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參數(shù)資料
| 型號(hào)： | LM1042N |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 模擬信號(hào)調(diào)理 |
| 英文描述： | Fluid Level Detector |
| 中文描述： | SPECIALTY ANALOG CIRCUIT, PDIP16 |
| 封裝： | N16A |
| 文件頁數(shù)： | 6/8頁 |
| 文件大小： | 170K |
| 代理商： | LM1042N |
Application Notes
(Continued)
CIRCUIT OPERATION
1) Thermo-Resistive Probes
These probes require measurements to be made of their
resistance before and after power has been dissipated in
them. With a probe connected as probe 1 in the connection
diagram the LM1042 will start a measurement when pin 8 is
taken to a logic low level (V
8
k
0.5V) and the internal time-
base ramp generator will start to generate the waveform
shown in Figure 5. At 0.7V, T
1
, the probe current drive is
switched on supplying a constant 200 mA via the external
PNP transistor and the probe failure circuit is enabled. At 1V
pin 1 is unclamped and C
1
stores the probe voltage corre-
sponding to this time, T
2
. The ramp charge rate is now re-
duced as C
T
charges toward 4V. As the 4.1V threshold is
passed a current sink is enabled and C
T
now discharges.
Between 1.3V and 1.0V, T
3
and T
4
, the amplified pin 1 volt-
age, representing the change in probe voltage since T
2
(and
as the current is constant this is proportional to the resist-
ance change) is gated onto the memory capacitor at pin 14.
At 0.7V, T
5
, the probe current is switched off and the mea-
surement cycle is complete. In the event of a faulty probe
being detected the memory capacitor is connected to the
regulated supply during the gate period. The device leakage
at pin 14 is a maximum of 2 nA to give a long memory
retention time. The voltage present on pin 14 is amplifed by
1.2 to drive pin 16 with a low impedance,
g
10 mA capabili-
ty, between 0.5V and 4.7V. A new measurement can only be
started by taking pin 8 to a low level again or by means of
the repeat oscillator.
TL/H/8709–8
FIGURE 5
2) Repetitive Measurement
With a capacitor connected between pin 9 and ground the
repeat oscillator will run with a waveform as shown inFigure
6 and a thermo-resistive probe measurement will be trig-
gered each time pin 9 reaches a threshold of 4.3V, provided
pin 8 is at a logic low level. The repeat oscillator runs inde-
pendently of the pin 8 control logic.
As the repetition rate is increased localized heating of the
probe and liquid being measured will be the main considera-
tion in determining the minimum acceptable measurement
intervals. Measurements will tend to become more depen-
dent on the amount of fluid movement changing the rate of
heat transfer away from the probe. The typical repeat time
versus timing capacitor value is shown in Figure 7.
TL/H/8709–9
FIGURE 6
TL/H/8709–10
FIGURE 7
3) Second Probe Input
A high impedance input for an alternative sensor is available
at pin 7. The voltage applied to this input is amplified and
output at pin 16 when the input is selected with a high level
on pin 8. The gain is defined by the feedback arrangement
shown in Figure 8 with adjustment possible at pin 10. With
pin 10 open the gain is set at a nominal value of 1.2, and
this may be increased by connecting a resistor between pin
10 and ground up to a maximum of 3.4 with pin 10 directly
grounded. A variable resistor may be used to calibrate for
the variations in sensitivity of the sensor used for probe 2.
TL/H/8709–11
FIGURE 8
POWER SUPPLY REGULATOR
The arrangement of the feedback for the supply regulator is
shown in Figure 9. The circuit acts to maintain pin 15 at a
constant 6V and when directly connected to pin 11 the reg-
ulated output is held at 6V. If required a resistor R may be
connected between pins 15 and 11 to increase the output
voltage by an amount corresponding typically to 1 mA flow-
ing in R. In this way a variable resistor may be used to trim
out the production tolerance of the regulator by adjusting for
V
REG
t
6.2V.
6
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