I want to vary the speed of a 12v dc fan using a mosfet with a DAQ 6008.Is this possible.

[shaggydog]

I am doing a  college project that involves using a DAQ to monitor and control temperature. I am using an LM35 temperature sensor.I need to control the speed of a12v dc fan.I am going to use an external 12 dc supply.I am hoping by to control the speed of the fan by varying the 5v voltage to the gate of the

mosfet.Can this be done.

[Adam.B · Accepted solution]

Hi shaggydog,

You can use one of the Analog Output channels of the USB 6008 to output a varying voltage to the MOSFET gate, this should provide the functionality you need.

[johnsold]

Make sure you choose a MOSFET which is turned on fully at 5 V. Many require somewhat higher voltages. Also consider that the portion of the transfer function of the FET over which the fan speed will very substantially may involve very small changes in gate voltage.

Lynn

[Henrik_Volkers]

Do you want to use the mosfet in PWM or linear mode?

How much current do you need for the fan?

The AO and DO have update rates of ~150Hz, however depending on the thermal mass of your system, a 1s PWM might be OK too 😉

With some more analog circuitry you can build a voltage controlled PWM (not shure but maybe a classical 555 can do that job, if it is a small fan, no mosfet needed) and use the AO to set the PWM ratio.

[shaggydog]

Hi

I am using labview to generate a vo;tage to control the speed of a 12dc fan with an N chanell mosfet (IRFZ44). I am getting a very unstable response.I am using a slider witha DAQ assistant to control the voltage to the gate. As advised I am using a 110k resistor at the gate to ground.  I have connected the +ve wire from the battery to the fan and the other wire from the fan to the drain. Then I have connected a wire from the source back to the -ve on the battery.I have used a second spare mosfet but still getting the same unstabe response.By unstable I mean the fan will run at full speed with or without a voltage at the gate.

[johnsold]

shaggydog,

1. How big is your fan? Using a 49 A FET to control a small muffin fan is a lot of overkill, and it makes the control almost impossible.

2. Do you have the wire from source and battery negative connected to the USB-6008 ground?  (My guess is that you do not have this connection).

This transfer characteristic curve is from the IRF datasheet for the IRFZ44N. Note that the behavior below 10 A of drain current is not shown. When you get down to tens or hundreds of mA (where a small fan probably operates), a few millivolts change in gate voltage probably turns the fan fully on or off.  Unless your fan is rated at 10 A or more, I would choose a smaller FET.

Lynn

[shaggydog]

Hi Lynn,

 Thanks for your reply.You were right I connected a resistor between the source and the DAQ gnd and it worked. But as you said the fan will go on/off and I have no control. My fan is rated 0.70 A. Can you suggest a suitable fet.

                                                                                                              Shaggydog.

[johnsold]

Shaggydog,

I do not have a part number in mind as I have not used anything in that voltage/current/power range for a long time.  Go to your favorite vendor's website and search for an FET with a current rating of 2-5 A, voltage rating of 50-200 V, power rating greater than 20 W, a low threshold voltage or "logig level" input, and a package type suitable for your application. That should get you a list of spossibly suitable devices.

Lynn