PWM VS Variable Voltage | For controling DC Fan Speed - Page 1
Forum section: electronics / beginners

author: ZeTeX
author_rank: Frequent Contributor
timestamp: April 07, 2016, 01:30:53 pm
content: Hi,I was wondering, is there any advantages to PWM a fan to control it speed instead of changing the input voltage?I know that 1 advantage is that you dont waste alot of power with PWM, But for 12V 160mA fan, its not a problem.So what are the advantages and disadvantages of PWM a fan and what are the advantages and disadvantages of changing directly the input voltage?

author: stmdude
author_rank: Frequent Contributor
timestamp: April 07, 2016, 07:20:48 pm
content: Well, it's a simpler circuit to build when using PWM, but, to be honest, it actually comes down to "saving power", or actually, "reducing heat".12V * 0.160A = 1.92Watts That's a lot of heat to get rid of in a small component. Building it with PWM reduces the losses, which reduces the heat. This means that you can build it without a heatsink (cheaper and smaller).

author: ZeTeX
author_rank: Frequent Contributor
timestamp: April 07, 2016, 07:41:48 pm
content: Quote from: stmdude on April 07, 2016, 07:20:48 pmso the problem is just heat? if I dont have to worry about heat, there is no reason to control a fan by PWM?I've heard that if you change the input voltage to a fan that is supposd to be controlled by pwm (4wire fan) it can damage it, is it true?

author: RGB255_0_0
author_rank: Frequent Contributor
timestamp: April 07, 2016, 07:43:43 pm
content: Quote from: ZeTeX on April 07, 2016, 07:41:48 pmI've heard it damages the fans also. And gives the fans a slightly different audio signature.http://www.overclock.net/t/1467480/running-a-pwm-fan-with-5v-or-7v-input-signal

author: ZeTeX
author_rank: Frequent Contributor
timestamp: April 07, 2016, 07:53:11 pm
content: Quote from: RGB255_0_0 on April 07, 2016, 07:43:43 pmso I should PWM my fans (4 wire fan)

author: RGB255_0_0
author_rank: Frequent Contributor
timestamp: April 07, 2016, 07:54:39 pm
content: Yes. I run my EK Vardars off a PWM splitter cable. Ideally you wouldn't mix different model fans with the same tach signal.

author: stmdude
author_rank: Frequent Contributor
timestamp: April 07, 2016, 08:05:20 pm
content: I'm no expert in PC fans, but a 4-wire fan doesn't seem like a _DC_ fan.I'd suspect it's a brushless 3-phase fan. That's a completely different thing all together, and what I said in my first post doesn't really apply.

author: ZeTeX
author_rank: Frequent Contributor
timestamp: April 08, 2016, 06:44:45 am
content: Quote from: stmdude on April 07, 2016, 08:05:20 pm4 wire -> +12V, GND, tach output (so your pc will know what speed it is running) and PWM Input.If you want to run the fan at full power you can always provide +12VDC and gone with it.Read here:http://formfactors.org/developer%5Cspecs%5C4_Wire_PWM_Spec.pdf

author: bktemp
author_rank: Super Contributor
timestamp: April 08, 2016, 06:57:34 am
content: Applying PWM to a regular (=non 4-wire) fan is a bad idea unless you know it tolerates it:Older and better fans have an input capacitor for smoothing the input supply current. This capacitor will screw up your PWM and create some high current spikes everytime the PWM is switched on.Cheaper and more modern ones use basically a single chip solution. The chip includes the hall sensor for commutation, the logic and the output switches. It even supplies itself via the non active coil, so it smoothes its suppy voltage internally. This type of fan runs fine if you apply PWM to the fan if the supply voltage seen by the ic is high enough.

author: ZeTeX
author_rank: Frequent Contributor
timestamp: April 08, 2016, 01:11:10 pm
content: Quote from: bktemp on April 08, 2016, 06:57:34 amAnd what if my fan has 4 wires?

author: bktemp
author_rank: Super Contributor
timestamp: April 08, 2016, 01:29:47 pm
content: Quote from: ZeTeX on April 08, 2016, 01:11:10 pmIf it has 4 wires, then you can apply a PWM with the correct frequency (21-28kHz) to the PWM input. Applying the PWM signal to the supply voltage can damage the fan.

author: ZeTeX
author_rank: Frequent Contributor
timestamp: April 08, 2016, 01:45:26 pm
content: Quote from: bktemp on April 08, 2016, 01:29:47 pmalright, but what if I change the supply voltage (and I dont PWM the fan).for example, I provide 7V and not 12V to the fan (and it spins), without touching the pwm wire. will the fan be damaged?

author: bktemp
author_rank: Super Contributor
timestamp: April 08, 2016, 01:56:59 pm
content: It depends on the fan. It is mandatory for 4-wire fans to work at 12V +/-5%. Many fans will work down to 5-8V, but it is not guaranteed.

author: ZeTeX
author_rank: Frequent Contributor
timestamp: April 08, 2016, 02:10:03 pm
content: Quote from: bktemp on April 08, 2016, 01:56:59 pmMy fan can start spinning at 3V, but it doesnt move alot of air.if I spin it at 7V (for example), will it be damaged? its spins no problem but some says it can damaged over time.

author: RGB255_0_0
author_rank: Frequent Contributor
timestamp: April 08, 2016, 02:19:42 pm
content: Quote from: ZeTeX on April 08, 2016, 02:10:03 pmWhat fans are you using?

author: ZeTeX
author_rank: Frequent Contributor
timestamp: April 08, 2016, 02:31:04 pm
content: Quote from: RGB255_0_0 on April 08, 2016, 02:19:42 pmMy fan is "arctic cooling F8 PWM REV2."

author: RGB255_0_0
author_rank: Frequent Contributor
timestamp: April 08, 2016, 03:09:17 pm
content: https://www.arctic.ac/uk_en/arctic-f8-pwm.htmlLooks to be suitable for down to 5.5v assuming it's fed a PWM signal.

author: ZeTeX
author_rank: Frequent Contributor
timestamp: April 08, 2016, 03:11:23 pm
content: Quote from: RGB255_0_0 on April 08, 2016, 03:09:17 pmBut I'm not powering it with PWM signal.

author: RGB255_0_0
author_rank: Frequent Contributor
timestamp: April 08, 2016, 03:16:52 pm
content: Quote from: ZeTeX on April 08, 2016, 03:11:23 pmThis is where I'd say, "they're £5 fans."But if you power a PWM fan from a non-PWM signal normally the fan controller will just set the speed to 100%.

author: ZeTeX
author_rank: Frequent Contributor
timestamp: April 08, 2016, 03:20:26 pm
content: Quote from: RGB255_0_0 on April 08, 2016, 03:16:52 pmwell the fan controller inside the fan cant set the speed to 100% because I'm underpowering the fan.they're £5 fans, but they are cheaper then noctua fans.