Centralion 400VA, Service Manual, Page: 10 / 33

D. Battery Charger
The action of charger is described as follows : (Figure S-3)
1.
When UPS connected to AC source, the CPU was active because the control power (+5V)
was established.
2.
The CPU turned on MAIN RELAY(RY01),AC power flow into Main Transformer.
3.
Source for the charging circuit comes from the charger coil of the Main Transformer.
4.
A traditional linear charging circuit is used for the UPS. Regulator U02(LM317) is used
to adjust the battery charging voltage. The charging voltage can be set by changing the
value of R28, R27, R29 and R62. The charger voltage is given by
Rp=(R27*R29*R62)/(R27*R29+R29*R62+R62*R27)
V
OUT
= V
REF
[1 + Rp/R28] + I
A
*Rp
I
A
= 50
µ
A typical (LM317 A pin offset
current)
Where V
REF
= 1.25 Volts, which is the voltage drop between LM317 A pin and the
Output pin. Thus the output voltage is
V
OUT
= 1.25 Volts [1+1050/100] + 50 *10
-6
* 1050 = 14.4275 Volts.
( If T=25 degree Rp=(2000*4020*4990)/
(2000*4020+4020*4990+4990*2000)=1.05kohm)
By considering the voltage drop of D13, the actual charging voltage is given by
14.4275-0.7 = 13.7275 V
DC
To prevent the charger current from over-charging and damaging the batteries,
R31,Q09 and associated components are used as the current limiter.
The current limiter process is described below. The process repeats itself until the
voltage for R31 drops to turn off Q09.
1.
Q09(2N3906) turns on when current of R31 too large.
2.
Q09 turns on Q08
3.
A (Adjust ) pin of U02(LM317) is pulled down to 0 Volts when Q08 turns on.
4.
As a result the output voltage of LM317 is pulled low from 13.7275V to
approximately 1.25 V causing Q09(V
E
< V
B
)and Q08 to be turned off.
5.
This cause the output voltage of LM317 return to 13.7275 V.
The CHG-OFF signal from CPU pin28 is used to turn off charger circuit at battery
mode. The CPU turns on Q08 via sends a HI signal to base of Q08. As a result the charger
be turned off.
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