# Simple fan control circuit

#### 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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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.

Ron

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#### 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.