Circuit Descriptions - TTI QL II Series Service Manual

Precision power supplies

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Table Of Contents

Table of Contents

The Power board contains the main power supply and the auxiliary supplies used internally, the

output series regulator transistors, the microcontroller, display drivers and some control logic.

The Control board contains voltage and current control, ADC and DAC and some control logic.

The linear regulator is in series with the positive output but, because of the way in which the

control circuit is referenced to the +ve output, it is convenient to consider the regulated output as

the negative side.

To help reading the circuit diagrams the two connectors between the power and control pcbs join

as follows. PJ14 to PJ18 and PJ20 to PJ19.

Power Pcb

The main windings are connected either in series (for the high voltage range) or in parallel (for the

low voltage range) by the range relays and an electronic tap change at approximately half range is

used to reduce dissipation. Bridge rectifier BR1 feeds C4 the reservoir capacitor when tap change

is low. When tap change is high, two of the diodes in BR1 are bypassed by SCR1 and SCR2.

Voltages are listed below at nominal mains (230V).

Secondary – high tap, no-load

Secondary – high tap, full-load

C4 – high tap, no-load

C4 – high tap, full-load

Secondary – high tap, no-load

Secondary – high tap, full-load

C4 – high tap, no-load

C4 – high tap, full-load

A 30Vrms centre tapped winding and full wave rectification provide the auxiliary supplies. IC1

generates +10V and IC2 –5V. A 10.4Vrms winding and full wave rectification power the relays and

fan and IC3 provides the +5V for the microcontroller, display and control logic.

Comparator IC6A provides the system reset signal and comparator IC6B provides the power fail

signal to the microcontroller to store the instrument settings at power down.

The fan speed is regulated by a pulse width modulator in the microcontroller; if the heatsink

exceeds a certain temperature measured by Q22, comparator IC50A switches the fan to full

speed. If the heatsink temperature continues to rise IC50B will pull the over-temperature line low

which will turn the output off.

Q15 and Q16 are the series regulator transistors; long-tailed pair Q17 and Q21 ensure current

sharing. The +10V supply to emitter follower Q14 is via switch Q13 which is turned on after a delay

to prevent output glitches when power is turned on.

Microcontroller IC9, which is factory programmed, writes the calibration values and instrument

settings to non-volatile memory IC31, reads the keyboard, spin-wheel and status of the instrument

via shift registers (4021s), reads the ADC, outputs data via shift registers (4094s) and display

drivers IC32, IC33 and IC38, and writes to the DAC.

QL355

QL564