Making a 12v fan work with a 24v applicatin - Page 1
Forum section: electronics / beginners

author: SirRage
author_rank: Contributor
timestamp: May 31, 2016, 07:56:05 am
content: Hey there,I have an oscilloscope that has a fan in it that buzzes on startup and drones like a swarm of bees, and I'd like to fix that. I can't seem to find a 24v fan in that size package outside of used pulls, so I had the brilliant idea that I could take a modern low noise 12v fan which had a similar footprint and same CFM and make some kind of adapt it to work for my application. The thing is I'm not really sure where I am suppose to start.First thing is, I'm not very sure how this is normally done. Taking a power source of 24v and stepping it down to 12v. I'm also not even sure what kind of shielding requirements might exist given that this is a piece of test equipment and adding any kind of additional circuitry might throw something out of spec.So if you guys have some input for me, or some reading material to direct me to, that would be most helpful. Thanks!Existing Unit:VOLTAGE IN VOLTS AND CURRENT TYPE: 24.0 DCVOLTAGE RANGE: 12 TO 28 VOLTS DCPRIME MOVER POWER RATING: 5.0 WATTSReplacement Unit:Voltage: 12vCurrent: 0.16 A (0.28 max)Power Consumption: 3.36w

author: danadak
author_rank: Super Contributor
timestamp: May 31, 2016, 09:52:53 am
content: You could -1) Use simple dropping R if its not a T controlled fan. Wastes power but is a low noisesolution.2) Do a simple 3 terminal regulator, with heat sink, also low noise but wastes power.Can be adapted to T controlled solution as well.3) Use a PWM controlled solution. Saves power but generates more noise. Can alsobe used in T control loop.If fan is brushless keep in mind it also gens noise, its current draw not a DC current.Regards, Dana.

author: Zero999
author_rank: Super Contributor
timestamp: May 31, 2016, 11:50:31 am
content: Quote from: danadak on May 31, 2016, 09:52:53 amThat may be a problem if it's a brushless fan, as the controller will probably not like 24V pulses on its power supply rail. If you're using PWM with a brushless fan, add an inductor and capacitor to form a buck regulator, which needn't be regulated, so long as the output voltage doesn't ever exceed the fan's absolute absolute maximum rating.

author: ebclr
author_rank: Super Contributor
timestamp: May 31, 2016, 11:53:05 am
content: http://www.aliexpress.com/item/1PCS-5A-Lithium-Charger-Step-down-5A-Power-Supply-Module-LED-Driver-DC-to-DC-CC/32347580371.html?spm=2114.30010308.3.70.4qgFc0&ws_ab_test=searchweb201556_8,searchweb201602_4_10037_507_10032_10020_10017_10021_10022_10009_9979_10008_10018_101_10019,searchweb201603_3&btsid=1f5d7c45-81f0-4b55-81a6-65b5a247ac13

author: danadak
author_rank: Super Contributor
timestamp: May 31, 2016, 09:21:49 pm
content: PWM design.http://ww1.microchip.com/downloads/en/appnotes/00857a.pdfhttp://www.st.com/content/ccc/resource/technical/document/application_note/5d/04/eb/5e/87/77/48/cb/CD00041736.pdf/files/CD00041736.pdf/jcr:content/translations/en.CD00041736.pdfRegards, Dana.

author: batteksystem
author_rank: Regular Contributor
timestamp: June 01, 2016, 03:23:58 am
content: Just use a DC-DC converter. 24V->12V conversion is very common, you do need to watch the size though.

author: jitter
author_rank: Frequent Contributor
timestamp: June 01, 2016, 05:06:20 am
content: Quote from: SirRage on May 31, 2016, 07:56:05 amI think a few questions should be answered first before you start swapping out stuff:- Did this fan always behave this way, or has this become worse over time?- If you start the scope, does it spin fast and drop down and settle on a lower speed?- If it seems to spin only at one speed, does it really or can a hot environment actually make it spin faster?

author: TheAmmoniacal
author_rank: Unknown
timestamp: June 01, 2016, 07:55:10 pm
content: I would just do a voltage divider and tap off across one resistor.Assuming a 300 mA 12V fan.. Not willing to do the math right now, but some rough values.. If you put a 30 ohm resistor and a 150 ohm resistor in series with the 24 VDC supply and then connect the fan across the 150 ohm resistor that should work..?You'll be dissipating like 5W or so in the 30 ohm resistor, which is a drawback.. And a watt or two in the other..Compared to how much power test gear draw on standby these days it's probably fine.

author: tautech
author_rank: Super Contributor
timestamp: June 01, 2016, 08:03:10 pm
content: Quote from: SirRage on May 31, 2016, 07:56:05 amTo avoid overlooking the obvious please state the brand and model ^^ as there may be a thread here with a simple/suitable remedy for you.

author: ZeTeX
author_rank: Frequent Contributor
timestamp: June 03, 2016, 08:03:03 am
content: I would use LM7812 and be done with it.12*0.3= 3.6W of dissipation, You will need a small heat sink.You would use any heat sink that has lower resistance then 10C/W.

author: Simon
author_rank: Unknown
timestamp: June 04, 2016, 05:49:33 pm
content: don't pwm a brushless fan unless as hero said you make a dirty smps converter by low pass filtering it to 12V

author: Unknown
author_rank: Unknown
timestamp: June 04, 2016, 07:39:25 pm
content: Quote from: danadak on May 31, 2016, 09:52:53 amOk, I'll bite. The "T" stands for transistor?

author: Simon
author_rank: Unknown
timestamp: June 04, 2016, 07:41:14 pm
content: No I think he meant temperature. A lot of equipment has a temperature -controlled fan to try and quieten the fan down when the machine is cool.

author: Circlotron
author_rank: Super Contributor
timestamp: June 05, 2016, 02:01:16 am
content: 12 volt 5 watt zener diode e.g. 1N5349B in series with the 12 volt fan.The End.

author: jitter
author_rank: Frequent Contributor
timestamp: June 05, 2016, 06:25:47 am
content: Quote from: Simon on June 04, 2016, 07:41:14 pmAgreed, hence the questions in my previous post, but OP doesn't seem to check back...