[NOOB] Choosing a potentiometer for a 12v fan - Page 1
Forum section: electronics / beginners

author: TDA81
author_rank: Newbie
timestamp: March 12, 2021, 06:47:21 pm
content: Hi, guys!I want to wire a 12v server fan directly to the power supply, inside a pc.I can't wire it directly to the motherboard due to it's power consumption (4.3W), and I want to use a potenciometer so I can control the fan speed.The specs of the potentiometer is what I need help about.I did my math (that is probably wrong) and the results are far, far away of the specs of the pots I find.The fan is:PAPST MULTIFAN 4212/2 G12v - DC 4.3w(7-14.5v DC)By my calculations (that might be wrong!!), I need a pot with about 40 ohm and rated for 1.5w (1.07w plus safety margin).But all I can find are pots with 1k, 10k, 50k... and really low power dissipation. Hence, I believe my calculations are wrong, and I deeply need your help.What potentiometer should I get for my fan??Thanks in advance.T. AtalaiaPS: Sorry for my bad english...

author: Benta
author_rank: Super Contributor
timestamp: March 12, 2021, 07:28:12 pm
content: Very 1960s and expensive solution. You need a wire wound power pot for this.Better solutions:1: LM1117-ADJ set for 7...12 V output.2: PWM control. I'm sure highly qualified experts will supply any number of 555 solutions.

author: themadhippy
author_rank: Super Contributor
timestamp: March 12, 2021, 07:47:42 pm
content: Quote50 ohm 5 watt pots are fairly common,for examplehttps://uk.rs-online.com/web/p/potentiometers/1790672/

author: Benta
author_rank: Super Contributor
timestamp: March 12, 2021, 09:20:54 pm
content: Quote from: themadhippy on March 12, 2021, 07:47:42 pmYes... at £3.10. Not really a bargain, and a lousy solution on top. The fan is not a simple brushed DC motor. It's a BLDC with integrated controller that stops working under 7 V.

author: themadhippy
author_rank: Super Contributor
timestamp: March 12, 2021, 09:34:14 pm
content: Did i say it was a good or bad solution?No, however i did show the o/p were to find something close to what he was asking for.

author: Gyro
author_rank: Super Contributor
timestamp: March 13, 2021, 10:23:06 am
content: For a resistor solution, I would go for a fixed resistor and a switch to bypass it, giving you two speeds. If you want more control, then you could use two resistors and a centre-off toggle switch to bypass one or both, giving you three speeds. Going beyond that is probably a bit anal.Using a couple of 2W resistors an toggle switch is probably the cheapest too, and avoids worries about the fan not starting if switched on with the pot turned too low.The LM317 solution would probably be the cheapest for fully variable control, as long as you restrict the minimum output voltage.

author: Benta
author_rank: Super Contributor
timestamp: March 13, 2021, 07:34:45 pm
content: Quote from: Gyro on March 13, 2021, 10:23:06 amI mentioned LM1117 on purpose. The LM317 has far too much voltage drop.

author: Gyro
author_rank: Super Contributor
timestamp: March 13, 2021, 07:42:26 pm
content: Good point.

author: Zero999
author_rank: Super Contributor
timestamp: March 13, 2021, 10:21:35 pm
content: Just use an emitter follower with the potentiometer and resistor on the base and fan on the emitter.

author: Benta
author_rank: Super Contributor
timestamp: March 13, 2021, 10:47:50 pm
content: Quote from: Zero999 on March 13, 2021, 10:21:35 pmAlso an idea, but I think you need to refine it a bit. Otherwise the B-E junction will pass all the current at full speed. To avoid this, you're into the same voltage drop as the LM1117.

author: amyk
author_rank: Super Contributor
timestamp: March 14, 2021, 05:05:54 am
content: A variable resistor of that power handling capability is usually referred to as a rheostat. As others have mentioned, more efficient solutions are available, and in this case you may want to use a MOSFET connected to the PWM output from the motherboard which will let you vary the speed but not overload it.

author: mariush
author_rank: Super Contributor
timestamp: March 14, 2021, 08:45:19 am
content: The easiest solution is to use a linear regulator with a low voltage drop and use a regular potentiometer to adjust the output voltage. 1117 has low voltage drop but there are regulators with even lower voltage drop.If you don't need super fine control and you'd be fine with just a few steps , then you could use a slide switch (or a sort of rotary control/switch) to switch between different resistors or different number of diodes.For example, see picture below ... by moving the slider, you short out a different number of diodes (remove them from circuit basically), so the voltage drop across changes. On the first position, there's no diode shorted, so you get n diodes x voltage drop per diode at that current ... one position to the right will short one diode, next position will short out two diodes and so on..with 1n5817...1n5719 and your fan that consumes around 0.35a, each diode should drop around 0.6v ... if you use 1n400x you should get 0.8v..1v drop.If you go with resistors, you can calculate your fan's internal resistance using this page :https://www.blackfiveservices.co.uk/fanspeedcalc.phpand then pick resistors to limit current at various levels.For example you say 12v 4.3w or 0.35A ... like the page shows you have V = IxR so your fan resistance is 12v / 0.35 = ~ 34 ohmIf you want maximum current, you set a 0 ohm resistor or just connect wires directly like the first position for the diode layout in the picture belowIf you want let's say 7.5v, then the resistor will have to be aroundtarget current for 7.5v V = IxR => I = 7.5 / 34 = 0.22Avoltage drop across resistor 12v - 7.5v = 4.5vSo again V = I x R ... 4.5v = 0.22A x R ... R = 4.5/0.22 = 20.45 ohm so a standard 21 ohm will do ....Power dissipated over the resistor would be 4.5v x 0.22a = 0.99w , so you'd want to use a 2-3w rated resistor or two 10 ohm resistors in series (for a total of 20 ohm) so that each resistor generates 0.5w of heat, making it possible to use 1w rated resistors.

author: Zero999
author_rank: Super Contributor
timestamp: March 14, 2021, 09:48:31 am
content: Quote from: Benta on March 13, 2021, 10:47:50 pmWhat makes you think that? The base will only pass too much current, if VBis taken above VC, which will only happen if there's a lot of resistance in Q1's collector, compared to R2, otherwise IB= IC/hFE.

author: bobbydazzler
author_rank: Regular Contributor
timestamp: March 14, 2021, 04:26:38 pm
content: I thought most motherboard fan headers could supply up to 1A current? Have you tried plugin it directly into the motherboard yet? Otherwise I think a simple lm317+trimpot regulator or make 3 different speed settings with resistors is easiest.

author: archil
author_rank: Contributor
timestamp: March 16, 2021, 08:33:58 am
content: right way!

author: perieanuo
author_rank: Frequent Contributor
timestamp: March 16, 2021, 02:03:39 pm
content: here you go, with automatic temperature control, you set the temp that starts the ventilatorI use that in some ps I designed

author: Benta
author_rank: Super Contributor
timestamp: March 16, 2021, 02:32:39 pm
content: Quote from: perieanuo on March 16, 2021, 02:03:39 pmShould work, unless the OP needs the tacho output. I that case it should be inverted to a P-channel circuit.

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: March 17, 2021, 06:27:56 pm
content: Hey op, if this is a server fan, consider using a PWM + duty cycle signal to control the speed of the fan, instead of controlling the voltage; most desktop/server fans don't respond to voltage changes for adjusting the fan speed.A stupid-cheap option for controlling the PWM + duty cycle would be the SG3525 module from eBay. These can be found for about $1.5USD.To control the fan, set the modules PWM to 25khz, and vary the duty-cycle between 0-100%.If you need more assistance, let me know!

author: Benta
author_rank: Super Contributor
timestamp: March 17, 2021, 06:46:29 pm
content: Quote from: LooseJunkHater on March 17, 2021, 06:27:56 pmThis is three wire fan, 25 kHz would never work (that's for 4-wire fans). 50 ... 100 Hz is OK.Three wire fans only have power input plus tacho output, so you'll need to PWM the supply voltage. Four wire fans have a separate PWM input, which is specified for 25 kHz.

author: james_s
author_rank: Super Contributor
timestamp: March 17, 2021, 06:47:49 pm
content: You can buy little devices specifically intended to control the speed of computer fans. They are a little box that plugs inline and has a pot on it, internally I'm sure they're a simple PWM driver.

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: March 17, 2021, 07:12:07 pm
content: Quote from: Benta on March 17, 2021, 06:46:29 pmI'd be very surprised if their server fan is 3-wire (12v, gnd, tacho), as 90% of the ones I've come across are 4-pin. Also, where was it mentioned that it's a 3-pin fan? I must have missed that part.

author: Benta
author_rank: Super Contributor
timestamp: March 17, 2021, 07:55:34 pm
content: Quote from: LooseJunkHater on March 17, 2021, 07:12:07 pmThe OP lists the fan part number IN THE VERY FIRST POST, and it's a three wire fan. OK?

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: March 17, 2021, 08:33:55 pm
content: Quote from: Benta on March 17, 2021, 07:55:34 pmWoops, you are 100% right. Sorry, I had a feeling I missed something :p

author: james_s
author_rank: Super Contributor
timestamp: March 17, 2021, 09:02:40 pm
content: I'm sure you can find something cheaper but I've used things like this a few times.https://www.ebay.com/itm/Gelid-Solutions-Fan-Speed-Controller-with-Manual-RPM-Control-3pin-Case-Fans-0-7A/233927307056