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

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

formula

2

Recalling some basic trigonometry

cos(i ))

3

TLH5263 – 4

in a direction i measured in a clockwise direction from

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

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

ference of the voltages at pins 8 (Norton outputfunction

generator input) and 4 Typically the reference voltage at pin

4 is 21V Therefore

4

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

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

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