Powering a regulated 12V Fan with a 24V PWM power supply, help please :-) - Page 1
Forum section: electronics / beginners

author: Prokktor
author_rank: Newbie
timestamp: November 28, 2018, 01:44:28 pm
content: Hi,I have a MKS GEN L 1.0 Board that has a regulated 2 pin FAN output on a 4.5A onboard Mosfet. Most probably generating a PWM signal to regulate.I happen to have some powerful 12V Fans lying arround that i want to use.3 ways sprang to my mind:1) I connect 2 12V Fans in series, but thats dangerous and I dont know if the board can handle the addtional stress so I rather not do that.2) I use some kind of voltage regulator between board and fan, but I do not know which one (linear or switching) or if I can regulate a PWM signal at all this way... would a 50% fan speed not result in 12V getting regulated to 12V thus giving me 100% fan speed? Or can I actually regulate every single pulse down to 12V?3) I use a MOSFET that is PWM compatible to work with the 24V PWM as input and connect a secondary true 12V power supply to actually power the fan. I like this method best since it relieves the MKS board from stress...Since i gathered all my electronics knowledge from the internet, I am not sure if all (any^^) of those methods will actually work.Can you please help me?regardsMartin

author: Zero999
author_rank: Super Contributor
timestamp: November 28, 2018, 01:52:20 pm
content: # 2 is correct, you need a proper 24V to 12V DC:DC converter.#1 is a bad idea because the fans will form a potential divider, but their resistances will vary depending on the loads, so the voltage won't be half the supply voltage.#3 is a bad idea because the fans will still be subject to double their rated voltage. They are not simple resistive loads.I wouldn't recommend any of the above. This is all a hack. The correct way is to get the correct voltage fans in the first place. There's no reason not to, as 24VDC fans are widely available and will no doubt be more reliable and possibly cheaper in the long run.

author: Prokktor
author_rank: Newbie
timestamp: November 28, 2018, 01:58:19 pm
content: Thanks for your quick reply.I have the correct fans, but they are to weak and I dont know if it is safe to stress the board with more regulated power consumption.I do not understand why #3 would subject the fans to more voltage. As far as i understand it they would get pulsed power from the 12V power supply, the pulsed 24V signal is only used to switch the mosfet.So basically they would get 12V pulses instead of 24V pulses, I thought that would be the best solution.

author: Zero999
author_rank: Super Contributor
timestamp: November 28, 2018, 02:04:54 pm
content: If the 24V fans are too weak, then they're not the correct ones for the job.I misread #3, which was to use the 24V PWM signal to switch a MOSFET which controls a 12V fan. Yes, that stands a chance of working, but it's still more complicated than getting a 24V fan with enough power.

author: Prokktor
author_rank: Newbie
timestamp: November 28, 2018, 02:15:48 pm
content: Thanks a lot for your help.I will still try the MosFet variant first, because that costs my 5€ for the Mosfet.The right way will require the board to deliver double the power it does at the moment, and that costs 70€ to replace...So cheap and complicated beats right and easy for me.But should that fail I'll buy powerfull 24V fans^^

author: Efe_114
author_rank: Contributor
timestamp: November 28, 2018, 05:53:33 pm
content: I would limit the max duty cycle to 50% and use a chunky cap to smooth it to 12 volts but it wouldnt be so proffesional

author: Zero999
author_rank: Super Contributor
timestamp: November 28, 2018, 06:59:33 pm
content: Quote from: Efe_114 on November 28, 2018, 05:53:33 pmAn inductor would be required to smooth a 24V 50% duty cycle waveform to 12VDC, just adding a huge capacitor results in massive current surges, every time it's charged and very little smoothing.

author: David Hess
author_rank: Super Contributor
timestamp: December 02, 2018, 12:24:56 am
content: Two fans in series will likely work fine. If necessary, a 15 to 18 volt zener diode or TVS can be placed across each one to limit peak voltage but if you do this, use a higher power zener or TVS.Is the 24 volt output actually chopped? If so, usually a simple LC filter will be on the output to produce rough DC because otherwise the PWM output end up driving any capacitance in the DC brushless fan which is a problem.

author: spec
author_rank: Frequent Contributor
timestamp: December 02, 2018, 02:01:25 am
content: Quote from: David Hess on December 02, 2018, 12:24:56 amWell said

author: Prokktor
author_rank: Newbie
timestamp: January 07, 2019, 09:37:44 am
content: Solution 3 (MosFet + 12V Power Supply) works absolutely perfect, so I did not even try the other.Thanks a lot for your help.