switching circuit to switch oN the fan. When the transformer windings
cool down to ≤ 40°C ± 5°C / 104°f ± 9°f, the switch will close and de-activate fan switch-
ing circuit to switch off the fan.
note: the fan may not switch on at all in case of low loads or in colder ambient
temperatures because the temperature of the transformer windings may not rise to
threshold of ≥ 60°c ± 5°c / 140°F ± 9°F under these conditions.
In case the fan fails or if the cooling is not adequate due to higher ambient tempera-
ture, inadequate air circulation or blockage of ventilation openings, the temperature of
the transformer windings will rise. At temperature ≥ 105°C ± 5°C / 221°f ± 9°f, Thermal
