TACH RC COMPONENT SELECTION

For a given maximum motor speed, the tach output has a maximum pulse width. For example, if a maximum motor speed of 3000

RPM were desired and the permanet magnet on the rotor of the motor being used has two pairs of poles, each mechanical

revolution would cause each hall effect sensor to deliver two pulses. The three sensors would generate six pulses. The MSK4410

tachometer would generate 12 pulses, one for each rising and falling edge of the sensor pulses. If a maximum motor speed in the

example were desired, this would be 50 revolutions per second. The tachometer will be producing 600 pulses per second or have

a frequency of 600Hz. Therefore, the maximum puse width would have to be less than 1.66 milliseconds. Figure one can be used

to select component values to achieve the desired pulse width.

6

APPLICATION NOTES CONTINUED

CURRENT LIMIT ADJUST

With the current limit adjust pin (pin 12) left open, the current will be limited to 1 amp ±0.7 amps. If a current limit of more than

1 amp but less than 15 amps is desired, a resistor between the pin and ground can be used to set a different limit. Figure two can

assist in the selection of the resistor value for this purpose.

FIGURE 1

FIGURE 2

Rev. B

11/04