ThermaChoice UBT System Service Manual
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P/N 01035 Rev. C
The current in the heater is constantly monitored while the heater voltage is
greater than 2.8 volts. If the current is less than 0.14 amps or greater than 2.5
amps, the controller will shut down with and generate an Error 65. If the heater
voltage varies more than +/- 10% from the desired control voltage, the same error
is generated.
The heater is powered from a 28 VDC SEPIC (Single Ended Primary Inductance
Converter) topology regulator. The heater load is nominally at 13.2 ohms.
Therefore, the nominal current of the heater is about two amps. The SEPIC
regulator has three control inputs. 1) 0 - 5 volts in to control; 0 - 28 volts out. 2)
HEATER_1_ON* and HEATER_2_ON*. HEATER_1_ON* enables the
switching IC in the power supply. HEATER_2_ON* enables a series pass
transistor to the heater and simultaneously disables a short circuit short transistor.
These transistors cannot both be on at the same time. If this were to happen, the
shunt transistor would draw all of the SEPIC output through a 4 amp fuse,
blowing it. This is to cover the short circuit failure of the series pass transistor.
During preheat, the temperature is checked to make sure it is rising at least 2
degrees every 10 seconds. This test is intended to catch the unlikely dual fault
condition of two shorted thermocouples. If the measured temperature persistently
fails to meet the criteria for 30 seconds, an error is declared.
The temperature must also reach the 87
°
target within four minutes to prevent a
hazard from shutting the system down.
Microprocessor Circuitry
The microprocessor used in the system is a Motorola 68HC11. The processor runs
at 8 MHz. The processor controls the digital displays, LCD, Alarm LED, Beeper,
Start LED, therapy sequence of events, timing, heater on/off control, and RS-232
port.
Support components are the 2KB Static Memory (RAM), 56KB Electronically
Programmable Read Only Memory (EPROM), and address decoding. These
devices are all contained in U15 on the microboard. This one-time programmable
part is a WSI PSD312-P. A special test mode or calibration mode places the unit
in constant therapy cycle. The constant therapy cycle is used for burn-in testing.
The Static RAM stores intermediate values being temporarily stored by the
microprocessor. This information is lost if power is turned off.
The EPROM stores the operational software for the processor.
The RS232 serial port is used only in manufacturing and for investigational
purposes. It is not intended to be used by the end user. It can be used to display
both thermocouples and both pressure transducer readings.