I'm trying to set up a relayed cooling mechanism for a mass spectrometer (i.e. cooling fans turn on when the MS exceeds a certain temperature). I have a 24V DC/25A power supply which converts 100-120V AC/12 A to 24V at multiple output leads, and that will be connected to several different instruments.
I just purchased 2 cooling fans – 12V DC/85mA – and would like to connect these in series to the power supply. However, when I connected them, they simply shuddered and did not turn on. Not to displace blame (I accept full accountability for the fans not working/not fully checking the specs), but it appears as though the company from whom I bought the fans did not specify that the nominal voltage range was about 10-14V.
The reason I bought the 12V fans specifically was because I thought the voltage would drop over each device – but perhaps not? I also want to make sure that I leave enough power for other small devices to operate off of the same supply.
So, what I would like to know if there is anything I can do? Should I simply return the fans for a 24V alternative? Or to save the cost of shipping, is there anything to make these fans work? I've read about using a diode to drop the voltage, but would that be the best option? It also appears to me as though a resistor would not do the trick.
Thanks very much in advance.
R
Best Answer
I suspect they will start working if you put a capacitor from the center point to ground. Maybe about 330 or 470uF.
Each of the fans has an electronic circuit which pulses the coil in the motor to start a rotating field. Without a good ground for the upper fan (or without a good Vcc for the other one) this logic circuit won't work well.
It's not an ideal situation, as on Power-On, the top motor will then receive 24V for a fraction of a second.
EDIT: @Asmyldof posted a good comment: Use two capacitors in series over the 24V supply and the center to the center of the fans. That'll keep the center..., well, centered.