TACH for DC Fan....
LUCA BRAZZI
Posts: 4
Instead of buying a 3 or 4 wire case fan, I would like to add a Tach output to a 2 wire fan I already have...
Anyone ever done this before? Any suggestions?
Anyone ever done this before? Any suggestions?
Comments
Its· a 12 volt PC case fan...
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Chris Savage
Parallax Tech Support
csavage@parallax.com
I think it was a little too early to make that call... I am planning to use a Basic Stamp to interface with this Fan/Tach
Here is something that may work for you....· The pulses are·short (10uS to 15uS), but should be enough to trigger a Flip-Flop into a Stamp
for a very nice square wave proportional to the fan speed.
The idea of operation is that instead of placing a resistor or·diode·in series with the fan and measure current spikes when the coils switch,
simply use the B-E junction of a transistor as a diode.· Remember the B-E junction of a transistor works as a diode.· Now, when the coils
within the brushless DC motor switch, the current will spike and turn the transistor "OFF" for 10uS to 15uS.· During this off time, the 4.7K
pull-up resistor will pull the output voltage level up and generate a pulse equal to the supply line.... in this case 12 Volts.
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Beau Schwabe
IC Layout Engineer
Parallax, Inc.
Im also planning on varying fan speed using PWM... how will that affect the output of this circuit?
I suspect that you can't simply do both and get a reliable speed measurement.
The PWM on its own might create a 'garbage in-garbage out' situation [noparse][[/noparse]Input pulses atop output pulses]
Much is dependent on what frequency and how long your PWM pulses are.
Why not try controlling the speed by reducing the voltage instead?
You may still use PWM, but you would have to smooth it with capacitors before it arrives at the fan.
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"When all think alike, no one is thinking very much.' - Walter Lippmann (1889-1974)
······································································ Warm regards,····· G. Herzog [noparse][[/noparse]·黃鶴 ]·in Taiwan
Since the current requirement is relatively low for the fan, what I would probably do here is use
an Op-Amp configured as a voltage follower with a gain of x2 to get at least 10V to the fan.
The Op-Amp input would receive PWM from the Stamp with proper resistor/capacitor filtering.
The Output of the Op-Amp would then drive the fan. (+ supply )
In the schematic that I provided above, the 4.7K resistor should go to a fixed +5V supply so
that it is more "Stamp friendly".
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Beau Schwabe
IC Layout Engineer
Parallax, Inc.
Post Edited (Beau Schwabe (Parallax)) : 12/14/2005 6:49:58 PM GMT