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PDF LM1042 Data sheet ( Hoja de datos )
| Número de pieza | LM1042 | |
| Descripción | Fluid Level Detector | |
| Fabricantes | National Semiconductor | |
| Logotipo |  |
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February 1995
LM1042 Fluid Level Detector
General Description
The LM1042 uses the thermal-resistive probe technique to
measure the level of non-flammable fluids An output is pro-
vided proportional to fluid level and single shot or repeating
measurements may be made All supervisory requirements
to control the thermal-resistive probe including short and
open circuit probe detection are incorporated within the de-
vice A second linear input for alternative sensor signals
may also be selected
Features
Y Selectable thermal-resistance or linear probe inputs
Y Control circuitry for thermal-resistive probe
Y Single-shot or repeating measurements
Y Switch on reset and delay to avoid transients
Y Output amplifier with 10 mA source and sink capability
Y Short or open probe detection
Y a50V transient protection on supply and control input
Y 7 5V to 18V supply range
Y Internally regulated supply
Y b40 C to a80 C operation
Block Diagram
C1995 National Semiconductor Corporation TL H 8709
TL H 8709 – 1
RRD-B30M115 Printed in U S A
1 page  
Application Notes
THERMO-RESISTIVE PROBES OPERATION AND
CONSTRUCTION
These probes work on the principle that when power is dis-
sipated within the probe the rise in probe temperature is
dependent on the thermal resistance of the surrounding ma-
terial and as air and other gases are much less efficient
conductors of heat than liquids such as water and oil it is
possible to obtain a measurement of the depth of immersion
of such a probe in a liquid medium This principle is illustrat-
ed in Figure 1
FIGURE 2
TL H 8709 – 5
current with very fine wires to avoid excessive heating and
this current may be optimized to suit a particular type of
wire The temperature changes involved will give rise to no-
ticeable length changes in the wire used and more sophisti-
cated holders with tensioning devices may be devised to
allow for this
FIGURE 1
TL H 8709 – 4
During the measurement period a constant current drive I is
applied to the probe and the voltage across the probe is
sampled both at the start and just before the end of the
measurement period to give DV RTH Air and RTH Oil repre-
sent the different thermal resistances from probe to ambient
in air or oil giving rise fo temperature changes DT1 and DT2
respectively As a result of these temperature changes the
probe resistance will change by DR1 or DR2 and give corre-
sponding voltage changes DV1 or DV2 per unit length
Hence
DV
e
LA
L
DV1
a
(L
b
L
LA)
DV2
and for DV1 l DV2 RTH Air l RTH Oil DV will increase as
the probe length in air increases For best results the probe
needs to have a high temperature coefficient and low ther-
mal time constant One way to achieve this is to make use
of resistance wires held in a suitable support frame allowing
free liquid access Nickel cobalt iron alloy resistance wires
are available with resistivity 50 mXcm and 3300 ppm tem-
perature coefficient which when made up into a probe with 4
c 2 cm 0 08 mm diameter strands between supports (10
cm total) can give the voltage vs time curve shown in Figure
2 for 200 mA probe current The effect of varying the probe
current is shown in Figure 3 To avoid triggering the probe
failure detection circuits the probe voltage must be between
0 7V and 5 3V (VREG b 6V) hence for 200 mA the permis-
sible probe resistance range is from 3 5X to 24X The ex-
ample given has a resistance at room temperature of 9X
which leaves plenty of room for increase during measure-
ments and changes in ambient temperature
Various arrangements of probe wire are possible for any
given wire gauge and probe current to suit the measurement
range required some examples are illustrated schematically
in Figure 4 Naturally it is necessary to reduce the probe
FIGURE 3
TL H 8709 – 6
Probes need not be limited to resistance wire types as any
device with a positive temperature coefficient and sufficient-
ly low thermal resistance to the encapsulation so as not to
mask the change due to the different surrounding mediums
could be used Positive temperature coefficient thermistors
are a possibility and while their thermal time constant is like-
ly to be longer than wire the measurement time may be
increased by changing CT to suit
FIGURE 4
TL H 8709 – 7
5
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| Páginas | Total 8 Páginas | |
| PDF Descargar | [ Datasheet LM1042.PDF ] | |
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