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Low Side Switching DC Fan; Tacho Signal

Arda , 02-26-2024, 10:44 AM

This is my first time in the Q/A forum. So sorry if I do sonething unintentioanally.

My question is about the Dc Fan. I am low side switching the DC Axial Fan. In some applications Tachometer returns the low side PWM in the exact way as a return path. How can I solve this issue? Does the Tacho signal needs a clear GND signal in order to read? How to come across this issue in low side switching?

Below you can see the schematics. I am open to suggestions. Thank you for your time.
SirJames , 02-26-2024, 05:11 PM
Could you show us a picture of how is your TACHO_5V signal connected to MCU? Tacho signal has to be pulled up by at least 1k resistor.
QDrives , 02-26-2024, 08:53 PM
The tacho output is an open collector output. So the only time when you can see the pulse is then the FET is driving the FAN.
No power, no signal.
As you are doing low side driving, you need an interface between the tacho output and your MCU.
Arda , 02-27-2024, 07:02 AM
Any suggestions for this interface?
Thank you all for your replies.
QDrives , 02-27-2024, 09:23 PM
Assuming the tacho output can handle 24V...
- A 22k pull-up to 24V (gives about 1mA)
- This going to the base of a NPN transistor, either directly (cheap, less protection) or through an additional 22k resistor (reduces base current when [ESD] spikes come on the line).
This is connecting two open collect circuits in series.
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