45
Service Manual
2-6
Inverter provides isolated positive and negative 5.25 V outputs for the in-guard
circuitry.
2-15. Raw DC Supply
The Raw DC Supply uses a power transformer (A1T3) that operates on input line
voltages ranging from 90 V to 264 V ac. Since there is no power switch in the
transformer input circuit, the Raw DC Supply is energized whenever the meter is
connected to line power. The transformer uses an internal 275 V ac MOV (metal-oxide
varistor) to clamp line transients. This MOV normally acts as an open circuit; when the
peak voltage exceeds approximately 400 V, the MOV turns on and, working with the
line impedance in series with the line fuse, limits the transient peak voltage to 400 to 500
V. All line voltages use a T 0.125 A, 250 V (slow blow) fuse.
On the secondary side of the transformer, the output is rectified by diodes A1CR2 and
A1CR3 and filtered by capacitor A1C27. In addition, A1C26 reduces rectifier diode
switching emi emissions from the meter. The meter power switch (A1S1) is also
connected in the output of the Raw DC Supply; it connects the Raw DC Supply either to
the 5-V Switching Supply (when the meter is ON) or to the Battery Charger Switching
Supply through A1J1-7 (when the meter is OFF.)
2-16. 5-Volt Switching Supply
The 5-Volt Switching Supply incorporates the A1U11 controller device and several
external components. Operating on an input of 7.5 V dc to 35 V dc, the 5-volt Switching
Supply uses a pulse-width modulation technique to regulate its output at 5.1 V dc. The
nominal switching frequency is 40 kHz.
With the controller, the output voltage is controlled by varying the duty cycle (ON time)
of the switch transistor in A1U11. (Controller device A1U11 contains the supply
reference, oscillator, switch transistor, pulse-width modulator comparator, switch drive
circuit, current-limit comparator, and current-limit reference.) Resistors A1R41 and
A1R42, in conjunction with the reference circuit, set the input levels to the pulse-width
modulating comparator. Resistors A1R35 (in parallel with A1R41) and A1R36 (in
parallel with A1R42) are used in production to adjust the 5.1 V supply output. Removing
A1R35 decreases the output by approximately 5%. Removing A1R36 increases the
output by approximately 7%.
Within the controller, the output of the comparator is combined with the oscillator signal
to form the drive signal for the switch transistor. Diode A1CR6 operates as a
complementary switch with the switch transistor. Dual inductor A1T1 is a magnetic
device that regulates current pulses as the switch transistor is turned on and off. A
current shunt (A1R47, A1R48, and A1R49) senses the overall current flowing through
diode A1CR5 and the switch transistor of A1U11. If this current rises too high, the duty
cycle (ON time) of the switch transistor is reduced to current-limit the supply. Capacitor
A1C33 serves as a filter capacitor and energy storage device, and A1C34 and A1C35 are
the output filter capacitors.
The boost circuit (A1CR4 and A1C32) supplies the controller A1U11 with sufficient
supply voltage when the switching supply input is low. For example, this condition may
occur when the meter is operating on low line voltage.
Resistor A1R40 and capacitors A1C28 and A1C29 are needed for proper dynamic
performance of the switching supply. Capacitor A1C30 sets the operating frequency of
the supply.
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