Circuit Description - Main Outputs - TTI PL Series Service Manual

Refer to the appropriate schematic at the back of this manual.
Power Supply - Mains
This consists of a full wave bridge rectifier feeding the reservoir capacitor C31. The full load
secondary winding voltage is 36V rms giving approximately 55 volts off load and 40V on full load
(30V and 24V on PL154).
Power Supply - Auxiliary
This consists of twin full wave rectified supplies from a 21V centre tapped winding. IC8 generates
+5V for the micro-Controller, display and A to D. Q7 and D10 generate +7.5V for the analogue
section. D11 generates -5V for the analogue section and A to D. 0V of this supply is connected to
the positive output at R51.
Voltage Control
IC7A buffers the voltage control pots and VR3 sets maximum output voltage of the unit. D6 is the
reference which is nominally 2.45V.
IC7B is a differential amplifier with a voltage gain of 13.3. For 30V output this means the voltage
at PJ2-8 will be 2.25V and for 15V output 1.125V. VR9 trims differential gain and effects voltage
regulation. This can be set more accurately by generating a few hundred millivolts between
positive output and positive sense, see calibration section.
Current Control
IC7C is the error amplifier and compares the voltage on the wiper of VR5 with the voltage
generated across the current sense resistor R51. When this limit is reached IC7C takes control
changing the supply output from constant voltage to constant current. Current limit is indicated to
the micro-controller by ILIMIT going high. The sense resistor is 100 milli-Ohm, therefore with the
current limit set to 2A the voltage on the wiper of VR5 will be typically 188mV when VR7 is set
central. VR8 adjusts the output current to be the same as the preset current at low levels. VR4
trims maximum output current.
Series Regulator
Q1 and Q2 form a triple Darlington. On 2 Amp and 4 Amp versions Q2 is in parallel with Q3, Q4
and Q5. On 3 Amp versions Q2 is in parallel with Q3, Q4, Q5, Q8 and Q9. Q1 is a Darlington and
is on a heatsink on the driver pcb.
A to D
IC2 is a 12 bit dual slope converter. Its reference is derived from D6 by R27 and R28 and is
typically 180mV. XL1 is either a 4MHz crystal or ceramic resonator. The buffered oscillator
output is also used by the microcontroller IC1. Analogue multiplexer IC3 selects measurement of
output voltage, preset current or output current. R65 and C38 provide the current meter damping
facility.
Microcontroller
The measurement system and display is controlled by a microcontroller IC1.
The two 4-digit LED displays are driven by IC1 via the segment latches IC4 and IC9 and the digit
latch IC5. Digit current is provided by IC6 and individual segment current is limited to 50mA by
the resistors R10 and R17. The digit on-time rate is 2ms and is controlled by IC1 which also
provides the inter digit blanking to prevent ghosting segments.
The measurement of output values of voltage and current is performed by the 12 bit analog to
digital converter IC2. The measurement rate is controlled by the 4.0MHz ceramic resonator XL1
connected between pins 22 and 23 and the buffered version of this 4MHz signal at pin 25 is used
as the clock to the microcontroller IC1. The ADC, IC2, is a dual slope converter and provides a
little over 8 readings per second when clocked at 4MHz. The ADC is run in continuous mode and
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Circuit Description – Main Outputs