Probe Warming (675 Oral Probes Only) - Welch Allyn SureTemp 670 Technical Manual

Welch Allyn, Inc.
Probe connector installation does not "wake" the instrument up. When the probe shaft is stowed, the probe
switch places voltage on U2 pin 23 via R24. This "wakes up" the processor which then reads the RAM flag
and sees that the probe logic lines are indicating a probe is connected. This situation triggers a probe
characterization routine as described in the Probe Warming section.
The probe type is determined by the probe logic lines at J1-B and E. Oral probes pull both lines low. Rectal
probes pull only J1-E low.

Probe Warming (675 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. To establish proper control parameters, the probe under control must be characterized.
The probe characterization process begins with a battery voltage reading. The battery voltage is needed in
the calculations to determine how much power will be applied to the probe during the 200mS test pulse.
After the battery voltage is read, the probe temperature is read once every 200 mS until the temperature is
found to be not changing.
Once a stable temperature is determined a 200 mS test pulse of power is applied to the probe via the
warming circuitry. The probe temperature change resulting from this pulse is used to determine parameters
needed for the PWM algorithm. This completes the probe characterization process.
The warming circuitry consists of Q7, Q8, Q9, R16, R18, R20, R21, R22, R23, C7, C8, D7, L1 and the
resistance in the probe connected to J1-D and J1-F. Power is applied to the probe connection only when
both Q7 and Q9 are turned on. When this occurs, both feedback lines to the processor (U2 pins 15 and 16)
are pulled high indicating proper operation. A fail safe hardware shut off circuit is incorporated consisting of
C8, D7, Q8 and R22. This circuit will shut off power to the probe if the software fails to refresh this circuit.
This prevents overheating due to any software errors leaving the warmer circuit on continuously.
At the start of a warming pulse, the processor drives both heater control lines (pins 54 and 55) low. This
turns on all three transistors Q7, Q8 and Q9. Q8 is turned on because the voltage across capacitor C8
cannot change instantly. Its positive side goes low when its negative side is pulled low by the processor.
Since Q9 E and B are pulled near the positive rail and Q8 B and E are pulled near the negative rail, the base
current from Q9 flows through R16 and mostly into Q8, but some current flows through R22 to satisfy the
diode drop across Q8 E to B. This current and Q8 base current flow into C8 charging it. As it charges, the
voltage rise eventually shuts off Q9.
This hardware shut off is only allowed to progress to completion during factory test and each time an oral
probe is installed as part of the instrument self test process. The thermometer checks that the hardware
shut off has occurred within one second of turn on.
Normal probe warming is controlled by the software and if the probe is well below the control temperature of
33.9 C (93 F), the pulse widths are at a maximum. Every 100 mS the hardware shutoff circuit is briefly
refreshed by the processor driving its pin 54 high. This momentarily shuts off power to the probe while
discharging C8 into the positive supply through D7.
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18 SureTemp Model 670/SureTemp 4 Model 675