Simple fan control circuit

Thread Starter

Gazer219

Joined Dec 17, 2018
5
I would like to build a simple control box to control two 12V DC fans, typical computer case type fans. I currently just connect them to a 12V Li-ion battery and use them to cool optics in a telescope. I wanted something that would allow me to control the speed and reverse the direction of the fans from a central control box, with a bonus ability to have a readout of the power applied to the fans so if I find a good speed I'd be able to approximate that setting next time around. I've been able to find most of what I need and I think I should be able to put it together in a smallish plastic enclosure that I would drill or cut holes for the connectors, knobs and displays.

My original thought was to use something like this: uniquegoods 6V 12V 24V 28V 3A 80W DC Motor Speed Controller (PWM) Speed Adjustable Reversible Switch 1203BB dc motor driver reversing to control the speed of the fans and provide the ability to reverse the direction, and then add something like this:
HOT Red LED Waterproof Monitor 12 Volt Battery Meter 2.5-30V DC Auto Gauge Digital Voltmeter in the circuit after the controller to show me the voltage applied to the fans. I want control for each fan independently so I'd have one path for each fan with an output connector on the box.

NOTE: I tried to put URL's for the about italicized parts above but get an error message when I do that so I had to make them plan text.

This seemed pretty straightforward but I starting thinking and I'm not even sure that the controller I've linked will really do what I want it to do and the meter I linked won't show negative voltage so if I reverse the fans I'm pretty sure that it won't show anything or possibly damage the meter. I've looked for similar LED meters that will show positive and negative volts but every one I've found so far is either out of stock or not made anymore. I'm not married to the idea of it showing VDC, I wouldn't care if it went from 1-100, I just want some reference point that I can go back to and VDC seemed like the obvious choice.

My questions are will this work the way I think it will and is there anything else I should be considering? I obviously have no experience in circuit design so there are probably factors I'm not even aware of. Also if there is a LED display meter that will show + and - VDC I'd appreciate direction where to look, or if that will even work. So far I've just been Googling parts and there are a lot of results to sort through. I appreciate any feedback or ideas.
 

ElectricSpidey

Joined Dec 2, 2017
1,176
Just be sure your design places the voltmeter before the direction switch. (to use positive only meter)

You can use a simple PWM controller, a meter, then a DPDT switch to reverse the fans. (in that order)

BUT…any random voltmeter may not be accurate in measuring the PWM output.

So you could replace the PWM with a simple voltage follower circuit.

Parts needed for the follower circuit…
1. Transistor (I would choose a Darlington)
2. Potentiometer
3. Resistor
4. Heat sink (depends)
Very simple.

You can also use a diode bridge before any meter to always give a positive voltage, but then you would have to allow for the voltage drop.

As far as cobbling together something from pre built parts, I will leave that to somebody else.
 
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Reloadron

Joined Jan 15, 2015
5,511
and provide the ability to reverse the direction
This is where you may have a problem. Most home computer fans use a BLDC motor (Brush Less Direct Current). They use a HALL Effect sensor triggered by the blade position. The windings are triggered in a sequence for rotation direction. In a nutshell they are not reversible by simply reversing input polarity which on some fans will destroy the fan. You want to make sure you get a reversible fan or it won't work. You may want to keep that in mind before trying this with standard BLDC motor fans. Speed control is relatively easy but reversal not so much.

Also the blade pitch is another consideration. The blades are designed with a pitch to move the most air in one direction. Reversing the motor won't give you the same air flow. Rotating the entire assembly 180 degrees will but that is a mechanical, not electrical solution.

This should be a link to your speed controller.

Ron
 
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Thread Starter

Gazer219

Joined Dec 17, 2018
5
Thank you both for the information, I wasn't aware of the fan reverse issue so I'll look into that and decide if reversible is something I will do without. Doing a quick search for reversible fans brought me to this one which I just had to share, note the shipping cost.


fan.jpg
 

Reloadron

Joined Jan 15, 2015
5,511
So let's see you have a minimum order of 100 pieces at $2.00 each and UPS Express Saver will fly them here for $5,736.03 which works out to be about $60 per fan. Not sure if any import tariffs apply? :) I think you can do better.

Now if you can get along with uni directional air flow and you want to be cool about it (no pun intended) most home computer fans, which are very inexpensive, have three wires. The third wire is a tachometer out and usually 2 pulses out per revolution. Rather than try to measure fan voltage which is pretty difficult you could measure fan speed. You can also buy inexpensive fan speed controllers with temperature sensors so as temp increases fan speed also increases. Pretty much whatever you want.

Ron
 

Thread Starter

Gazer219

Joined Dec 17, 2018
5
reverse the direction of the fans
why?

Honestly, mostly just to have the option. One of the fans sits on the back of the primary mirror and is used to bring to, and keep the glass at ambient temperature while the other fan is blowing across the front surface of the mirror to prevent areas where air can pool and adversely diffract light coming down to the mirror and back out. I thought having the option on this fan to either blow across the mirror or blow out away from the mirror might produce better results different times of the year depending on whether the ambient temperature was hot or cold and initially thought this would be pretty easy to do, but it appears that isn't the case. This fan isn't in a place that's easy to get to once I'm set up and observing. The back fan is pretty easily accessible and if I want to switch the airflow I can just physically switch the direction it's blowing but a switch that did that would have been nice.

I found a reversible ventilation fan but I don't think I'll pay $40 each for the two fans, especially not knowing if it will work. I'm worried about vibration and with the fans I use now there's no visible vibration so I may just forget the reversible piece and just add variable speed. I started thinking about this as a way to clean up the wiring on the current fans as it's fairly sloppy and in doing that get some centralized control where I could turn the fans on and off and adjust the speed and direction of the fans independently, right now they just plug into the battery and they're either on or off with no control.
 

AnalogKid

Joined Aug 1, 2013
8,493
Small computer chassis fans usually have 5 blades and 3 or 4 struts supporting the motor. This sets up a beat pattern that is the majority of the acoustic noise from the fan, and is the reason you hear the fan noise change pitch as it speeds up and slows down. As always, where there's noise there's vibration. The 11-blade, super-curvy fan in post #4 should be much quieter than normal, especially if you run it on 8-9 V instead of 12 V.

ak
 
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