Circuit Operation; Probe Warming ( Oral Probes Only); Theory Of Operation - Welch Allyn SureTemp 678 Technical Manual

Circuit Operation

R10, R11 (both 1 M
C(PROBE_0) and J1-E(GND) are used to provide logic 0 or 1 inputs to U1-17 and U1-
18 depending on whether a probe or cal key has been plugged into the Model 678
probe connector receptacle. When a probe has been plugged in, the software
determines whether it is Oral, Axillary, Rectal, or a Cal Key as follows:
When J1-C (CAL) and J1-F (RCTL) are both connected to J1-E (GND) (by
jumpers installed in the Model 678 probe PCB), the software determines that the
probe is an Oral probe;
When J1-C (CAL) is connected to J1-E (GND), but J1-F (RCTL) is not connected
to J1-E (GND), the software determines that the probe is Rectal probe;
When J1-F (RCTL) is connected to J1-E (GND), but J1-C (CAL) is not connected
to J1-E (GND), the software determines that the probe is a Calibration Key;
When neither J1-C (CAL) or J1-F (RCTL) are connected to J1-E (GND), the software
determines that no probe has been plugged in.
C19 and C20, both 0.01 uF capacitors, are bypass capacitors used to filter out
spurious noise to the microprocessor on the probe input lines J1-F(PROBE_1) and
J1-C(PROBE_2).

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 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, 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 region and then supplies progressively shorter pulses as the probe temperature
converges to about 93 F. Once 93 F is reached, software continues to send a "control"
pulse to maintain the temperature.
®
SureTemp Model 678/SureTemp
pullup resistors to reduce battery drain), J1-F(PROBE_1), J1-
®
Model 679
Theory of Operation
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