I am wondering if anyone else has seen this problem. I have an offshore-made 7W FM transmitter that has a tiny 35mm fan in it.
Lately I find that the fan runs erratically. I took the unit apart and tested the fan using ONLY the Red and Black wires. I found it runs perfectly on the 12V battery but when the RF is switched on it starts running at varying speeds or stops all together. It is a 3 wire fan with built-in commutation circuitry (I can see the board inside the hub housing). The wires are red(+12V), black(gnd) and white. Does anyone know what the white does? I am wondering if installing a ferrite ring around the wires would help this.

 

Does anyone know what the white does?

My guess and only a guess is the white wire could be a tachometer output which on most basic 12 volt fans outputs 2 pulses per revolution which can be used to derive fan speed or tell a system if the fan is running or not. I have seen the third wire as yellow and white and they are generally an open collector output..

Another possible is a fan speed control. When this is done a sensor in a system detects a rising temperature and increases fan speed accordingly. I would be reluctant to apply that in this case since you removed the fan and just applying 12 volts to red and black it runs fine.

If it can be done easily I guess you could monitor the fan voltage on the red and black wires while it is in circuit and acting up. Again, this is pure speculation on my part and hopefully another member has some thoughts.

Ron

 

Thanks, Ron. It acts up when either plugged into the circuit board or with the red/black wires connected
directly to the 12V board input so I don't think that the white wire is some kind of control input. I connected
the white wire to +12 briefly and it made no difference. I read elsewhere that it is usually a tach ouput as you suggested.

 

Thanks, Ron. It acts up when either plugged into the circuit board or with the red/black wires connected
directly to the 12V board input so I don't think that the white wire is some kind of control input. I connected
the white wire to +12 briefly and it made no difference. I read elsewhere that it is usually a tach ouput as you suggested.

Then we get to the part of remove and replace with known good fan. Sometimes we get lucky and the fan has a manufacturer's part number on it. Sunon is a popular name used by many manufacturers but there are dozens. Short of having the fan running in circuit and probing the white wire it's hard to say what it might actually be for sure. A 35mm X 35mm is a pretty popular size and if you look for a replacement you also want to make sure you get the right diameter (thickness) as they can vary.

Ron

 

I think I will go out tomorrow and buy a couple of ferrite cylinders and try slipping those over the fan wire before
replacing the fan.

 

Can't hurt but didn't it previously run fine less ferrite cores? Won't hurt to try them though.

Ron