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 part of the SCR assembly 010. 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 comparator 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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