Picking the Correct Pull-Up Resistor Value for Your DC Fan Tach Output Sensor
What Is a Pulse Tach Output Sensor?
The pulse tach output sensor from our DC fans provides real-time RPM measurements and monitors the health of the fan. It helps determine when the fan’s service life is approaching its end, as well as detecting any abnormalities —making it a valuable tool for scheduling maintenance.
See related article: An Overlooked but Useful Benefit of the Pulse Tach Sensor
Why Do Designers Ask About Measuring the Output
We often receive questions from customers who are starting new designs, particularly around how to measure the pulse tach output. This article explains what to consider when designing a circuitry to properly read this output.
Note that, unlike the PWM Input which has an internal Pull-Up resistor (see article: Get to know Protection Features in a Sanyo Denki DC Fan ) the pulse tach output does not have an internal pull-up resistor. It must be added externally by the customer, please refer to the schematic below for proper setup.
Setup
Sizing the Pull-Up Resistor: Don’t Exceed the Maximum lc Value
For example: 5mA Max
When choosing an external pull-up resistor, make sure Ic (collector current) does not exceed 5mA—going above this will damage the fan.
Use the Ohm’s Law formula to determine the appropriate resistor:
R = V / I, or specifically, Rpu = VCE / IC
For example:
- If you’re using a 12V supply:
R = 12V / 5mA = 2.4kΩ Min.
Choosing a Common Resistor Value
Common standard values like 3.3kΩ, 3.6kΩ, or 4.7kΩ can also be used. These values (with a VCE of 12V) result in the following collector currents which are all below the 5mA maximum limit:
- 3.3kΩ → 3.6mA
- 3.6kΩ → 3.3mA
- 4.7kΩ → 2.5mA
What to Expect for the Output Signal
With the correct pull-up resistor in place, you can now measure the pulse tach output. You should see two pulses for every impeller rotation, as shown in the waveform diagram below.
In Closing
To properly use the pulse tach output sensor:
- You must add an external pull-up resistor
- Ensure Ic stays under 5mA
Failing to do this may result in permanent damage to the fan.
Need Help?
Contact your local SANYO DENKI representative or sales engineers if you have further questions.
Written by Gary Masessa
About SANYO DENKI
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