LM903 Fluid Level Detector General Description Features 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 circuit has possible applications in the detection of hydraulic fluid, oil level, etc., and may be used with partially conducting fluids. Y Y Y Y Y Y Y Y Y Y Flashing or continuous warning indication Warning threshold externally adjustable Control circuitry for thermal-resistive probe Switch on reset and delay to avoid transients 600 mA flashing lamp drive capability Short and open circuit probe detection 70V transient protection on supply and control input 7V – 18V supply range Internally regulated supply 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 TL/H/5699 RRD-B30M115/Printed in U. S. A. LM903 Fluid Level Detector February 1995 Absolute Maximum Ratings Operating Temperature Range If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/Distributors for availability and specifications. Supply Voltage, VCC b 55§ C to a 150§ C Maximum Junction Temperature Lead Temperature (Soldering, 10 sec.) 18V 18V 70V 10 mA Control Input Voltage (Pin 7) Transient Voltage (Pins, 6, 7, 9) 10 ms (Note 1) Output Current (Pin 4) I4 (Sink) b 40§ C to a 85§ C Storage Temperature a 150§ C 260§ C Electrical Characteristics VCC e 12V, CT e 33 mF, RT e 7.5 kX, TA within operating range except where stated otherwise Symbol Parameter VCC Supply Voltage IS Supply Current VREG Regulated Voltage Regulation Temperature Drift Tested Limits (Note 2) Conditions Design Limits (Note 3) Min Max Min Typ 7.0 18 7.0 13 50 5.5 VCC e 7.2V–18V 6.2 5.3 5.8 Units Max 18 V 50 mA 6.3 105 V mV mV/§ C 500 V6 – V3 Probe Current Reference Voltage 2.0 2.35 1.95 2.20 2.40 V VREF Measurement Reference Voltage 790 900 780 850 910 mV RREF Reference Input Resistor V7 Start Input Logic High Level V7 Start Input Logic Low Level I7 High Input Current Latch Off I7 Latch Holding Current Latch On 2.5 nA R7 Resistance Pin 7 Latch On 22 kX I12 Ramp Current See Timing Diagram Charging Discharging V12 Ramp Threshold 1.2 kX 1.6 V 1.0 V 100 nA V12 e 0V–1V 600 1100 590 1100 V12 e 1V – 4V 53 93 50 96 mA V12 e 4.1V b 700 b 450 b 710 b 440 mA V12 e 0.5V b 650 b 400 b 660 b 390 mA mA See Timing Diagram Probe Current Start 570 850 550 710 870 mV First Measurement 910 1200 890 1055 1220 mV 910 1240 890 1080 1270 mV VREGb1.0 V Second Measurement V1 Probe Input Voltage Range VCC e 7.5V–18V V5 Probe Open-Circuit Threshold At Pin 5 V5 Probe Short-Circuit Threshold 1 VREGb0.85 VREGb0.6 0.6 I1 Pin 1 Input Leakage Current Pin 1 e 300 mV I15 Pin 15 Leakage Current V15 e 2V, V7 e 12V b 3.5 60 Pin 15 Charging Current V15 e 4V, V7 e 12V f9 Lamp Oscillation Frequency CL e 3.3 mF I9 Lamp Driver Current Flashing Mode V9 Lamp Driver Saturation I9 e 200 mA b 3.5 a 3.5 3.5 a 5.0 V nA mA mA 0.5 200 2 V 0.85 1.5 2.5 Hz 600 mA 250 mA Electrical Characteristics (Continued) VCC e 12V, CT e 33 mF, RT e 7.5 kX, TA within operating range except where stated otherwise Symbol Parameter Tested Limits (Note 2) Conditions Min V14 V1 Auxiliary Output Voltage Alarm Level Max Lamp OFF Design Limits (Note 3) Min Typ Units Max 5.0 V Lamp ON (Difference Between First and Second Measurement) 230 280 1.2 V 330 mV 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. TA e b 40§ C to a 80§ C and from VCC e 7.5V–18V. 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, I9 should be limited to 600 mA. Block and Application Circuit TL/H/5699 – 3 Memory capacitor on pin 15 is set HighÐLamp off LowÐLamp 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 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 temperature 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 measurements. The ramp timing could be adjusted to assist this. Probes in liquids must be able to drain freely. A measurement is initiated when the supply is applied, provided 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 transients 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 sampled and held on external capacitor C1 (leakage current at pin 1 less than 1 nA). After a further 1.5 seconds the difference 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 after 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 medium (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 provide 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 designed to drain freely to avoid clogging. A possible arrangement 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. FIGURE 1. Typical Thermo-Resistive Probe TL/H/5699 – 5 FIGURE 2 4 TL/H/5699 – 6 Equivalent Schematic Diagram 5 Application Hints 3V, the memory capacitor will be refreshed on powering up again. There is no internal pull down on detecting an incorrect measurement. If it is required to use pin 15 as an output indicating the measurement result, an external pull down resistor and buffer will be required. INTERNAL COMBUSTION ENGINE OIL LEVEL The basic system provides a single shot measurement when the supply is applied and has a primary application in automotive oil, hydraulic fluid and coolant monitoring. Particularly in the case of the oil level, a valid measurement is only possible before the oil is disturbed. The application circuit shown is arranged such that the measurement is made when the ignition is switched on via switch A. Switch B is the oil pressure sensor and is closed before the engine starts, keeping pin 7 low and enabling the measurement. CONTINUOUS WARNING LAMP The lamp can be arranged to light continuously by disabling the oscillator with a resistor of 150k or less, connected between pins 10 and 11. REPETITIVE MEASUREMENTS Measurements may be repeated by strobing the supply to pin 6. The probe current regulator transistor must have the same supply as pin 6, but the warning lamp can be permanently powered. The lamp will light during each measurement and will flash in between measurements when incorrect conditions are detected. STALLING AND RESTART PROTECTION The 4M7 resistor and 10 mF capacitor connected to pin 7 provide the restart protection. When oil pressure builds up, switch B opens and the 10 mF capacitor charges through the bulb. At switch-off, the capacitor discharges slowly and is capable of preventing a low state on pin 7 for 1 minute. Unless pin 7 is low, a new measurement can not be made and the previous measurement result stored in the memory capacitor on pin 15 is used to control the output. ALTERNATIVE APPLICATIONS Gas flow detection: The cooling effect of gas flowing over a probe could be used to provide a warning signal from the LM903 in the event of gas failure. Automatic top up: With the LM903 strobed continuously, the output may be stored, buffered, and used to drive solenoid valves to correct a fluid level as required. MEMORY The pin 15 memory output goes high if a correct measurement is made (lamp off). If the power is removed, pin 15 leakage is less than 3 mA and the memory status is retained for some time. Provided pin 15 voltage does not fall below 6 7 LM903 Fluid Level Detector Physical Dimensions inches (millimeters) Molded Dual-in-Line Package (N) Order Number LM903N NS Package N16E LIFE SUPPORT POLICY NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein: 1. Life support devices or systems are devices or systems which, (a) are intended for surgical implant into the body, or (b) support or sustain life, and whose failure to perform, when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in a significant injury to the user. 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