Power-Regulation-Battery Description

Theory of Operation

This section contains detailed circuit descriptions of the Power Supply PCB, Power Conversion PCI3, Interconnect PCB, Main PCB, and Display PCB/CRT Assemblies. Refer to schematic diagrams of these electrical circuits in Section 5 while reading the circuit descriptions. Information about selected integrated circuits (indicated by a * on the schematic) is provided in Section 6

Power Supply PCB Assembly (803726)

The Power Supply PCB converts input ac voltage to a nominal dc voltage for the Power Conversion PCB and for charging the battery.

Power-On

When ac current flows through J1, it is conditioned by RT1, L1, C2, C3, and CR1. Resistor divider R 1 , R3, and R4 in series with parallel resistors R 11 and R12 turn on Q l. Current builds in T1 primary windings (pins 4 and 6) and is coupled with pins 1 and 2. Capacitor C6 couples the current build-up into Q1, causing saturation.

Increasing current through the Ti primary produces voltage on the base of Q2 and a voltage drop across parallel resistors RI1 and R12. Q2 activation overrides Q l and terminates the primary current. Transformer T2 sustains base drive current to Q2.

Regulation

When battery voltage exceeds the voltage threshold set by VR1, Q4 turns on and couples the voltage from T2 pins 4 and 5 to T2 pins 2 and 7. When current subsides to proper levels, Q4 turns off, providing current feedback for regulation

Overvoltage Protection

If the regulation circuit fails, Silicon Controlled Rectifier (SCR) CR12, together with CR13, R17, and R 18, protects the load from damage. Diode CR13 sets a voltage threshold which, when exceeded, triggers SCR CR12 to short the Power Supply PCB circuit output. Removing ac power resets the protection circuit.

Output Isolation

Diode CR11 isolates the Power Supply PCB output from the filter network to protect it from the battery voltage when ac power is removed. This is necessary because the battery is connected in parallel with the Power Supply PCB.

Battery

The 16Vdc, lead-acid battery is wired directly to the Power Supply PCI3 output. The battery receives a constant charging current whenever the instrument is connected to ac power and the rear panel mains power switch is on. As the battery approaches full charge, the supply operates in constant voltage mode. Current limiting sets the maximum charge current available to the battery at approximately +4.5A. This limit is implemented when the Power Supply PCB sources current into a dead short, a requirement when charging a severely depleted battery.

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