LM903 NSC [National Semiconductor], LM903 Datasheet - Page 4

• Manufacturer Part Number: LM903
• Description: Fluid Level Detector
• Manufacturer: NSC [National Semiconductor]

Circuit Timing Diagram
Circuit Operation
A measurement is initiated when the supply is applied pro-
vided the control input pin 7 is low Once a measurement is
commenced pin 7 is latched low and the ramp capacitor on
pin 12 begins to charge After 25 ms when switch-on tran-
sients have subsided a constant current is applied to the
thermo-resistive probe The value of probe current which is
supplied by an external PNP transistor is set by an external
resistor across an internally generated 21V reference The
lamp current is applied at the start of probe current
35 ms after switch-on the voltage across the probe is sam-
pled and held on external capacitor C1 (leakage current at
pin 1 less than 1 nA) After a further 1 5 seconds the differ-
ence between the present probe voltage and the initial
probe voltage is measured multiplied by 3 and compared
with a reference voltage of 850 mV (externally adjustable
via pin 16) If the amplified voltage difference is less than
the reference voltage the lamp is switched off otherwise
the lamp commences flashing at 1 Hz to 2 Hz 10 ms later
the measurement latch operates to store the result and af-
ter a further 8 ms the probe current is switched off
A second measurement can only be initiated by interrupting
the supply An external CR can be arranged on pin 7 to
prevent a second measurement attempt for 1 minute The
measurement condition stored in the latch will control the
lamp
PROBES
The circuit effectively measures the thermal resistance of
the probe This varies depending on the surrounding medi-
um ( Figure 1 ) It is necessary to be able to heat the probe
with the current applied and for there to be sufficient
change in resistance with the temperature change to pro-
vide the voltage to be measured
Probes require resistance wire with a high resistivity and
temperature coefficient Nickel cobalt alloy resistance wires
are available with resistivity of 50
coefficient of 3300 ppm which can be made into suitable
probes Wires used in probes for use in liquids must be de-
signed to drain freely to avoid clogging A possible arrange-
ment is shown in Figure 2
The probe voltage has to be greater than 0 7V to prevent
short circuit probe detection less than 5V to avoid open
circuit detection With a 200 mA probe current this gives a
probe resistance range of 4
it possible to use the probe in partially conducting fluids
to 25
This low value makes
cm and temperature
4
Using resistance wire of 50
mm (40 AWG) give approximately 8
will give about 500 mV change between first and second
measurements in air and 100 mV change with oil hydraulic
fluid etc in the application circuit With an alarm threshold
of 280 mV (typ) lack of fluid can readily be detected As the
probe current measurement reference and measurement
period are all externally adjustable there is freedom to use
different probes and fluids
Another possibility is the use of high temperature coefficient
resistors made for special applications and positive temper-
ature coefficient thermistors The encapsulation must have
a sufficiently low thermal resistance so as not to mask the
change due to the different surrounding mediums and the
thermal time constant must be quick enough to enable the
temperature change to take place between the two mea-
surements The ramp timing could be adjusted to assist this
Probes in liquids must be able to drain freely
t1
t2
t3
t4
t5
25 ms
35 ms
t2
t3
14
FIGURE 1 Typical Thermo-Resistive Probe
a
a
a
1 5s
10 ms
8 ms
0 7V
1 0V
1 0V
0 8V
0 7V
FIGURE 2
Threshold
1st Measurement
2nd Measurement
Measurement Latched
Probe Current Off
cm resistivity 8 cm of 0 08
at 25 C Such a probe
TL H 5699– 4
TL H 5699 – 5