THEORY OF OPERATION MODEL MBC8 BATTERY CHARGER
GENERAL - Refer to drawing 10003 for schematic representation. Note that while the transformer winding configuration
changes with Charger voltage and current ratings, the typical winding configuration is representative of the Charger
operation. This discussion will assume the unit is a 24 volt model.
A.C. PATH - The A.C. power is applied to the power transformer primary through fuse Fl. Note that the primary taps
shown are factory wiring options to accommodate different battery types. The 'GND' terminal is tied directly to the chassis
which is in electrical contact with the cabinet. The primary and secondary circuits are electrically isolated from each other
and from chassis or cabinet ground. The transformer, then, isolates the A.C. power and transforms the voltage to the
level required by the battery.
The regulator board has its own regulated power supply which is supplied via the 'AC' pins and terminals 15 and 16. The
wiring options shown on drawing 10003 are to provide approximately 12 V AC to the regulator board. The return line for
this supply is the 'RET' pin which connects to the transformer center tap via terminal '-B'. The local regulated supply on
the board supplies the reference element providing the double regulated reference voltage for low line regulation error.
D.C. PATH - The main power rectifiers are port of the SCR assembly Q10. Rectification is full wave center tap with the
center tap leads extending to the -B1 terminal (battery negative). The rectifier output goes directly to the anode of the
SCR which is also part of the SCR assembly (Q10). Battery charge current will flow, then, when the SCR (Silicon
Controlled Rectifier) is turned on and vice versa. The resistor and capacitor of Q10 SCR assembly form a "snubber"
network to prevent false turn on of the SCR with line surges or transients. When the SCR is turned on via the '+PT' output
from the regulator P.C. (Printed Circuit) board via terminal 14 of TS2 (blue) charge current will flow through the SCR to
terminal 13 (green), terminal 23 (F2), Fuse F2, Ammeter (A) terminal 24 and to battery positive from the '+B' terminal.
The SCR turn on signal is a pulse supplied from a pulse transformer on the regulator board. The return lead from this
transformer is '-PT' which is connected to the cathode of the SCR via terminal 13. The shutdown sense line (SD) is also
connected to terminal 13.
VOLTAGE SENSING - The regulator board is the controlling element of the whole Charger. The SCR assembly is a
"Slave" to the regulator. The regulator senses the battery terminal voltage via terminal 24 (+V), the "Float" switch (SW-1),
or resistor R30, and 25 (+RS) and terminal 26 (-V). The regulator will turn on the SCR sooner in each half line cycle when
the sensed battery voltage is below the internal regulated reference voltage. Turning on the SCR sooner in each half
cycle will allow more charge current to flow. When the battery voltage increases (with charge level) above the fixed
reference voltage, the regulator will turn on the SCR later in each half cycle causing reduced charges currents. With the
"Float-Equalize" switch in the "Equalize" position resistor R30 is in series with the regulator voltage sensing circuit, which
reduces the battery terminal voltage seen by the regulator voltage comparing circuit. This causes the Charger output
voltage to increase to equalize the battery. With the "Float-Equalize" switch in the "Float" position resistor R30 is shorted
out causing the regulator voltage comparitor to see a true battery terminal voltage reducing the Charger output voltage to
the "Float" level.
The battery voltage sensing network is high in impedance (approximately 100K ohm) and will not cause battery discharge
on power outage.
MBC8(10003) iss2
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