參數(shù)資料
型號(hào): LM1830N
廠商: NATIONAL SEMICONDUCTOR CORP
元件分類: 模擬信號(hào)調(diào)理
英文描述: Fluid Detector
中文描述: SPECIALTY ANALOG CIRCUIT, PDIP14
封裝: 0.300 INCH, PLASTIC, DIP-14
文件頁數(shù): 4/8頁
文件大?。?/td> 150K
代理商: LM1830N
Application Hints
The LM1830 requires only an external capacitor to com-
plete the oscillator circuit. The frequency of oscillation is
inversely proportional to the external capacitor value. Using
0.001
m
F capacitor, the output frequency is approximately 6
kHz. The output from the oscillator is available at pin 5. In
normal applications, the output is taken from pin 13 so that
the internal 13k resistor can be used to compare with the
probe resistance. Pin 13 is coupled to the probe by a block-
ing capacitor so that there is no net dc on the probe.
Since the output amplitude from the oscillator is approxi-
mately 4 V
BE
, the detector (which is an emitter base junc-
tion) will be turned ‘‘ON’’ when the probe resistance to
ground is equal to the internal 13 k
X
resistor. An internal
diode across the detector emitter base junction provides
symmetrical limiting of the detector input signal so that the
probe is excited with
g
2 V
BE
from a 13 k
X
source. In cases
where the 13 k
X
resistor is not compatible with the probe
resistance range, an external resistor may be added by cou-
pling the probe to pin 5 through the external resistor as
shown in Figure 2. The collector of the detecting transistor
is brought out to pin 9 enabling a filter capacitor to be con-
nected so that the output will switch ‘‘ON’’ or ‘‘OFF’’ de-
pending on the probe resistance. If this capacitor is omitted,
the output will be switched at approximately 50% duty cycle
when the probe resistance exceeds the reference resist-
ance. This can be useful when an audio output is required
and the output transistor can be used to directly drive a loud
speaker. In addition, LED indicators do not require dc exci-
tation. Therefore, the cost of a capacitor for filtering can be
saved.
In the case of inductive loads or incandescent lamp loads, it
is recommended that a filter capacitor be employed.
In a typical application where the device is employed for
sensing low water level in a tank, a simple steel probe may
be inserted in the top of the tank with the tank grounded.
Then when the water level drops below the tip of the probe,
the resistance will rise between the probe and the tank and
the alarm will be operated. This is illustrated in Figure 3. In
situations where a non-conductive container is used, the
probe may be designed in a number of ways. In some cases
a simple phono plug can be employed. Other probe designs
include conductive parallel strips on printed circuit boards.
It is possible to calculate the resistance of any aqueous
solution of an electrolyte for different concentrations, pro-
vided the dimensions of the electrodes and their spacing is
known.
The resistance of a simple parallel plate probe is given by:
R
e
1000
c.p
#
d
A
X
where
A
e
area of plates (cm
2
)
d
e
separation of plates (cm)
c
e
concentration (gm. mol. equivalent/litre)
p
e
equivalent conductance
(
X
b
1
cm
2
equiv.
b
1
)
(An equivalent is the number of moles of a substance that
gives one mole of positive charge and one mole of negative
charge. For example, one mole of NaCl gives Na
a
a
Cl
b
so
the equivalent is 1. One mole of CaCl
2
gives Ca
aa
a
2Cl
b
so the equivalent is 1/2.)
Usually the probe dimensions are not measured physically,
but the ratio d/A is determined by measuring the resistance
of a cell of known concentration c and equivalent conduct-
ance of 1. A graph of common solutions and their equivalent
conductances is shown for reference. The data was derived
from D.A. Maclnnes, ‘‘The Principles of Electrochemistry,’’
Reinhold Publishing Corp., New York., 1939.
In automotive and other applications where the power
source is known to contain significant transient voltages, the
internal regulator on the LM1830 allows protection to be
provided by the simple means of using a series resistor in
the power supply line as illustrated inFigure 4. If the output
load is required to be returned directly to the power supply
because of the high current required, it will be necessary to
provide protection for the output transistor if the voltages
are expected to exceed the data sheet limits.
Although the LM1830 is designed primarily for use in sens-
ing conductive fluids, it can be used with any variable resist-
ance device, such as light dependent resistor or thermistor
or resistive position transducer.
The following table lists some common fluids which may
and may not be detected by resistive probe techniques.
Conductive Fluids
Non-Conductive Fluids
City water
Sea water
Copper sulphate solution
Weak acid
Weak base
Household ammonia
Water and glycol mixture
Wet soil
Coffee
Pure water
Gasoline
Oil
Brake fluid
Alcohol
Ethylene glycol
Paraffin
Dry soil
Whiskey
4
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