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LM1830N Arkusz danych(PDF) 4 Page - National Semiconductor (TI) |
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LM1830N Arkusz danych(HTML) 4 Page - National Semiconductor (TI) |
4 / 8 page 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 0001mF 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 VBE 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 kX 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 g2VBE from a 13 kX source In cases where the 13 kX 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 Re 1000 cp d A X where Aearea of plates (cm2) deseparation of plates (cm) ceconcentration (gm mol equivalentlitre) peequivalent conductance (Xb1 cm2 equiv b1) (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 Naa aClb so the equivalent is 1 One mole of CaCl2 gives Caaa a 2Clb so the equivalent is 12) Usually the probe dimensions are not measured physically but the ratio dA 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 DA 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 in Figure 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 Pure water Sea water Gasoline Copper sulphate solution Oil Weak acid Brake fluid Weak base Alcohol Household ammonia Ethylene glycol Water and glycol mixture Paraffin Wet soil Dry soil Coffee Whiskey 4 |
Podobny numer części - LM1830N |
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Podobny opis - LM1830N |
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