how do you measure fan speed  - Page 1
Forum section: electronics / projects

author: odell
author_rank: Contributor
timestamp: January 08, 2012, 08:59:28 am
content: how do you measure fan speed dose is need it be pwn

author: joelby
author_rank: Frequent Contributor
timestamp: January 08, 2012, 09:09:42 am
content: Use a fan with a speed output wire that pulses with each revolution.

author: odell
author_rank: Contributor
timestamp: January 08, 2012, 09:14:32 am
content: what would I need to get the fan speed. Do I need a chip on the board or what

author: Rerouter
author_rank: Super Contributor
timestamp: January 08, 2012, 10:44:04 am
content: well if this is for a computer, then all you need is software, as it records it, e.g. occt,however if this is for a project, then it depends on what you plan to do with the speed info,for instance you could feed it into a frequency to voltage chip, then use an op amp to drive the power transistor for the fan, to maintain a rather perfect speed by playing with the gain, if you want it that way,

author: Rufus
author_rank: Super Contributor
timestamp: January 08, 2012, 12:34:18 pm
content: Quote from: odell on January 08, 2012, 08:59:28 amStick your fingers in the blades. How much it hurts will give an indication of fan speed.

author: vtl
author_rank: Regular Contributor
timestamp: January 08, 2012, 12:42:04 pm
content: A novel way of measuring fan speed is to use an an adjustable strobe light. You flash the object at a range of speeds until the fan appears still to the human vision. Your strobe flash rate would then indicate what the RPM is.

author: mobbarley
author_rank: Regular Contributor
timestamp: January 08, 2012, 01:04:38 pm
content: Quote from: vtl on January 08, 2012, 12:42:04 pmNot a good idea - there will be many speeds where the fan appears still.Use the sense wire and figure out how many pulses per revolution / Paint a spot white and use an optical tachometer / whack a small magnet on it and use a reed switch and frequency counter / do some power analysis with an oscilloscope and a shunt resistor and figure out how many current spikes per revolution

author: saturation
author_rank: Super Contributor
timestamp: January 08, 2012, 01:21:33 pm
content: The best way today is an optical tachometer, For just 'ball park' values eBay variety suffices.http://www.ebay.com/itm/Digital-Laser-Photo-Tachometer-Non-Contact-RPM-Tester-/300645164201?pt=BI_Electrical_Equipment_Tools&hash=item45ffd924a9

author: amspire
author_rank: Super Contributor
timestamp: January 08, 2012, 02:31:08 pm
content: If you just want to measure the speed, you use what is at hand. If you lightly touch a small piece of paper to the fan blades so it makes a sound and record the sound on a PC, you can look at it in an audio editor like Audacity, and measure the exact speed. As accurate as any tachometer.You would only buy a tachometer if you needed it.If you want the circuit to measure fan speed, you need a fan with the speed sensor wire, and something like a micro has to measure the pulse frequency.Richard.

author: odell
author_rank: Contributor
timestamp: January 08, 2012, 05:59:09 pm
content: It is going to be on a circuit board. I like to have the fan speed to come up on a lcd

author: IanB
author_rank: Super Contributor
timestamp: January 08, 2012, 08:01:27 pm
content: Quote from: odell on January 08, 2012, 05:59:09 pmThen you need a fan with a speed sense wire.Otherwise you would probably have to construct an optical sensor with a light path interrupted by the fan blades. You could deduce the speed from that.

author: sonicj
author_rank: Frequent Contributor
timestamp: January 10, 2012, 03:20:19 am
content: connect a microphone to your spectrum analyzer, look for the frequency peak, multiply that by 60, divide by the number of blades on the fan. problem solved-sj

author: Zad
author_rank: Super Contributor
timestamp: January 10, 2012, 05:20:36 am
content: If you are driving it with PWM, you measure the motor voltage when the drive transistor is off. The voltage it produces is proportional to the speed of the motor.

author: hacklordsniper
author_rank: Frequent Contributor
timestamp: January 10, 2012, 07:58:21 am
content: Most of the fans today have the third sense wire, pull that up with few kohms resistor and measure the frequency with scope. Most fans output 2 pulses per rotation, measure the frequency, divide by 2 and multiply by 60 to get the actual rpm

author: amyk
author_rank: Super Contributor
timestamp: January 10, 2012, 11:21:23 am
content: Even if it has no integrated sense function you can measure the current draw and determine the time between successive peaks.I searched a little and it seems someone has actually patented this somewhat obvious method:http://www.freepatentsonline.com/7728581.html

author: metalphreak
author_rank: Frequent Contributor
timestamp: January 10, 2012, 01:59:55 pm
content: Quote from: amyk on January 10, 2012, 11:21:23 amHave you seen those "V8 Engine Noise" things you plug into the cigarette lighter socket in your car. It makes a V8 revving sound in sync with your actual engine speed, by detecting the pulses on the power supply due to the rotating speed of the alternatorI love the genius solutions people come up with for novelty items.