Question about switches and 12v leads.

[SaiZo]

My electronics is a bit rusty, but what I'm trying to do is to connect an SPDT switch (a three-way switch infact..) to act as an on/off for my fans.

The fans are not powered by the system anymore, but an external source, since I only have 1 PWM header and to that one another fan is already connected.

Ok, so to the question: Should I use the positive or the negative lead connected to the switch, I just can't remember how to connect them.


Thanks

[Geofrancis]

it shouldnt matter but i think the positive lead is the usual standard.

[Aquinus]

Your common should be your circuit, make L1 +12v, and ground L2. So when it's off, both ends of the circuit are grounded leaving zero potential and little resistance. Then when you turn it on, it will switch from ground to on. I just recommend using a small cap and inductor to smooth the output voltage and current. When you first turn a SPDT switch on or off, it "bounces" due to the mechanical nature of the switch. I'm sure it won't hurt the fan to just run it right off, but it doesn't hurt since there a short period of transience induced by the switch when the state changes.

[Frick]

Fans are usually sturdy, jumps in the switch probably won't hurt them. Still a good idea with the cap/inductor (which incidentally is a filter, I don't remember exactly what that kind of filter does though :P).

[Aquinus]

Originally Posted by Frick

Still a good idea with the cap/inductor (which incidentally is a filter, I don't remember exactly what that kind of filter does though :P).

The cap smooths fluctuation in voltage and the inductor smooths the current. Just be careful where you put that cap because you may introduce resonance between the cap and the inductor. Your best option would probably be to ignore the inductor just go with a small 20-30v cap. I suspect current smoothing won't be necessary. Nothing too big for the cap though, keep in mind that cap has to charge when it's switched on. All in all, you're driving fans, not circuitry so it probably isn't necessary.

[SaiZo]

Thanks for the replies. I forgot to mention it is an ON-OFF-ON switch, and what I was trying to do, is that in one position fan #1 is on, and in the other position fan #1 and fan #2 is on.
Sad part is that they are single pole. Good part is that I have 4 identical.

When the switch is in the mid position, well then both fans are off. I need to a small experiment PCB for this to work (I guess).

I think I could manage this on my own, otherwise I'll come back here. I tried to google my "thinking" but it was hard to find a correct answer.

Was speaking earlier with the people at our local electrical shop, they sell that Arduino Uno, and said that it could be programmed to control fans if you want to (yes, I'll have to program it myself..)

Is there anything else I should know of before starting to conduct my experiment?


EDIT: Ok, so let's just say I have soldered two PWM headers to the PCB, I have the input from the external source being 12v. Now, when I throw the switch in one direction, only one fan will get power, and in the other direction both fans get power. And in the middle position it is OFF. Would this work in reality, or will I destroy anything?? Should I let Electronics Workbench print the schematic for soldering (so I'll get everything in the right direction)?

Also, I do have some caps around here, but only one is rated 35v & 10 microfarad. Other than that I have some 50v, 16v caps. I think I need some new ones.


Thanks everyone for the replies.


EDIT: Forgot to mention, I actually have two 12v AC adaptors and one 7v. However, one of the 12v adapters actually gives as much as 14v. I think it was for charging something. I also have a small 12v to 9v transformer, that can be surface mounted. I do not know it's condition right now (have to test it first). Going tomorrow to buy usefull hardware. Just keep the idéas coming!

[1freedude]

would a diode work for this?

[Aquinus]

Originally Posted by 1freedude

would a diode work for this?

Won't the diode drop the voltage by a couple volts or so because of the forward bias in the diode? I think he would like to hold on to 12v if at all possible.

[1freedude]

dont know, it was a question on my part, too.

[Aquinus]

I think something like this would be ideal for what you're trying to do. Just keep in mind that your SPDT switch has a third option for off or ground, but I would do something like this. A tiny bit of logic could eliminate the second power transistor, but this is what I was thinking.

[SaiZo]

I want to thank everyone for their efforts in helping me with this.
Unfortunately, I have the flu, so I couldn't go and buy the stuff needed (fan headers, PCB etc.).

Will start constructing it as soon as I get better, and yes, ofcourse with pictures!