Samlex America SEC-4825BRM - Manual (Page 9)

8 | SAMLEX AMERICA INC. SAMLEX AMERICA INC. | 9
common DC bus bar (before the Schottky Diode D1) is tightly regulated at the preset
value of 55VDC, the voltage at the output terminals Load (+) and Load (−) will vary
slightly due to the forward voltage drop “Vf” of the isolating Schottky Diode D1 and
the drop along the DC bus bar and wiring. The voltage at the output terminals LOAD
(+) and LOAD (−) will be as follows:
• At no external load Approximately 54.8VDC
• At 25A load Approximately 54.2VDC
When there is a requirement of un-interrupted DC power to the load, an external 48V
battery should be connected at the terminals Battery (+) and Battery (−). When input
AC power is available, the load current is supplied by the Power Supply Section through
Isolating Schottky Diode D1. At the same time, the battery is charged to Float Voltage
Level by “Taper Type of Charging” through 2 x 12 Ohm resistors R1 & R2 in parallel with
an effective series resistance of 6 Ohm. This effective series resistance of 6 Ohm will limit
the maximum charging current to 2A. The charging current will be determined by the
following equation (Battery’s internal impedance and battery cable resistance have been
disregarded as their values are negligible as compared to the 6 Ohm effective series
resistance):
Charging Current = (Load Terminal Voltage – Battery’s Intrinsic
State of Charge Voltage) ÷ 6 Ohm
Maximum Charging Current (At completely
discharged voltage of 42.8V) = (54.8V – 42.8V) ÷ 6 Ohm = 2A
Charging current when fully charged to maxi-
mum Float Voltage of 54.6V 25 mA (0.1% of Ah capacity of the battery to
compensate for self-discharge. Battery with
25Ah capacity has been considered)
R2
D2 D1
BATTERY
BATTERY
LOAD
LOAD
+
+
POSITIVE BUS BAR
NEGATIVE BUS BAR
+
R
55V
from
PSM
R1
D3
Fig 3.1 Schematic - Battery Backup
LEGEND
D1, D2 Schottky Diode, 40CPQ060 by International Rectifier
D3 10A, 600V Diode “10A6” for reverse polarity protection
R1, R2 Power Resistor 12 Ohm, 35W
R
Static Load Resistor, 1K, 5W
Section 3 | Design & Principle of operation
11001-SEC-4825BRM-0114.indd 9 14-01-07 1:15 PM