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Author Topic: Use of MX+B Math Function - Fluke 8846A - DC fan speed RPM measurement  (Read 4585 times)

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Online Kiriakos GR

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For some time I was looking for a way to measure DC fan speed RPM, as this is read at every computer motherboard.

Occasionally I am in need to verify used DC fan over all condition, so this to become usable (reusable) at a new application.
Many DC fan with three wires and or four wires, them using Hydro-bearing, they have significantly longer life-cycle than other DC fan.

DC fan motor rotation speed this is measured in RPM = Rotation Pen Minute

An DC fan with third wire, this is output of square wave pulse, in the unit of time, an Oscilloscope will measure signal width and this is not useful for as with the expectation to measure the number of pulses occurred in the unit of time.
   
Bench top multimeter of advanced product design they come along with Math functions, but the most important at our application this is MX+B .
By the activation of Frequency measuring mode at Fluke 8846A (DMM4050) this will measure the frequency of square wave pulse as it does the oscilloscope too. 

Our goal this is to measure amount of square wave pulse in the unit of time.
Time = 60 seconds at every single minute = RPM     

Oscilloscope and quality multimeter will do that HZ measurement by calculating plain signal repeat ability and will measure single square wave pulse as this to be a square block with two sides.
At the application of RPM measurement circuit, this is after frequency measurement too, but of total amount of pulses per minute.

And therefore our much more expensive measuring gear this now requiring a math compensation function, so this to ignore double triggers counting and the measurement to become equal to what an RPM meter dedicated circuit will deliver.

At the Linear Equation  MX+B We need to alter MX+B in half value than 60 seconds per minute.
New  MX+B value should now become 30 seconds per minute.
Compensation can be saved at one of five available Store to memory positions at Fluke 8846A, and this be activated when ever you need it. 8)   

Linear Equation MX+B this is about X and Y lines, by the use of it at our multimeter, we convert actual measured frequency in to occurrence of lines in the unit of time.
 
 
While both Fluke 8846A (DMM4050) they offer tremendously rich display resolution at frequency measurement, what we should now compensate by the use of our eyes, this is amount of digits of interest.  :)

Application example:    DC Fan product code XXX1225 this has known power related specifications and highest RPM this is 3600 (this be defined at 12V or 13.8V DC) according the datasheet.
Therefore at Fluke 8846A display (screen) only first four digits they should be taken in to account.  ;)

PC DC fan with four wires they include Pulse modulation input, and an external benchtop functions generator this can used as source of such a signal.
The combination of Fluke 8846A along with a functions generator, this is the ultimate test bed for inspection of four wires (Four Pin) DC fan at it entire RPM range.   8)

I am currently working at making my own RPM control and RPM measurement project box -  this be dedicated to DC fan inspection, this including all popular DC fan PCB connectors.
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Online Kiriakos GR

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Re: Use of MX+B Math Function - Fluke 8846A - DC fan speed RPM measurement
« Reply #1 on: October 03, 2022, 12:51:31 PM »
At about this November I am thinking to accomplish this project.

Name: DC Fan RPM controller and speedometer (all in one).

DC fan headers support:

1) INTEL four wire fan
2) VGA card mini connector, four wire fan
3) INTEL three wire fan
4) PC PSU mini plug, DC fan two wire (No RPM data).

Connectivity input/output:
a) Output to DMM (for RPM measuring)
b) BNC input for speed-control signal (External Signal generator).

Plus a dedicated power supply of 12V for the DC fan.
And all these to be combined in to a small size project box. 

At the moment I did collect and or own, most of necessary parts. :)


 
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Online Kiriakos GR

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This is hart of Summer time in Greece, and found the opportunity to activate any low in priority projects.  :)

The most hard to find component, this was a dead mini Hi-Fi sound system from LG. 
The photographs below speak by them self's.

Other than a bunch of DC fan electrical connectivity options, there is another huge pack of DC fan mounting options.
140mm, 120mm, 80mm, 60mm ... Vertical , Horizontal, testing of High-speed 3000 rpm or higher also possible.  ;)

FLUKE 8846A : Freq/Period Accuracy 0.01 %
Ranges 3 Hz to 1 MHz
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Online Kiriakos GR

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Photographs continued ..
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Online Kiriakos GR

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There is no need to say of how delighted I do feel today.

I did compose here, a true scientific DC fan testing ring, for basic two wires DC fan motor along Pulse Width Modulation (PWM) DC fan motor. 
Wide PWM Frequency Range Input 30Hz- 30kHz
With extreme measuring accuracy at Rotation Per minute RPM

In summary:
With this setup or test ring, I am now able to test and or evaluate, new or used DC fan for specifications compliance.
Even a used DC fan with lost label this can now be charted and have a second life in to another application.

In the occasion that cleaning and lubrication of used DC fan this is possible, DC motor this can be safely tested after repair, and now test if this DC fan motor has a good chance so to be reused.
 
My last thought, think Green it’s not just about energy saving’s, I have a pair of DC fan this working trouble free for eleven years at my personal computer.
At the recent years several computer accessory brands, elevated DC fan unit pricing by asking outrageous amount of money.
My new goal this is rescue and reuse of products which were falsely characterized as heavily damaged or bad.   ;)


   
Recently I did a review of Seasonic MagFlow 120mm  three pack, and I did discover something unusual, at daisy chain configuration PWM signal strengths shown as not enough, so this to drive all three fans due a single signal source.  :(
https://www.ittsb.eu/forum/index.php?topic=1775.0
 
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Offline Andrew-88

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The most hard to find component, this was a dead mini Hi-Fi sound system from LG. 

Its good to see Kiriakos been stroked by inspiration.  8)
He worth the position of manager, at quality control testing and inspection lab of a BIG brand. 

Online Kiriakos GR

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The most hard to find component, this was a dead mini Hi-Fi sound system from LG. 

Its good to see Kiriakos been stroked by inspiration.  8)
He worth the position of manager, at quality control testing and inspection lab of a BIG brand.

For the right price, I might do it.  ;D
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