EMP OP80 - Page 29

PWM Variable Speed Control
© 2019 EMP, Inc. 12
Tachometer Output Parameters
Tachometer output is a pin that produces a variable frequency output based on motor speed.
This pin will output a TACH_OUT signal when the PWM signal pin receives a valid input or the PWM signal pin
is held low.
The TACH_OUT is an open drain output. The TACH_OUT needs an external pull-up to be read by an external
controller (See the example schematic). Tachometer output represents component speed according to the
following formula:
 =

 ∗ 
Tachometer Output Parameters
Parameter
Min
Nom
Max
Units
Vol (No Load)
0
V
V Pull Up
32
V
I Short Circuit
1.5
A
I Operating (Sink)
500
mA
Example TACH_OUT Schematic
At 4000 rpm, the TACH_OUT signal will have a time on (TimeOn) of 625 µs and a frequency of 800 Hz:
TimeOn = 625 µs = 0.000625 seconds
Period = 2 * 625 µs = 1.25 ms = 0.00125 seconds
Frequency = 1/0.00125 = 800 Hz
To recover the motor speed from the TACH_OUT signal, input the TimeOn into the rpm formula given above.
 =
60
24 ∗ 
 =
60
24 ∗ 0.000625
 = 4000