TL H 5699
LM903
Fluid
Level
Detector
February 1995
LM903 Fluid Level Detector
General Description
The LM903 uses the thermal-resistive probe technique to
measure the level of nonflammable fluids A low fluid level is
indicated by a warning lamp operating in continuous or
flashing mode All supervisory requirements to control the
thermal-resistive probe including short and open circuit
probe detection are incorporated within the device The cir-
cuit has possible applications in the detection of hydraulic
fluid oil level etc and may be used with partially conduct-
ing fluids
Features
Y
Flashing or continuous warning indication
Y
Warning threshold externally adjustable
Y
Control circuitry for thermal-resistive probe
Y
Switch on reset and delay to avoid transients
Y
600 mA flashing lamp drive capability
Y
Short and open circuit probe detection
Y
70V transient protection on supply and control input
Y
7V 18V supply range
Y
Internally regulated supply
Y
b
40 C to
a
80 C operation
Connection Diagram
Dual-In-Line Package
TL H 5699 1
Order Number LM903N
See NS Package Number N16E
C1995 National Semiconductor Corporation
RRD-B30M115 Printed in U S A
Absolute Maximum Ratings
If Military Aerospace specified devices are required
please contact the National Semiconductor Sales
Office Distributors for availability and specifications
Supply Voltage V
CC
18V
Control Input Voltage (Pin 7)
18V
Transient Voltage (Pins 6 7 9) 10 ms (Note 1)
70V
Output Current (Pin 4) I
4
(Sink)
10 mA
Operating Temperature Range
b
40 C to
a
85 C
Storage Temperature
b
55 C to
a
150 C
Maximum Junction Temperature
a
150 C
Lead Temperature (Soldering 10 sec )
260 C
Electrical Characteristics
V
CC
e
12V C
T
e
33 mF R
T
e
7 5 kX T
A
within operating range except where stated otherwise
Tested Limits
Design Limits
Symbol
Parameter
Conditions
(Note 2)
(Note 3)
Units
Min
Max
Min
Typ
Max
V
CC
Supply Voltage
7 0
18
7 0
13
18
V
I
S
Supply Current
50
50
mA
V
REG
Regulated Voltage
5 5
6 2
5 3
5 8
6 3
V
Regulation
V
CC
e
7 2V 18V
105
mV
Temperature Drift
500
m
V C
V
6
V
3
Probe Current
2 0
2 35
1 95
2 20
2 40
V
Reference Voltage
V
REF
Measurement Reference Voltage
790
900
780
850
910
mV
R
REF
Reference Input Resistor
1 2
kX
V
7
Start Input Logic High Level
1 6
V
V
7
Start Input Logic Low Level
1 0
V
I
7
High Input Current
Latch Off
100
nA
I
7
Latch Holding Current
Latch On
2 5
nA
R
7
Resistance Pin 7
Latch On
22
kX
I
12
Ramp Current
See Timing Diagram
Charging
V
12
e
0V 1V
600
1100
590
1100
m
A
V
12
e
1V 4V
53
93
50
96
m
A
Discharging
V
12
e
4 1V
b
700
b
450
b
710
b
440
m
A
V
12
e
0 5V
b
650
b
400
b
660
b
390
m
A
V
12
Ramp Threshold
See Timing Diagram
Probe Current Start
570
850
550
710
870
mV
First Measurement
910
1200
890
1055
1220
mV
Second Measurement
910
1240
890
1080
1270
mV
V
1
Probe Input Voltage Range
V
CC
e
7 5V 18V
1
V
REG
b
1 0
V
V
5
Probe Open-Circuit Threshold
At Pin 5
V
REG
b
0 85 V
REG
b
0 6
V
V
5
Probe Short-Circuit Threshold
0 6
0 85
V
I
1
Pin 1 Input Leakage Current
Pin 1
e
300 mV
b
3 5
a
3 5
a
5 0
nA
I
15
Pin 15 Leakage Current
V
15
e
2V V
7
e
12V
b
3 5
3 5
m
A
Pin 15 Charging Current
V
15
e
4V V
7
e
12V
60
m
A
f
9
Lamp Oscillation Frequency
C
L
e
3 3 mF
0 5
1 5
2 5
Hz
I
9
Lamp Driver Current
Flashing Mode
600
mA
V
9
Lamp Driver Saturation
I
9
e
200 mA
200
250
mA
2
Electrical Characteristics
(Continued)
V
CC
e
12V C
T
e
33 mF R
T
e
7 5 kX T
A
within operating range except where stated otherwise
Tested Limits
Design Limits
Symbol
Parameter
Conditions
(Note 2)
(Note 3)
Units
Min
Max
Min
Typ
Max
V
14
Auxiliary Output
Lamp OFF
5 0
V
Voltage
Lamp ON
1 2
V
V
1
Alarm Level
(Difference Between First
230
280
330
mV
and Second Measurement)
Sensitivity to Electrostatic Discharge
Pins 7 10 13 and 14 will withstand greater than 1500V when tested using 100 pF and 1500X in accordance with National
Semiconductor standard ESD test procedures All other pins will withstand in excess of 2 kV
Note 1
Test circuit for overvoltage capability at pins 3 6 7
Note 2
Guaranteed 100% production tested at 25 C These limits are used to calculate outgoing quality levels
Note 3
Limits guaranteed to include parametric variations T
A
e
b
40 C to
a
80 C and from V
CC
e
7 5V18V These limits are not used to calculate AOQL
figures
Note 4
Variations over temperature range are not production tested
TL H 5699 2
In Lamp ON condition I
9
should be limited to 600 mA
Block and Application Circuit
TL H 5699 3
Memory capacitor on pin 15 is set
High
Lamp off
Low
Lamp on
3
Circuit Timing Diagram
t1
25 ms
0 7V
Threshold
t2
35 ms
1 0V
1st Measurement
t3
t2
a
1 5s
1 0V
2nd Measurement
t4
t3
a
10 ms
0 8V
Measurement Latched
t5
14
a
8 ms
0 7V
Probe Current Off
TL H 5699 4
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 mXcm and temperature
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 4X to 25X This low value makes
it possible to use the probe in partially conducting fluids
Using resistance wire of 50 mXcm resistivity 8 cm of 0 08
mm (40 AWG) give approximately 8X at 25 C Such a probe
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
FIGURE 1 Typical Thermo-Resistive Probe
TL H 5699 5
FIGURE 2
4
Equivalent Schematic Diagram
TLH5699
6
5
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