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need electrical help with fanbus...

I

illig

2[H]4U
Joined
Nov 13, 2000
Messages
3,046
fan%20switches.jpg


OK... here's what i built for a friend... it's a regular 7/12v switched fanbus, and the design was blatantly stolen from http://www.fanbus.com and i only added an extra fan on each switch...

now... all 4 fans work perfectly at 12V (measured with multimeter) but when either switch is lowered to the 7V position, they only get 5V!!! and these fans (Panaflo L1A) don't spin too well at 5V....

can someone point out why they're only getting 5V? I'm not good with electrical principles, but I've successfully built several baybusses/fanbusses before with no such problems...

this was tested on an ATX PSU that was started with the wire trick, if that matters

...any help appreciated!
 
well ... what you've shown in the diagram will provide 12/7 volt switching ... but you have to make sure you get the correct type of switch.

Personally .. I recommend a switch like this:

http://doc.union.edu/18/Labs/lab1_fig2.gif

Make sure it is only a two positition switch. If it's 3 position then it will short Y and X in the diagram ... not good.

So ... provided you have the correct switch and that you've hooked it up correctly, then it should be fine.

How exactly do you know its only getting 5 volts? How did you measure it? What I mean is ... did you measure the voltages right across the fan wires, or did you measure the voltage from one of the wires to ground?

To produce 7 volts, the "12V" wire has to be at 12V and the ground wire has to be at 5 volts ... so if you simply measured from the ground wire to ground you'd get 5 volts.
 
the switches are regular SPDT 3 position ON-OFF-ON switches... and it's not shorting (i checked for continuity before powering up)

and i measured the voltage across the fan wires with a multimeter... the voltage showed ~12V for one switch position, and ~5V for the other
 
Yup, measure the drop across the fan - you should see your 7v.
 
Originally posted by PS-RagE
Yup, measure the drop across the fan - you should see your 7v.
Click to expand...

yeah... i measured across the fan.. and got a bit less than 5V :(
 
Well if the 12V line is connected to the fan and is NOT switched then there is no way the fans could ever get 5V across them unless you fed 7V into it. Make sure the 12V line is always connected and not switched, and that the 5V and ground are switched and run into the fan ground.
 
are you sure you wired the +12 directly to the + line on the fan?

if you switched the ground and +12 it would become a 5/off/12 fanbus...

make sure your switches have +5 and GND going to them
 
thanks for the replies guys, but i found that my problem was the PSU

i completely forgot that ATX PSU's get weird when not connected to a computer (i.e. they don't put out correct voltages)

turns out the fans worked perfectly at 7V once installed in the case and the PSU was connected to the mobo...


the stupid thing is, i ran into the same problem when building my baybus 2+ years ago... i just didn't remember it :D
 
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