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LM1819N Datasheet(PDF) 3 Page - National Semiconductor (TI) |
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LM1819N Datasheet(HTML) 3 Page - National Semiconductor (TI) |
3 / 10 page Application Hints (Continued) TLH5263 – 3 (c) (b) (a) FIGURE 3 Magnet and pointer position are controlled by the H field generated by the two drive coils In an air-core meter the axle assembly is supported by two nylon bushings The torque exerted on the pointer is much greater than that found in a typical d’Arsonval movement In contrast to a d’Arsonval movement where calibration is a function of spring and magnet characteristics air-core me- ter calibration is only affected by the mechanical alignment of the drive coils Mechanical calibration once set at manu- facture can not change Making pointer position a linear function of some input is a matter of properly ratioing the drive to each coil The H field contributed by each coil is a function of the applied current and the current is a function of the coil voltage Our desired result is to have i (pointer deflection measured in degrees) proportional to an input voltage ie kVIN 1 where k is a constant of proportionality with units of de- greesvolt The vector sum of each coils’ H field must follow the deflection angle i We know that the axle assembly always points in the direction of the vector sum of HSINE and HCOSINE This direction (see Figure 4 ) is found from the formula (i)earctan l HSINE l l HCOSINE l 2 Recalling some basic trigonometry (i)earctan(sin (i) cos(i )) 3 TLH5263 – 4 FIGURE 4 The vector sum of HCOSINE and HSINE points in a direction i measured in a clockwise direction from HCOSINE Comparing 3 to 2 we see that if HSINE varies as the sine of i and HCOSINE varies as the cosine of i we will gener- ate a net H field whose direction is the same as i And since the axle assembly aligns itself with the net H field the point- er will always point in the direction of i THE LM1819 Included in the LM1819 is a function generator whose two outputs are designed to vary approximately as the sine and cosine of an input A minimum drive of g20 mA at g4V is available at pins 2 (sine) and 12 (cosine) The common side of each coil is returned to a 51V zener diode reference and fed back to pin 1 For the function generator kj54 V (in equation 1) The input (pin 8) is internally connected to the Norton amplifier’s output VIN as considered in equation 1 is actually the dif- ference of the voltages at pins 8 (Norton outputfunction generator input) and 4 Typically the reference voltage at pin 4 is 21V Therefore ie k(V8bVREF) e 54 (V8b21) 4 As V8 varies from 21V to 775V the function generator will drive the meter through the chip’s rated 305 range Air-core meters are mechanically zeroed during manufac- ture such that when only the cosine coil is driven the point- er indicates zero degrees deflection However in some ap- plications a slight trim or offset may be required This is accomplished by sourcing or sinking a DC current of a few microamperes at pin 4 A Norton amplifier is available for conditioning various input signals and driving the function generator A Norton amplifi- er was chosen since it makes a simple frequency to voltage converter While the non-inverting input (pin 6) bias is at one diode drop above ground the inverting input (5) is at 21V equal to the pin 4 reference Mirror gain remains essentially flat to IMIRRORe5 mA The Norton amplifier’s output (8) is designed to source current into its load To bypass the Nor- ton amplifier simply ground the non-inverting input tie the inverting input to the reference and drive pin 8 (Norton out- putfunction generator input) directly An NPN transistor is included on chip for buffering and squaring input signals Its usefulness is exemplified in Fig- ures 1 6 where an ignition pulse is converted to a rectan- gular waveform by an RC network and the transistor The emitter is internally connected to ground It is important not to allow the base to drop below b5Vdc as damage may occur The 21V reference previously described is derived from an 85V regulator at pin 11 Pin 11 is used as a stable supply for collector loads and currents of up to 5 mA are easily accommodated 3 |
Número de pieza similar - LM1819N |
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Descripción similar - LM1819N |
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