Welch Allyn SureTemp 670 Specifications download pdf (Page 30)

Technical Manual

4-8 Welch-Allyn, Inc.

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 J1B and J1E.

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 (470K) 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.

During normal operation, software turns the warmer circuit on and off. The width of the

pulse on HTRC determines how long Q5 is turned on, thereby determining how long the

heater is heating. Once the HTRC pulse goes high again, the base of Q5 is pulled high

turning it off, and the capacitor discharges to VCC through D1.

Q13 and Q14 are selected for their low saturation on voltage. D1 is a diode clamp

used to keep the base of Q5 from attaining a much higher voltage than VCC. R4(47K)

and R2(4.7K) in combination with R3(47K), serve as pull down resistors ensuring that

the processor feedback lines (U1-34 and U1-35) go low immediately upon warmer

component shut off. C2 serves as an RFI suppression component.

Other Components

Liquid Crystal Display

The model 678 and Model 679 use a liquid crystal display to display data to the user.

Three communication lines and 18 segment lines connect the LCD to the display

driver (U1-60 to U1-80) internal to the NEC microprocessor. The LCD is 3:1

multiplexed with 1/2 bias. The bias voltages (1.5 volts, and 3.0 volts) are supplied to

the display driver by the voltage reference circuit

The LCD glass is electrically tied to the display PCB via an elastomeric connector

sandwiched and compressed between the glass and the PCB by the frame. This

assembly, if taken apart, cannot be reassembled without replacing the frame.

LCD Backlight (Model 678 Only)

The backlight is a low power LED which illuminates the back of the LCD display. The

backlight will be turned on automatically in any mode except Low Power. Once the

backlight is turned on, it will automatically shut off whenever the instrument goes into

Low Power mode.

Battery power is applied to current limiter resistor R23 and LED D3. When

microprocessor signal /BLIGHTCTL at pin 42 goes low, current is allowed to flow

through the LED. The amount of current is approximately 2 milliamps, depending on

the Battery voltage Vcc. When the instrument is in Low Power mode / BLIGHTCTL

goes high, turning the LED backlight off to conserve power.

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Welch Allyn SureTemp 670 Specifications Page 30

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