Thought Problem: cheapest way to regulate 12v dc fan.

[Geoncic]

This is a very beginner question, but I am just getting back into circuits after 10+ years of a 101 class.

What would be the cheapest (and reliably safe) way to drop the speed of a 12v dc fan? Assuming all the components are purchased.

Assumptions:
Fan draws 4.5 watts at 12v dc
Would like to drop the fan to 7-9 volts
Fixed drop is fine.

1. What would be the absolutely cheapest solution?
2. What would be the most practical solution?
3. What if I wanted to have variable speed?

My first thought was a resistor, but then I figured that would just burn out. 5w resistors seem to be pricey when not buying in bulk. My 2nd thought was a 10k 5w potentiometer?

Are there any other things that I might be missing?

Thanks!

[ebclr]

https://www.fairchildsemi.com/products/power-management/voltage-regulators/positive-voltage-linear-regulators/LM7808.html

[Ian.M]

A 10K 5W  pot will cost a fortune, and will probably burn up.  Pot wattage ratings are for the whole track, so use P=I²R to get the current rating. - A 10K 5W  pot will burn up if you try to put much over 22mA through any part of its track.    An adjustable linear regulator would work, but then you need to heatsink it and get rid of the heat from that heatsink which all adds to the cost.

The best option (not for commercial production) is an Ebay $0.99 adjustable buck converter module.  e.g. http://www.ebay.com/itm/DC-DC-Buck-Converter-Step-Down-Module-LM2596-Power-Supply-Output-1-25V-35V/172130630915.  Set it for whatever fan voltage <12V you want.

At that price point there's no way you are getting a genuine LM2596 switching regulator, nor is the catch diode likely to have a full 3A rating, but as your fan draws under 400mA it wont significantly stress the module.   

The most likely failure mode is for the chopper to short giving full power to the fan but there is a small possibility of the catch diode shorting so if the PSU isn't over-current protected, fuse the input at 1A!  If you *always* want it to fail safe with the fan on, it gets a lot more complicated.  For that situation its better to have multiple redundant fans and an over-temperature alarm.

If you are using it in a PC, don't let it flap around inside the case - stick it down with 3M foam VHB tape - accept no substitutes as inferior foam tapes tend to let go with age and then your PC goes Bang! when the module shorts to the motherboard or some other critical part.  It would be nice to insulate it with heatshrink tubing but that can cause the module to overheat - some experimentation is required with a temperature probe on the chopper chip if you do that.

See http://www.hobbyelectronics.net/review_dctodcmodule.html for a thorough review of a typical module of this type.

[edpalmer42]

Do you have access to 5V?  If so, connect the + lead from the fan to +12V and the - lead to +5V.  This runs the fan off of 7 volts which reduces the speed and noise.

Ed

[mariush]

limit the current with a resistor.

4.5 watts at 12v = > current = 4.5w/12v  = 0.375 A  ... but for simplicity let's round it to 0.4A


Now use   V=IR (Ohm's Law) to figure fan's "resistance", so 12 = 0.4R   =>   R = 12 / 0.4   =   30 ohms

Target voltage: 9V

    V=IR (Ohm's Law), so 9 = 30I   =>   I = 9 / 30   =  0.3 A

Voltage drop across resistor: 12V - 9V  =  3V

    V=IR (Ohm's Law), so 3 = 0.3R    =>    R = 3 / 0.3    =  10 ohm

Power dissipated by resistor:  3V * 0.3A  =  0.9W

0.9w is a bit too close to 1w which is a standard rating for resistors, so I'd probably use a 3w resistor, or I'd use two x 18-22 ohm 1w resistors in parallel

Have fun with this one : http://www.blackfiveservices.co.uk/fanspeedcalc.php?Voltage=12&Current=0.4&Target=9

If you use a linear regulator, you won't get better efficiency.

If you don't have resistors , you could always connect multiple diodes in series , and at that current each should drop around 0.6-0.8v  and the power dissipated per diode will be small enough to not worry about it.  Put 4-5 x 1n400x in series and you have around 2.5v - 3v drop.  And you could have the added benefit of being able to adjust voltage  - just short out 1 or more diodes in the series with a wire to take them out of the equation and reduce the voltage drop in the series of diodes.

[Rick Law]

Get one of those cheap adjustable buck converter, and adjust away.  Your maximum current draw is less than 1/2amp, (0.375mA at 12volt for 4.5W).  The buck will not be dissipating that much power.  You can find cheap ones at about $1 plus shipping on AliExpress.

I have a few of those old cell phone car charger (12V buck to 5v, mc34063a chip at 1watt max dissipation), I changed a few to adjustable voltage bucking and I can run those kind of buck converter at 0.5amp all day without it getting hot.

If you have some of those old cell phone car charger (mc34063/ka34063), converting it to adjustable voltage is rather easy but they are not very efficient.  You do need to get some of those cheap VR pots.

[john_c]

Do you have access to 5V?  If so, connect the + lead from the fan to +12V and the - lead to +5V.  This runs the fan off of 7 volts which reduces the speed and noise.

Ed

Well, that answers #1, since it doesn't get any cheaper than free.

#2 is what would be the most practical. You should look to Ian M's very practical answer.

And 3, well, mariush's suggestion of a string of diodes soldered together in series and an alligator clip for setting the value is clearly the niftiest way to do variable speed!

[Ian.M]

I should point out that #1, running the fan between +12V and +5V could easily end up not being free, but being the *MOST* *EXPENSIVE* decision you will ever make in this project.

The fan dumps current from the 12V rail into the 5V rail. As long as there are other loads on the 5V rail that *ALWAYS* take more than the peak fan current (assume that's the startup current immediately after a stalled motor is released or power is switched on, which may be quite a bit more than the running current), as nearly all PSUs are designed to only source current and will loose regulation if their load drops too low or attempts to push current back into them, you risk the 5V rail voltage rapidly rising and blowing all voltage-sensitive loads on the 5V rail.

TLDR: running a fan like that is very risky unless the *MINIMUM* 5V rail load is *MANY* times the fan operating current.

#3 (diode dropper) is the best of the simple suggestions.  The voltage drop varys far less with current than if you use dropper resistors, so the fan has better startup torque, and the wasted power is well distributed with no single part getting dangerously hot.   However the long chain of diodes to get down to 7V is a PITA so unless you need instant air movement gratification, you'd do better to order the buck module.

You can also find the buck modules on Amazon, (at a premium) if you don't want to mess with Ebay, but unless you pay a LOT more for a supplier that carries local stock and offers 3 day delivery, they'll still be shipped by post fron China.

[BradC]

Nobody seems to have mentioned the fan will draw far less than 4.5w at 7V. Plug it in and measure it.

edit : just to put my money where my mouth is I grabbed a random fan off the shelf.
12.0V 138mA - 1.6W
7.0V 74mA - 0.52W

Found a Sunon 4.3W finger mucher in my fan pile.
12.0V 330mA - 3.96W
7.0V 233mA - 1.61W

So a 2W resistor would be stacks to throttle that to 7W.

[Zero999]

Nobody seems to have mentioned the fan will draw far less than 4.5w at 7V. Plug it in and measure it.

edit : just to put my money where my mouth is I grabbed a random fan off the shelf.
12.0V 138mA - 1.6W
7.0V 74mA - 0.52W

Found a Sunon 4.3W finger mucher in my fan pile.
12.0V 330mA - 3.96W
7.0V 233mA - 1.61W

So a 2W resistor would be stacks to throttle that to 7W.

The ratio is roughly the square of the input voltages.

7²/12² = 0.34

0.34*1.6 = 0.544W
0.34*3.96 = 1.35W

Not exact, as a fan isn't a purely resistive load, but it's within 80% of your measurements.

[BradC]

The ratio is roughly the square of the input voltages.

Pretty much. This generally applies to anything moving a fluid (air, water, whatever). I just happened to have a similarly rated fan within reach of a decent DMM and bench supply. Easy to demonstrate using real world figures.

I've got the 24V fan in one of my APC SU2200 UPS set up to tick over whenever it's not supposed to be running. The UPS uses a FET to switch the low side of the fan, so I put together a couple of 1/4W resistors in parallel between the fan and chassis until it was guaranteed to start. Put them on a choccie block in the direct airflow and it has been apples for about 7 years now. Keeps a steady but small airflow untill it is supposed to run, then goes like a scalded cat.

[Mechatrommer]

buy 10 to get cheaper per module... and smaller footprint too...
http://www.ebay.com.my/itm/10PCS-Super-mini-3A-DC-DC-Converter-Step-Down-Power-Supply-Module-3V-5V-16V-M21-/301724335371?hash=item46402bfd0b:g:SXkAAOSw1x1UMeab

[BradC]

buy 10 to get cheaper per module... and smaller footprint too...
http://www.ebay.com.my/itm/10PCS-Super-mini-3A-DC-DC-Converter-Step-Down-Power-Supply-Module-3V-5V-16V-M21-/301724335371?hash=item46402bfd0b:g:SXkAAOSw1x1UMeab

With the greatest of respect, how on earth is that cheaper or easier than a 30c resistor?

[Mechatrommer]

With the greatest of respect, how on earth is that cheaper or easier than a 30c resistor?

its not cheaper than a resistor (10c more expensive per module) but its another way for the solution. its >80-90% energy efficient, variable speed ready with mini trimpot, take variable power supply input without much tweaking and runs cooler. without knowing it, using linear regulated resistor make the electric bill 1c more per month, 12c per year, and has been 84c in the 7 years of operation, well nevermind dont take that seriously i'm just giving an opinion.

[Codebird]

1. A simple resistor. Cheap as can be. But every time you turn the fan on, Al Gore rings your doorbell and hits you on the head with a dead penguin.
2. Crude little PWM circuit - there are about a hundred ways to do this. Don't forget to check the rating of your power transistor!
3. Slightly less crude little PWM circuit. May involve a 555 or compatible substitute.

[Seekonk]

I do feel beginners always ask the wrong questions, like what is cheap or easy. Clients would always come in and say I just want it to do this. I have to beat them over the head with what would your customer really like, how can we add value to this. So what would you really like this to do?

Our camp has a 12V fan that my wife likes to go to sleep with. The nights cool off quickly and you would be cold if the fan stayed on. I used a NANO and a FET. I don't really like building things anymore and there is nothing easier than writing a couple lines of code and it is already a breadboard.  So this fan comes on for about 50 minutes.  After that it ramps down in speed for 20 minutes and then shuts off.

No one has mentioned this yet, DIODES.  Each drops the voltage about .7 volts, just make a string. Then you have speed selection and much better starting.  They cost less than 2 cents each and the heat is distributed.

[Cliff Matthews]

..No one has mentioned this yet, DIODES.  Each drops the voltage about .7 volts, just make a string..

Already given (last part of #4) and IMO the 40-cent convertor is cheapest, since it adds variable-speed.

[Geoncic]

I just want to thank everybody for contributing to this question. This is amazing.

I just wanted to clarify, I wasn't looking for a single best solution, but wanted to see what people with different thought processes would suggest, and you guys/gals delivered!

Now I have several different ideas to play around with and learn the pro's and con's of each. Maybe document along the way as I learn.

I'm planning on setting up 4 fans for some localized ventilation on my workbench, and two of them are small fans that are loud and power hungry.

I really like mariush's suggestion on diodes, mainly because I don't have much experience using them.

Thanks again!

[james_s]

They sell little devices that plug inline with the fan and have a little knob for PWM speed control, I have a couple of them in my PC and they work fine. I've also used those 99c buck regulator modules and they also work fine. In my experience there is no problem running a 12V brushless fan on 5V, I've yet to encounter one that doesn't work.

[Ian.M]

Another reason to use the DC-DC converter is that it converts power, so when you reduce the voltage, it trades voltage for current and thew load on the PSU feeding it is reduced.  The other solutions (apart from the 555 PWM) are linear and throw away the excess voltage as heat, so their input current is the same as their output current. This means you can probably get away with a lower current PSU, though if you are ever going to run them full speed it should be rated for the max power they can draw.  Whenever one or more fans are slowed down you'll have more safety margin (on the PSU current rating) than the linear solutions.

You can also hack the DC-DC converter boards to add a pot or switched resistors to set the speed, with the on-board preset used to set the minimum speed - if you want to do that, once you've got a fan running, find the minimum speed it starts at reliably, then measure the output voltage and the preset wiper voltage on a reasonably good DMM, and ask here how to mod it, posting a good closeup photo of each side of the board, and also the value marked on the preset.

@James_s: the problem isn't running fans at 5V, the problem is people running fans at 7V between the 12V and 5V rails of PC PSUs.  Do that with barely enough load on the 5V rail, and when a fan jams or gets clogged with dirt, it can blow everything on the 5V rail.

[Rick Law]

Another reason to use the DC-DC converter is that it converts power, so when you reduce the voltage, it trades voltage for current and thew load on the PSU feeding it is reduced.
...
You can also hack the DC-DC converter boards to add a pot or switched resistors to set the speed, with the on-board preset used to set the minimum speed - if you want to do that, once you've got a fan running, find the minimum speed it starts at reliably, then measure the output voltage and the preset wiper voltage on a reasonably good DMM, and ask here how to mod it, posting a good closeup photo of each side of the board, and also the value marked on the preset.
...

_{re: "DC-DC converter is that it converts power"}

That it converts power not only makes it more efficient, it eliminate other problems too.  Linear, be it via diode or resistor, will be burning that power off resulting in heat.  So you need a higher dissipation rating for the diode or resistor.

Now that you have a high enough power dissipation capability, you will have to eliminate the heat resulted from it.  Of course, it heating up more means that the voltage will change more from when it was cold to warm to hot.

_{re: "You can also hack the DC-DC converter boards"}

If you have some of those old cell phone car-charger, they really are fun to modify.  If the are the MC34063/KA34063 types, you need just a 3296 trimpot (25turns) at 10kohms and you can get it converted from fixed to variable output.  Then, as you learn more, you can make that more fancy.

Attached top/bottom side photo of such modifications to three of my old cell phone car chargers to show how it could be done the most simple way.  They all have no added part except the trimpot added-to or replaced the fix resistor.   (In case you don't know, the big blue box with a screw on the top is the trimpot)

Two of them (if my labels are right) are converted to boost (increase V) instead of buck (decrease V).  The soldering job is awful - probably done before I upgrade my eyeballs by adding a pair of reading glasses when I solder.  But it does the job in showing how the mod can be in its simplest form: add trimport, cut a few traces, add a few new connection...  there you go, a new boost or a new buck dc-dc converter.

By the way, if you are hacking... always check the capacitor voltage rating and always center the trimpot before soldering when converting to boost.  Typically, the output caps are 10V or less since they typically output 5V-ish.  The trim could be left in max (center tab is practically at start/end) which results in max voltage output which is a good bit higher than 30V.  Poof, there goes the cap.  Puffing off the cap is kind of fun and it does leave a nice aroma in the room.

[mariush]

Unless the inductor and the diode and the switching frequency are carefully chosen for the average current you're going to have, these switching regulators will be fairly inefficient , they could be as low as 70% efficient.

I've played with  a cheap car charger which was using a mc34063 and was rated for 12v input and 5v at 0.5a ...  the most efficient was at around 75% with 9v in and 0.45A out.   So compare that with linear regulator that would have 9 x 100 / 12 = 75% efficiency at 12v in 9v out ... so basically same amount of heat just more concentrated in a single spot in the case of linear regulators

Most of these cheap dc-dc converters pick the cheapest inductors and diodes that would still make the circuit work.

Those LM25xx or xl300x dc-dc converter from ebay also use components picked for wide output range and current and not least because they're cheap, not for highest efficiency. imho pretty much the benefit of the cheap dc-dc converters would be that you don't have quite a bit voltage drop as you have with linear regulators (typically 1v to 1.5v for cheap regulators like 7809 or LM317)



with just the output capacitor changed to a higher quality one

[Rick Law]

Unless the inductor and the diode and the switching frequency are carefully chosen for the average current you're going to have, these switching regulators will be fairly inefficient , they could be as low as 70% efficient.
...
Most of these cheap dc-dc converters pick the cheapest inductors and diodes that would still make the circuit work.
...
Those LM25xx or xl300x dc-dc converter from ebay also use components picked for wire output range and current, not for highest efficiency.
...

Certainly agree.  They are not efficient at all.  Just fun to play with.  And yes, they are cheap - really cheap.  The voltage is not as stable and not as clean as desired.  There is always a triangular wave form of a few tens mV riding on top of the dc.  So all in all, fun, usable, not good or best but adequate for some applications.

Efficiency with buck is bad, the efficiency for boost is ungodly!  Using original everything as depicted, 40%-ish boosting approx 4V to 7.5V (for Arduino +V input).

To power a small fan, it is probably more efficient than linear voltage drop be it diode or resister - basically, burn away the excess.

[Zero999]

All of this is a bodge. There should be no need for a dropper resistor or regulator.

How about simply looking for a slower fan?

Another possibility is a 24V fan, assuming it will run off 12VDC. That will give roughly a quarter of the fan's rated power.

[Geoncic]

Thanks for the suggestion, but I'm asking the question because I wanted to learn something about implementing different solutions with a circuit, even if they aren't ideal or efficient. I'm mainly wanting to learn with a very simple project with different ways to approach it.