Modifying Fan Tach Signal

[Ifixtheinternet]

I want to replace an inferior fan in my prized television with a top of the line Noctua fan.
The issue is the OEM fan outputs a constant DC voltage at speed, whereas the Noctuas are standard PC fans and output an RPM TACH Signal with a hall-effect sensor.
I could easily defeat the monitoring system by connecting the sensor to one of the other fans, but I want to retain the Televisions ability to alert me if the fan should fail (however unlikely).

I found a circuit for creating a fan alarm on this page:   http://pcbheaven.com/circuitpages/PC_Fan_Failure_Alarm

It seems I only need a portion of this circuit, (up to C2) to convert the rpm signal from the fan into a constant DC voltage. (Please correct me if I'm wrong here)

I measured VDC coming from the original fan at full speed.
With the positive lead of my multi meter connected to the Fan signal output wire, and the negative lead connected to my PC fan Header, I am getting -10.88V.
Of course if I reverse the polarity I get +10.88V.
The voltage remains -10.88V at any RPM, then when I stop the fan, I am getting 0v.

This means the TV will only alert me if the fan stops. If possible I would like it to trigger if the fan is not spinning fast enough.
Depending on what the TV expects, this may be achieved already; If the rpm starts dropping the voltage will start dropping, and trigger the fan error message.
But if the TV only reacts when there is 0V, then I will need to set the circuit so that 0V is produced at a fixed RPM, let's say 1500.

So I want to put the circuit together so I am getting about -10.88V from the circuit when the new fan is at full speed, and 0v when it is at 3/4 speed.
The new fan runs at a full speed of 2000 RPM.

Now for the questions. (feel free to only answer what you know)

How do I adjust this circuit to output -10.88V at full RPM?
If the TV only responds when there is 0V, then how do I set the circuit to produce 0V at, let's say, 1500RPM?
Do I need to reverse the polarity of this circuit to get a negative voltage sent from the fan, or will the design achieve this as is?

This will be the most in depth electronics DIY project for me so far, so I appreciate any help for a novice.

Thanks!

[kam]

The idea of this circuit is to use R4/C2 as a low pass filter. C1 removes any DC voltage, the transistor then amplifies the pulses along with R1/R3. So the transistor acts as a buffer input. So as you said, up to C2 is everything you need to convert the PWM pulses from the fan to a DC voltage.
I'd suggest though to keep the circuit up to R8. You need to have an output buffer as well, this is where the comparator comes into play.
You can set the "alarm rpm" threshold by trimming R2. This sets the threshold voltage at the + input of the comparator. If the voltage at the - input (fro the low pass filter) does not change significantly when the rpm changes, then you should test other R4/C2 values. I'd start with a lower R4, say 22K for testing.

If you want to have negative voltage at the output, then you should have a negative power supply as well. So instead of using the positive and ground, use the ground and negative supply terminals.

[Ifixtheinternet]

Thanks for the reply, and much respect for your work.

So you're right. it seems easier to just built the whole circuit minus the buzzer, and then just adjust what voltage should be output from R8.
Would I just use a different (or add) resistors in front of R8 to achieve the 10.88 volts I need?

Also, this circuit is designed to supply power to the buzzer when the alarm is triggered, but I need it to do the opposite.
I need the circuit to output a constant voltage always, and cut power to the third pin when it is triggered by the fan not spinning (or not spinning fast enough)
What part do I need to change to reverse the operation?

I still need to verify the negative voltage.
I took these readings at my PC, but I assumed the pinout for the TV is the same, and maybe it's not.
Maybe the TV allows voltage to flow to the fan, and the PC allows voltage to flow from the fan to the fan header.
In that case I would need a positive voltage, I'll need to test this later.

Thanks for your help!

[Ifixtheinternet]

So I read the article again, and I was intrigued by the part about the 555 chip.
It mentioned the 555 timer wont work because of the fans behavior when it stops, may not trigger the alarm.

But my desire is not to trigger when the fan stops, but to trigger when the RPM falls below a threshold.
For my application it would be:

RPM > 1500 = ON
RPM < 1500 = OFF

And what I am turning on and off is a supply voltage to my TV of 10.88 VDC.

Would a 555 timer work for this or would there still be an issue?

[kam]

Ok, first regarding the output. The Pnp transistor at the output is to reverse the comparatior's output. So just replace the transistor with an npn (and reverse polarities).

Which 555 are you talking about???

[Ifixtheinternet]

OK, I will get a couple extra NPN transistors, thanks.
I assume the BC547 already on the list will do?

I was talking about the Texas Instruments NE555P timer chip.
It was suggested to me that I could use one for this application.
At first I disregarded that option because the author of the article I found here mentioned that a 555 timer chip would not work due to the way a brushless fan sends voltage when it is stopped.

I dont quite understand how that would matter if the circuit is designed to switch a signal based on if detected RPM is less than X.

My circuit will not be triggered by the fan stopping, but whenever the fan falls below an RPM threshold. The reason I bring this up is I want to build the circuit as simply and as compact as possible, as I have to try to fit it on a small board to fit inside my TV.

I just wanted to know if it was possible to use a timer chip since my needs are different, and if that might be a better solution for what I am trying to do, if it would require less parts or a smaller footprint.

[kam]

Oh now I get it. (i am the writer of the article  )
Yes the 555 will not work. That is because the 555 waits for a HIGH to reset. So if the fan stops with the output at HIGH (that is 50% chance since the tacho output is 50% duty cycle) the 555 will always reset and will never set a fail signal output. I originally began designing this circuit with the 555 but i faced this problem and therefore decided to make this more complex but 100% reliable circuit.