Epson PX-8 Technical Manual page 113

REV.-A
2.9.3 Temperature Detector Circuit
When the power switch is off, battery power consumption is minimized by optinizing the refresh
current according to ambient temperatures. The ambient temperature is sensed by a thermistor
and fed to channel ANO of the A-O converter which detects the reference temperatures.
2.9.3.1 Circuit Operations
Fig. 2-91 shows the temperature detector circuit, including the sensing section. The reference
voltage (VRF) is adjusted to +2.0V by VR2. Thus, the potential at point (A) is given as follows, be-
cause of the voltage divider circuit consisting of R76 and Rlll :
RlllVx
VRF = -R-7-6-+-R-l-l-l-
Vx=
VRE(R76+Rlll)
Rlll
C
K
C
5
5 C
o
L
o
I
(ID)UPD 7001
~
2
ERT COJR56JAA
Fig. 2-91 Temperature Detector Circuit
Vx = 2 (34.8 x 10
3
+ 100 x 10
3 )
100 x 10
3
= 2.696 (V)
Potential at point (A)
010
MA
153
RS8
10K
GND
The potential (A) is also applied to the voltage divider circuit consisting of R86 and the thermistor
TH 1, which is the temperature sensor. The potential at the junction of R86 and TH 1, VTH, which
is supplied to channel ANO, is therefore represented by the following expression:
VTH = R (TH1)· 2.696
R86 + R (TH1)
where the thermister resistance R (TH 1) has the following temperature characteristic:
R (TH 1)
=
56 K exp 4.3 (
0.27~2
+ t -
0.2~82
) [t:
DC]
As the temperature rises, the thermister resistance decreases, causing the voltage supply to the
ANO channel (VTH) to be lowered.
2-89