Fan speed control using PWM signal

Hi,
This is a circuit to control the speed of fan using PWM signal.
Pin 4 is the input from controller which generates the PWM signal.
Pin 3 is the output from fan tachometer which provides the RPM to controller.
Pin 2 in power supply to the fan and pin 1 is the reference voltage.

"Can anyone please tell me what these two diode (D25 and D29) are doing here?"
Are they used to protect the circuit from back EMF or there is something else?
Please explain the use and working.

Thanks in advance

Rahul Soni said:

View attachment 116523

Hi,
This is a circuit to control the speed of fan using PWM signal.
Pin 4 is the input from controller which generates the PWM signal.
Pin 3 is the output from fan tachometer which provides the RPM to controller.
Pin 2 in power supply to the fan and pin 1 is the reference voltage.

"Can anyone please tell me what these two diode (D25 and D29) are doing here?"
Are they used to protect the circuit from back EMF or there is something else?
Please explain the use and working.

Thanks in advance

Click to expand...

D25 and D29 limits (clamps) the voltages to 5v.
this is for logic protection.

srizbf said:

D25 and D29 limits (clamps) the voltages to 5v.
this is for logic protection.

Click to expand...

Since they are connected reversed biased, the voltage to be limited must be the voltage generated by back EMF only? Am I right ?

Hi,

as srizbf pointed out.

It calmps it to 5V while the fan is supplied with 12V only.

12V is higher than 5V, back EMF may be higher than 5V.

Klaus

The diodes are to protect the electronics inside the fan.
For back EMF you have to add your own protection for the 12V drive.
Some fans have internal back EMF protection combined with reverse voltage protection.

I won't expect "back-EMF" at the logic pins in normal operation. The protection has been more likely provided to protect in case of hot insertion or when a fan with wrong pin assignment is connected.

The signals where the diodes are present do not get connected to any windings/ coil. So no back emf here. May be because of mutual coupling due to wire routing or some other incidents which may result in higher than 5V on these lines, the diodes are used to limit them to not more than 5V. The series 100 ohm resistor limits the current to safe value in such incidents.

FvM said:

The protection has been more likely provided to protect in case of hot insertion or when a fan with wrong pin assignment is connected.

Click to expand...

hi,

How will this diode protect the circuit in case of hot insertion.
I don't think pin assignment will be any problem since they will be connected through proper connector only.
Am I right ?

Thanks in advance.

- - - Updated - - -

ravindragudi said:

The signals where the diodes are present do not get connected to any windings/ coil. So no back emf here. May be because of mutual coupling due to wire routing or some other incidents which may result in higher than 5V on these lines, the diodes are used to limit them to not more than 5V. The series 100 ohm resistor limits the current to safe value in such incidents.

Click to expand...

So you are saying that any back EMF will be generated only if the coil is connected directly to the IC ? If it is connected through any connector, then this back EMF will have no effect on the IC connected to the coil via connector?
Am I right?

Can you please explain how this diode will protect the circuit in case of over voltage?

Thanks a lot

Wiki gives info on back emf. its associated with motors, windings .. https://en.wikipedia.org/wiki/Counter-electromotive_force
Anything which gets in contact with the windings will see back emf when it happens. In your circuit neither Tacho has a coil/ winding nor the PWM output.
Connector is a way of making physical contact .. it doesn't introduce back emf by itself.

When the diode is present on signal line with K @ 5V and A at signal level <5V. In over voltage condition when the voltage exceeds by 5V + Vf (0.7V typ) of diode the diode starts conducting and helps this signal line on mcu to be limited to safer value.