Can't run a 24V DC fan with NPN PWM output signal

[D.A.(Tony)Stewart]

What is the current rating or Pd on fan and load current on DAC?

You can't simply use a FET or a BJT to drive the fan unless you use a very small control range.
It is better to add resistors to control the gain the convert 0 to 10V to fan speed 0 to 100% using negative feedback.

Can you design this or do you need help?

Fan = 3W? DAC can drive 1kohm ?

 

[Akanimo]

The fan may be internally controlled at a fixed speed irrespective of the input DC voltage.

Disregard this senseless statement that I made here.

--- Updated Jan 24, 2024 ---

You can actually vary the speed of the two-wire fan but you are not going to be able to control it because you don't have a means of getting feedback about its instantaneous speed. To vary the speed, you can vary the supply voltage.

To control it you need to be able to tell what the actual speed is. That requires one with three wires. To be able to use your board to control the speed by PWM, you need a fan that has four wires.

 

[D.A.(Tony)Stewart]

You do not need a precision tach PWM to control cooling. You just want a variable DC to control the speed of the fan according to some thermal sensor. . This may be achieved easily as I indicated. Below I have assumed the Requiv of a 24V fan in your photo and made a linear amp out of a power transistor. Choose a 2W heatsink to keep it cool or near the fan airflow and 1/4W resistors ought to be fine.
Each value is computed based on the ratios of the equivalent load R of the fan with 0 to 10V in for a 24V fan.

I used a sawtooth ramp to illustrate the transfer functions for power but did not simulate the stall starting voltage which ought to be 30 ~ 40%.

 

[Akanimo]

It is a school project so the fan may not necessarily be for cooling control. Since the OP specifically mentioned speed control without mention of temperature control or temperature sensor, I decided to provide guidance on speed control.

However, all of the suggestions are okay. The OP will just take what is most suitable for their project and leave the rest.

Let's not forget that the primary purpose of the 3-wire and 4-wire fans is still cooling control.

 

[eagle1109]

Option 2 can work with an amplifier using a transistor to fan V-

That's what I'm planning to do on my next step in this project.

--- Updated Jan 24, 2024 ---

The fan may be internally controlled at a fixed speed irrespective of the input DC voltage. So, first of all, you need to check whether the speed of the fan depends on the input DC voltage. You can take like 4 or 5 equally-spaced points along the DC voltage range to check whether the speed of the fan varies with input DC voltage or not.

Yep, I tested it today, there's of course a threshold voltage has to be delivered so the fan kicks off, which is around > 4.8 when I raise the voltage slowly. When I raise the voltage fast enough to > 3.5V this fast raise of voltage can kick off the fan to a relatively slow speed but not very slow I consider it a medium speed.

As you mentioned, if I would use 4-5 spaced points, then I won't have the speed at the 1st stage, I can't reach it with either the bench power supply or even with using the PWM module. It would run at stage 2 to the last stage with max 24V.

--- Updated Jan 24, 2024 ---

What is the current rating or Pd on fan and load current on DAC?

I actually don't know how much current the DAC pin can source. Also, it's not mentioning the power or current of the fan, only the voltage.

You can't simply use a FET or a BJT to drive the fan unless you use a very small control range.

Do you mean because I might damage the base/gate of the transistor with the full 10V range ?

If you mean that, then I'm taking into account the base/gate - current/voltage driving specs.

As I recall for example, I must use a proper resistor in series with a BJT base with the full base voltage to deliver the saturation current to full turn on the BJT transistor. Also use a resistor with the power line if it would exceed the max transistor collector current.

It is better to add resistors to control the gain the convert 0 to 10V to fan speed 0 to 100% using negative feedback.

Can you design this or do you need help?

I might start designing the required circuit on Proteus:

These are the parameters of the fan, I only can change the voltage, but not the current or the power ratings.

Fan = 3W? DAC can drive 1kohm ?

I don't know what is the power rating of the fan, these the specs from the Aliexpress page:

So if the current is 200mA @24V then if I'm right the power should be around 4.8W.

--- Updated Jan 24, 2024 ---

I did some experiments on the fan with only direct DC voltage.

1. This is the sticker

2. Here I was raising the voltage slowly to know which voltage kicks off the fan, and I reached 5V and the fan didn't move, but if I touch the fan it will move.

3. At this voltage, the fan started to move, without moving the fan with my hand.

4. After it moved, this is the lowest voltage I can get before it stops.

5. Max speed, it's saying on the power supply the current 140mA but I didn't measure it with the DMM.

 

[Akanimo]

That's what I'm planning to do on my next step in this project.

Good.

So you will take note of the starting voltage and stalling voltage and, maybe add soome headroom, and use that to provide minimum starting DAC voltage = (5.3V + Headroom)/amplifier_gain. And minimum stalling DAC voltage = (3.5V + Headroom)/amplifier_gain.

 

[Akanimo]

Hi all,

I have just noticed that the OP had mentioned that they were using a clone PLC in their project. I am not in support of their use of a clone PLC. All help that I have provided to the OP on this project were given without me noticing that the OP had mentioned that they were using a clone PLC. All such help were provided in error.

 

[eagle1109]

Hi all,

I have just noticed that the OP had mentioned that they were using a clone PLC in their project. I am not in support of their use of a clone PLC. All help that I have provided to the OP on this project were given without me noticing that the OP had mentioned that they were using a clone PLC. All such help were provided in error.

I really understand your obligation of using original devices and tools.

But the PLC is for training purposes and not for industrial/real life applications.

Even the Arduinos purchased for training are also clones, yes Arduino is an open source platform but still not original boards. Also the PIC programmers ICSD are also clones.

Anyway thank you for your help and support I really learned a lot in this thread.

 

[D.A.(Tony)Stewart]

My guess was correct or close enough for an unloaded fan at 180 ohms so use my design for best linear control using the DAC. You can adjust the 3.3k to suit your needs.

Dont worry about stall speed as the power dissipated will be low.

- otherwise, just using base resistor will effectively be an on or off control with a very small window if linearity.

 

[eagle1109]

You do not need a precision tach PWM to control cooling. You just want a variable DC to control the speed of the fan according to some thermal sensor. This may be achieved easily as I indicated. Below I have assumed the Requiv of a 24V fan in your photo and made a linear amp out of a power transistor. Choose a 2W heatsink to keep it cool or near the fan airflow and 1/4W resistors ought to be fine.
Each value is computed based on the ratios of the equivalent load R of the fan with 0 to 10V in for a 24V fan.
View attachment 188135

Thank you for the support.

Of course any NPN transistor that can handle the max current of the fan should be fine.

If the fan draws 140mA on 24V, then I guess any transistor can handle that current without a heatsink, am I right here ?

I used a sawtooth ramp to illustrate the transfer functions for power but did not simulate the stall starting voltage which ought to be 30 ~ 40%.

It's ok I can do the necessary bench tests, and learn the outcome.