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If the customer wishes to set the output amplitude, Comair Rotron can provide an open collector output. There is no output on the sensor lead until an external pull-up resistance is attached. The value of the resistance will need to be determined so that the current is limited to 15mA. (see figure 3)
Figure 3 - Non-Isolated, Open Collector Tachometer Circuit
Non-Isolated/Isolated Circuit
On most models, the fan is offered with either an isolated or non-isolated sensor circuit. A fan with an isolated sensor would have five lead wires. Two solid coloured power leads [ Red (+), Black (-) ] provide power to the motor windings. There would also be three sensor circuit leads [Red/white (+), Black/white (-), and Blue/white (sensor output)] to power the tachometer and the signal return. See Figure 4. A fan with a non-isolated sensor would have only three lead wires. Two solid coloured motor power leads [ Red (+), Black (-), and Blue/White (sensor lead)]. The power for the sensor circuitry is internally derived from the power of the motor windings.
Figure 4 - Isolated, 5 Volt Tachometer Circuit
An isolated tachometer is typically used where a negative voltage potential is used to power the fan and a positive voltage potential is used for the tachometer monitoring circuitry. The two different voltage potentials have different references to ground return and therefore have to be isolated. A non-isolated tachometer output is typical and can be found on almost every fan model.
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