Probe Warming ( Oral Probes Only); Theory Of Operation; Circuit Operation - Welch Allyn SureTemp 767 Technical Manual

Welch Allyn Model 767
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D4 and R16 protect the comparator from input undervoltage when the timing capacitor (C21) is
discharged. D4 also reduces the recovery time by limiting the voltage at the beginning of recovery to
0.6 volts instead of 2/3VREF.
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C5 provides the first line of defense from EMI (Electro Magnetic Interference) coming in on the
probe. R33 and C18 protect Q1 from rectifying EMI in the substrate diode. R16 protects the
comparator input from overcurrent. C32, C33, C34, C35, and C37 are also needed for protection from
EMI.

Probe Warming ( Oral probes only)

Probe characteristics vary somewhat due to normal production process variations. It is desirable to
warm the probe as efficiently as possible from a time-to-ready standpoint and from a temperature
stability standpoint when the probe is up to temperature.
The probe warming process is a closed loop feedback control system incorporating PWM (pulse width
modulation) control.
The probe warmer circuitry is used to heat the probe tip prior to taking a temperature reading in
order to speed the convergence of the prediction algorithm. This allows quicker temperature
readings. A fail safe hardware shutoff circuit is included to ensure the heater will shut off in the
event of a software failure.

Theory of Operation

The microprocessor sends pulses via /HTRC to drive the probe heater resistor which heats the
probe tip. A temperature of about 93°F(33.9°C) is maintained prior to taking a temperature.
A software algorithm calculates the width of the HTRC pulse as a function of the difference
between the probe temperature and 93°F(33.9°C), and as a function of the probe temperature
rate of change. It provides an initial pulse to rapidly heat up the probe tip to the 90°F(32.2°C)
region and then supplies progressively shorter pulses as the probe temperature converges to
about 93°F(33.9°C). Once 93°F(33.9°C) is reached, software continues to send a "control" pulse
to maintain the temperature.

Circuit Operation

The warmer circuitry consists of Q5, Q13, Q14, C1, C2, L1, D1, R1, R2, R3, R4, R5, R31, R32,
and the heater resistor (27 ohms) connected across J1-2 and J1-5.
Line /HTRQ is pulled low by the uP which enables Q13 to turn on. /HTRC is then pulsed low
which brings the base of Q5 low via capacitor C1. Q5 turns on which in turn enables Q14 on.
Base current from Q14 flows through R31 (1K) and R32 (1K). Most of this current flows through
the emitter of Q5 while some flows through R1 (3M) to satisfy the diode drop of Q5's BE junction.
The base current of Q5 along with the current through R1 flows into C1(1uF), charging it up. This
sets up the mechanism for the hardware shutoff. As this capacitor charges up, the base voltage
of Q5 approaches the emitter voltage and the transistor shuts off, thereby shutting down the
probe warmer. As long as Q5 remains on, Q14 has a base current flowing which allows current
to flow from its collector through R2 (4.7K) and the heater resistor. With about 150mA flowing
through it ([VCC-2VCEsat]/29 ohms), the heater resistor heats up the probe tip.