Sharp R-9570 Service Manual page 19

RL957o
TEMPERATURE MEASUREMENT CIRCUIT
Fig. T-7 shows the temperature measurement circuit. The ther-
mistor in the temperature probe varies in resistance accord-
ing to the cooking temperature and this variation is converted
to an analog voltage variation to be delivered to the AN0 ter-
minal of LSI which contains an A/D converter. In the LSI there
are produced "H" level and "L" level signals at its output ter-
minal R8, as shown in Fig. T-8.
When the output signal of the terminal R8 is at "H" level, tran-
sistor Q60 turns on to short R63, and the circuit equivalent
to that shown in Fig. T-9(a) is set up. With this equivalent cir-
cuit in operation, the LSI can perform the temperature meas-
urement through the temperature probe.
TE MP
PROBE
When the output signal of terminal R8 is at "L" level, the tran-
sistor 060 turns off to get R61 and R63 connected in series
with each other, and the circuit equivalent to that shown in
Fig. T-9(b) is set up. With this equivalent circuit in operation,
the LSI judges whether the temperature probe is opened or
not: that is, when the temperature probe is opened, the vol-
tage almost equal to
VREF/2
is applied to the AN0 terminal of
LSI; when it is not opened (at its normal state), the voltage
lower than
VREF/2
is applied thereto. With the temperature
probe opened, the temperature indicator and numeral indica-
tor flash to indicate this state.
Figure T-7. Temperature Measurement Circuit
'::'#----J ~1 ------j-----.1:: :I
I
18
msec. 1
I 1-1
Figure T-8. Signal Output Timing at Q2 Terminal
PR.
PR
R62
R62
-[To LSI( ICl 11
I -WCTo
LSI ( ICl 13
R61
R61
R63
VC s
VC z-
(a) R8 tcrminal"H"
(bl
I?8 terminal "C'
Figure T-9. Circuit Equivalent to Temperature Measurement Circuit
17