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Zakładka z wyszukiwarką danych komponentów |
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LM1819N Arkusz danych(PDF) 4 Page - National Semiconductor (TI) |
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LM1819N Arkusz danych(HTML) 4 Page - National Semiconductor (TI) |
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4 / 10 page  Application Hints (Continued) TACHOMETER APPLICATION A measure of the operating level of any motor or engine is the rotational velocity of its output shaft In the case of an automotive engine the crankshaft speed is measured using the units ‘‘revolutions per minute’’ (RPM) It is possible to indirectly measure the speed of the crankshaft by using the signal present on the engine’s ignition coil The fundamental frequency of this signal is a function of engine speed and the number of cylinders and is calculated (for a four-stroke engine) from the formula f en0 120 (Hz) (5) where nenumber of cylinders and 0erotational velocity of the crankshaft in RPM From this formula the maximum fre- quency normally expected (for an 8 cylinder engine turning 4500RPM) is 300 Hz In certain specialized ignition systems (motorcycles and some automobiles) where the coil wave- form is operated at twice this frequency ( f e0 60) These systems are identified by the fact that multiple coils are used in lieu of a single coil and distributor Also the coils have two outputs instead of one A typical automotive tachometer application is shown in Fig- ure 1 The coil waveform is filtered squared and limited by the RC network and NPN transistor The frequency of the pulse train at pin 9 is converted to a proportional voltage by the Norton amplifier’s charge pump configuration The igni- tion circuit shown in Figure 5 is typical of automotive sys- tems The switching element ‘‘S’’ is opened and closed in synchronism with engine rotation When ‘‘S’’ is closed en- ergy is stored in Lp When opened the current in Lp diverts from ‘‘S’’ into C The high voltage produced in Ls when ‘‘S’’ is opened is responsible for the arcing at the spark plug The coil voltage (see Figure 6 ) can be used as an input to the LM1819 tachometer circuit This waveform is essentially constant duty cycle D4 rectifies this waveform thereby pre- venting negative voltages from reaching the chip C4 and R5 form a low pass filter which attenuates the high frequen- cy ringing and R7 limits the input current to about 25mA R6 acts as a base bleed to shut the transistor OFF when ‘‘S’’ is closed The collector is pulled up to the internal regu- lator by RREG The output at pin 9 is a clean rectangular pulse Many ignition systems use magnetic hall effect or optical sensors to trigger a solid state switching element at ‘‘S’’ These systems (see the LM1815) typically generate pulses of constant width and amplitude suitable for driving the charge pump directly The charge pump circuit in Figure 7 can be operated in two modes constant input pulse width (C1 acts as a coupling capacitor) and constant input duty cycle (C1 acts as a differ- entiating capacitor) The transfer functions for these two modes are quite diverse However deflection is always di- rectly proportional to R2 and ripple is proportional to C2 The following variables are used in the calculation of meter deflection symbol description n number of cylinders 0 0IDLE engine speed at redline and idle RPM i pointer deflection at redline degrees e charge pump input pulse width seconds VIN peak to peak input voltages volts Di maximum desired ripple degrees k function generator gain degreesvolt f f IDLE input frequency at redline and idle Hz Where the NPN transistor and regulator are used to create a pulse VINe85V Acceptable ripple ranges from 3 to 10 de- grees (a typical pointer is about 3 degrees wide) depending on meter damping and the input frequency The constant pulse width circuit is designed using the fol- lowing equations (1) 100 mAk VIN R1 k 3mA (2) C1t 10e R1 (3) R2e R1i VINek f e 120R1i VINn0ek (4) C2e 1 R2Di fIDLE e 1 R2Din0IDLE The constant duty cycle equations are as follows RREG t 3kX R1 s VINx104 bRREG C1 s e 10(RREGaR1) RZ e i 354n0C1ei 425 fC1 C2 e 425C1 Di The values in Figure 1 were calculated with ne4 0e6000RPM ie 270 degrees ee 1 ms VIN is VREGb07V and Die3 degrees in the constant duty cycle mode For distributorless ignitions these same equations will apply if 0 60 is substituted for f 4 |
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