Hello All,

I have a fan - DC brushless CFM-6025BF-235-274

I am driving it using just an NMOS, but I need to have a circuitry that verifies that the fan is actually spinning and set a High signal to the AND gate, which will turn other part of the circuitry.
The purpose of this check is to make sure that the remaining circuitry is unpowered until fan is spinning due to the thermal issues

What circuitry I can you to verify that?
Here is the current waveform that I see for the Fan

Best regards,
Timur

 

How big/powerful is the fan?

 

I used to work on some power supplies which had a vane in the airflow of the fan. The air moved the vane and operated a microswitch. If there was no airflow the switch shut down the supply.

Another possibility is to use an LED and photodiode/photocell either side of the fan blades so each blade interrupts the light. You could also measure the fan speed this way.

 

If you would have purchased the fan with the "-20" at the end of the model number, it would have a tachometer wire for you...
https://www.digikey.com/en/products...CBcDaIMIDECyBaAbABjAVgEIJTAGZtCB2AFkIxAF0BfIA

 

And don't do it with a "digital" tester like I did!

Use an LED and photo detector.
You can also use an air flow sensor.

 

I used to work on some power supplies which had a vane in the airflow of the fan. The air moved the vane and operated a microswitch. If there was no airflow the switch shut down the supply.

Another possibility is to use an LED and photodiode/photocell either side of the fan blades so each blade interrupts the light. You could also measure the fan speed this way.

Hi Albert,

Fan is 24V and 50mA, quite small one
This fan is blowing on the PCB, therefore I need to use electronics to verify that the fan is spinning.
I can't use external devices for that

Best regards,
Timur

 

If you would have purchased the fan with the "-20" at the end of the model number, it would have a tachometer wire for you...
https://www.digikey.com/en/products...CBcDaIMIDECyBaAbABjAVgEIJTAGZtCB2AFkIxAF0BfIA

I know that there are fans with Tacho output and PWM input signal, but I specified the following, which has only 2 power cables
Any thoughts on how to measure that one?
Obviously I can use a driver, but I am would like to avoid it, due to the issue with the components shortages

Best regards,
Timur

 

I know that there are fans with Tacho output and PWM input signal, but I specified the following, which has only 2 power cables
Any thoughts on how to measure that one?
Obviously I can use a driver, but I am would like to avoid it, due to the issue with the components shortages

Best regards,
Timur

All the good options have been suggested. Any response of why/why not those other suggestions could work?

 

All the good options have been suggested. Any response of why/why not those other suggestions could work?

Because this is a fan that is blowing to the PCB and I want to eliminate usage of any other external components, somehow verify that with the electronics on the board level

 

Why not use a Temperature-Sensor to control the Fan ?
This will indicate what actually matters.

You can have the Fan running at maximum-Speed and
at the same time,
still have an overheated-Board which needs to be shut-down,
so in this type of situation,
detecting whether or not the Fan is spinning is useless information, and a moot-point.
.
.
.

 

You could detect the fan current and rectify the signal to detect the current variation when it is running.

Fan is 24V and 50mA, quite small one

That doesn't jib with the current graph in post #1, which shows a average current of about 0.4 A.
So which is correct?

 

I would wait for the cpu to approach steady state temp when heavily loaded with some stressTestCode (StressBerry, for example) and then turn the fan off for 10 seconds to see if the CPU temp rises without changing the load on the cpu. Then turn the fan back on to see if the CPU goes back down. If the fan wasn't spinning, there is no worries about turning the fan off. If it is spinning, you can keep pinning the cpu temp to make sure you're not going to get out of control in 10 seconds.

 

Here is the current waveform that I see for the Fan

Since you specified a two wire fan your options are limited. Looking at your current plot above you may be able to amplify that signal and feed it into a basic missing pulse detector circuit. That or measure the current and measuring current won't tell you if the fan is actually turning. Next as mentioned measure temperature rise.

Ron

 

Why not use a Temperature-Sensor to control the Fan ?
This will indicate what actually matters.

You can have the Fan running at maximum-Speed and
at the same time,
still have an overheated-Board which needs to be shut-down,
so in this type of situation,
detecting whether or not the Fan is spinning is useless information, and a moot-point.
.
.
.

That is a good point, I thought about that already as a solution

 

You could detect the fan current and rectify the signal to detect the current variation when it is running.
That doesn't jib with the current graph in post #1, which shows a average current of about 0.4 A.
So which is correct?

Waveform in the Figure is just an example of the current waveform I see.
How do I rectify the signal to get the average current?

 

I would wait for the cpu to approach steady state temp when heavily loaded with some stressTestCode (StressBerry, for example) and then turn the fan off for 10 seconds to see if the CPU temp rises without changing the load on the cpu. Then turn the fan back on to see if the CPU goes back down. If the fan wasn't spinning, there is no worries about turning the fan off. If it is spinning, you can keep pinning the cpu temp to make sure you're not going to get out of control in 10 seconds.

This fan is used for the LED driver, not CPU temperature control
It is to cool down power electronics devices

 

The only way you are going to know if the fan is spinning is to check for a condition for which spinning is only explanation like the tach, photointerrupter, or airflow detector precisely mentioned.

Any other mention will suggest the fan is moving but not guarantee it. There are three failure modes to account for: dead (doesn’t spin because it’s got a problem that isn’t mechanical), stalled (like a bad nearing or something in the way of the blades—such as part of @MrChips—preventing motion), and weak (moving but not enough to cool).

You could probably tell from the current if the fan is stalled because it should be higher than when it is operational. For the other two, unless the fault results in reduced current draw (no guarantee of that), you won’t know.

Despite wanting to keep parts count down, if this is a vital function, you really should consider adding a sensor that check for actual rotation. One possibility that comes to mind is a variation on the photointerrupter previously mentioned. That would be a retroreflective sensor mounted in front of the fan and a strategic white spot on at the rim of the hub to create a tachometer.

Alternatively, a simple thermal sensor that is in the airflow could be checked at startup before the fan is spun up and you could look for a drop in temperature due to convective cooling. The problem with that would be if you also want to “watch” the fan and shutdown if it fails. I don’t think it would have a good response time to a failure but that’s just surmise.

 

Any thoughts on how to measure that one?

Use a pulse detector to monitor the fan current. If it's pulsing then the fan must be rotating (assuming it's internally commutated).

 

The only way you are going to know if the fan is spinning is to check for a condition for which spinning is only explanation like the tach, photointerrupter, or airflow detector precisely mentioned.

Any other mention will suggest the fan is moving but not guarantee it. There are three failure modes to account for: dead (doesn’t spin because it’s got a problem that isn’t mechanical), stalled (like a bad nearing or something in the way of the blades—such as part of @MrChips—preventing motion), and weak (moving but not enough to cool).

You could probably tell from the current if the fan is stalled because it should be higher than when it is operational. For the other two, unless the fault results in reduced current draw (no guarantee of that), you won’t know.

Despite wanting to keep parts count down, if this is a vital function, you really should consider adding a sensor that check for actual rotation. One possibility that comes to mind is a variation on the photointerrupter previously mentioned. That would be a retroreflective sensor mounted in front of the fan and a strategic white spot on at the rim of the hub to create a tachometer.

Alternatively, a simple thermal sensor that is in the airflow could be checked at startup before the fan is spun up and you could look for a drop in temperature due to convective cooling. The problem with that would be if you also want to “watch” the fan and shutdown if it fails. I don’t think it would have a good response time to a failure but that’s just surmise.

I see.
External components is not an option for me unfortunately
I do not have any brain on the boards, so it must be a hardware shutdown if the fan is not spinning, I believe that using thermal sensor on the board next to the hottest components will do the job for this purpose

 

Use a pulse detector to monitor the fan current. If it's pulsing then the fan must be rotating (assuming it's internally commutated).

Do you have a circuitry or part number I can check for that purpose?