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Author Topic: 3 or 4 wire fan: How to?  (Read 599 times)
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Hello,

for a project I want to apply two 120 mm 12V fans (not for computer cooling). The ones I got have only two wires and work always on full speed (about 90 cfm). The full speed is not enough for me, so I looked for fans with about 150 cfm (at full speed). All I found have three or four wires (eg. this one http://www.frozencpu.com/products/12944/fan-858/Sunon_120mm_x_25mm_Precision_Ball_Bearing_Fan_-1500_CFM_PMD1212PTB1-A.html?tl=g36c435s1106). Since I have already done the wiring and don't want to change, I wonder if I could use such fans (my Arduino only controls, wether the fans will run or not, i.e. wether they get power or not). At the end of my wiring solution I added a box for the remaining wires. If necessary I could add some electronics inside to make the fans running at full speed, but if possible I want to avoid to send control signals through the third or third and fourth wire.

Can anyone help me solving my problem?
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Yes, those 3 & 4 wire computer fans will usually run at full speed just fine by only connecting the +V & GND wires. Mine do, anyway.

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There is no spec sheet on the fan you provided the link to. I suspect the fan runs on 12V so there is a 12V pin, and gnd pin. Then there is an optical encoder that needs 5V input and output pulses to tell you how fast it is running at.
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If it is a standard computer-type fan, then the red wire is power (+12V) and black is ground. On a 3-wire fan, the third (usually yellow) wire is an open-collector output from the tacho. Leave it disconnected if you are not interested in measuring the fan speed. On a 4-wire fan, the 4th wire is the PWM input, used to control the fan speed. If you leave it disconnected, the fan runs at full speed.
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Sorry my guess was wrong. Good to know. Thanks dc42
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Thank you all for your information. This helps a lot.
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