12v fans and 24 power supply - Page 1
Forum section: electronics / beginners

author: Godzilla
author_rank: Contributor
timestamp: March 12, 2021, 02:55:08 pm
content: Hello guys, I've a bunch of 12v fans that I don't use, and I wanted a little fumes extractor, I've a 24v smps that I recovered from an old printer and I wanted to use that to power the fans, is it possible to reduce the voltage of the power supply from 24 to 12 without schematics? Also is it possible to use 2 fans in series with the 24 power supply?

author: Andree Henkel
author_rank: Regular Contributor
timestamp: March 12, 2021, 03:29:01 pm
content: Series should work fine. Assuming it´just for your personal private use.

author: Godzilla
author_rank: Contributor
timestamp: March 12, 2021, 03:54:15 pm
content: Yeah its for personal use, im gonna use it very rarely, Should i put a capacitor in parallel with each fan?

author: tunk
author_rank: Super Contributor
timestamp: March 12, 2021, 04:08:05 pm
content: Assuming they have the same amp-rating (or better, are identical)I guess it should be ok. As a precaution, measure the voltageacross them.Edit: Photos may help to see if it's possible to modify the PSU.

author: Godzilla
author_rank: Contributor
timestamp: March 12, 2021, 04:26:28 pm
content: They are identical fans, if i measure voltage one is 11 and the other is 13Ill try to post the pictures asap!

author: Jeff1946
author_rank: Regular Contributor
timestamp: March 12, 2021, 05:08:31 pm
content: Try three in series and see air flow is adequate.

author: Zero999
author_rank: Super Contributor
timestamp: March 12, 2021, 06:35:04 pm
content: Connecting fans in series is generally not a good idea, because if one of them jams, or they don't start at the same time, one of them will see an excessive voltage. If it's just for a personal project and you can accept the risk of damaging one of them, then go for it, but don't do it for a professional project.

author: Godzilla
author_rank: Contributor
timestamp: March 13, 2021, 02:56:05 pm
content: Here are the SMPS pictures. Im gonna use it at home and they are very old fans, its fine if one of them dies. I sadly dont have 3 of them that are identicals...

author: madires
author_rank: Super Contributor
timestamp: March 13, 2021, 03:26:43 pm
content: You can try to the change the feedback voltage divider for the TL431 or get an inexpensive buck converter from your preferred shopping platform.

author: Godzilla
author_rank: Contributor
timestamp: March 13, 2021, 04:35:44 pm
content: I guess the TL431 is the IC2 in the board right? so the divider is made from r22 and R20 i guess, and the Vo equation is Vo=(1+R20/R22)*Vref so if Vref is 2.5V and R20 is 24K and R22 is 56K i should have a Vo that is 3.6V but i cant measure it anywhere near the IC... So whats exactly the role of the IC here? Im probably using the wrong formulas...

author: tunk
author_rank: Super Contributor
timestamp: March 13, 2021, 05:53:33 pm
content: No expert, but it could be R24 instead of R22.Doesn't make much sense though, as it should give 8.5V [(1+24k/10k)*2.5)].Or it could be a higher voltage reference.I guess you could measure the voltages across the resistors and IC2 (be careful not to short anything).And you could try e.g. a 100k resistor in parallel with R20 to see if the output changes.

author: Godzilla
author_rank: Contributor
timestamp: March 13, 2021, 07:41:37 pm
content: I will try to check the voltages tomorrow, im not much expert on SMPS, the voltage pass on the opto and go back to the switching IC right? In this case the IC wants the full ouput voltage as reference or a scaled one?

author: Refrigerator
author_rank: Super Contributor
timestamp: March 13, 2021, 07:59:33 pm
content: R26A and B go off somewhere and they're also connected to the feedback of the IC.Actually they go to the collector of Q5 so they might offset the feedback in some way?The voltage across R24 should be the reference voltage of the IC, you should measure it and find out if it's a TL432 or some other IC.

author: David Hess
author_rank: Super Contributor
timestamp: March 14, 2021, 08:41:35 am
content: I might at least place a big 15 volt zener diode across each fan which is in series. In more critical applications, I have used a pair of transistors and a voltage divider to force the midpoint to half of the supply voltage.

author: Godzilla
author_rank: Contributor
timestamp: March 14, 2021, 02:18:27 pm
content: The voltage across R24 is 2.5V. On R26A ive 2.35V, and on R26B ive 120mV. On the opto ive around 24V so i guess i feed all the output voltage on the feedback... I just dont understand what the voltage reference do in the circuit...

author: madires
author_rank: Super Contributor
timestamp: March 14, 2021, 02:57:14 pm
content: Start at the TL431's reference pin and look for a resistor to Gnd and second one to the 24V output. That's the voltage divider. There could be another resistor plus a capacitor in series between reference pin and cathode.

author: tunk
author_rank: Super Contributor
timestamp: March 14, 2021, 03:18:45 pm
content: What's the voltages across the other resistors?Could look like R20 goes between 24V and the TL431,and if so, the voltage could be around 21.5V.

author: Godzilla
author_rank: Contributor
timestamp: March 14, 2021, 04:51:18 pm
content: So, R20 should be R1 (connected to 24V) and R24 should be R2 (connected to GND), so i should have 8.5V on the cathode, but it is connected to the Q5 with R26A and R26B resistors so maybe it has an offset...

author: Godzilla
author_rank: Contributor
timestamp: March 14, 2021, 04:52:37 pm
content: Quote from: tunk on March 14, 2021, 03:18:45 pmThe voltage on R20 is 21.5V.

author: Ian.M
author_rank: Super Contributor
timestamp: March 14, 2021, 05:05:38 pm
content: Most AC input SMPSes are designed for a specific output voltage and may become unstable if you modify the feedback circuit for a large change in output voltage. You can generally get away with a change of under 20%e.g. modding a 12V PSU for 13.8V out to power radios designed for automotive use.Fake LM2596 DC-DC buck modules are dirt cheap, readily available on Ebay, Amazon and the usual far-east marketplace sites, and are typically capable of 0.5A out continuously, with a significantly higher surge rating (but nowhere near the nominal 3A rating frequently claimed for them), so unless the 12V fans draw over 0.5A, I'd simply use one buck module set to 12V per fan. You can also adjust the fan speed by reducing the output voltage.

author: tunk
author_rank: Super Contributor
timestamp: March 14, 2021, 05:07:42 pm
content: You could try e.g. 220k in parallel with R20 (i.e position R21)and see if that lowers the output voltage. If it does, then youcould experiment with lower values and see if you can get 12V.NB: No expert on SMPSs; it is designed for 24V, so it may putextra/excessive stress on it, maybe not work at all, or blow up.

author: Godzilla
author_rank: Contributor
timestamp: March 14, 2021, 05:49:12 pm
content: Quote from: tunk on March 14, 2021, 05:07:42 pmI tried with a 150k in parallel, now it provides 21V, it doesnt look like it can go down to 12V...

author: tunk
author_rank: Super Contributor
timestamp: March 14, 2021, 05:55:44 pm
content: Try lower values, e.g. 47k or 22k.

author: Godzilla
author_rank: Contributor
timestamp: March 14, 2021, 06:32:37 pm
content: Quote from: tunk on March 14, 2021, 05:55:44 pmWith 22K i reached 13V, i think i will stop here, im gonna use a couple of diodes if i need 12V precisely i think...

author: Godzilla
author_rank: Contributor
timestamp: March 14, 2021, 06:59:11 pm
content: Just one more thing, in the output ive 3 wires, teh traces of the 2 external are connected to the output capacitor and they are big so i guess they are my 12V, but why do i have a 3rd output (center)l?

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: March 14, 2021, 09:55:13 pm
content: I'm with David Hess on this one.Put a 12 (or 15 Volt) Zener Diode in series with each fan.Put the Zener BEFORE the fan and put it in with cathode on your 24 volts POS. You will get 12 volts on the other side of the zener with the fan in place.Fans do not draw very much so I don't think you need a really big ZenerSimple and cheap.

author: Zero999
author_rank: Super Contributor
timestamp: March 14, 2021, 10:17:14 pm
content: Quote from: Godzilla on March 14, 2021, 06:32:37 pm13V is fine. Don't bother with diodes. The fans will be able to run off 13V, with no problem.Some data sheets.https://www.cuidevices.com/product/resource/cfm-70-series.pdfhttp://www.farnell.com/datasheets/2310930.pdfhttps://www.ebmpapst.co.uk/media/content/infocentre_media/downloads/content_dl/catalogues/Compact_fans_for_AC_and_DC_2014.pdf

author: Ian.M
author_rank: Super Contributor
timestamp: March 14, 2021, 10:41:24 pm
content: Quote from: Wallace Gasiewicz on March 14, 2021, 09:55:13 pmThat's *NOT* what David said to do:Quote from: David Hess on March 14, 2021, 08:41:35 amIf you put a Zener in series with each fan, you have to get rid of all the resulting waste heat. Many common 12V fans have running currents in the range 0.2A to 0.3A, so you are looking at between 2.4W and 3.6W of heat. Yes a 1N5349 12V 5W Zener can handle that, but why dissipate the extra heat?David's second suggestion is far more energy efficient as it only has to handle the current difference which if you group the fans to divide the total running current as near as possible in half can be quite small.

author: james_s
author_rank: Super Contributor
timestamp: March 14, 2021, 11:00:45 pm
content: I've used those little 99c Chinese buck regulator boards to run 12V fans from 24V on a number of occasions, it's a cheap and effective solution. An added bonus is they have a multiturn pot on them to adjust the voltage so you can tweak the fan speed.

author: David Hess
author_rank: Super Contributor
timestamp: March 15, 2021, 03:27:56 am
content: I am sorry if I was not clear; I mean put the 12 volt fans in series across the 24 volt supply, with a 15 volt zener diode in parallel with each fan to maintain some balance with unequal loading. Since it is much easier to find power transistors than power zener diodes these days, the alternative using transistors is needed at higher power levels.

author: Godzilla
author_rank: Contributor
timestamp: March 15, 2021, 09:01:37 am
content: OK so, it seems like that the SMPS struggle at holding the 13V, the fans turn, but not as fast as they should, so i think im gonna revert it back to 24V and use zener diodes or the 12V converters. Since i dont have neither of them i think im gonna order the buck converters, now, should i run the fans in parallel from one converter or should i buy one of them for every fan? They are 80mm fans and they consume a very low amount of current...

author: Ian.M
author_rank: Super Contributor
timestamp: March 15, 2021, 10:38:11 am
content: That depends on *HOW* *LOW* is "a very low amount of current...". Give us numbers!

author: Godzilla
author_rank: Contributor
timestamp: March 15, 2021, 10:55:57 am
content: Around 150mA (200mA max) for each fan. They are 80mm PC fans.

author: Ian.M
author_rank: Super Contributor
timestamp: March 15, 2021, 12:15:40 pm
content: Three in parallel for a load current of 450mA *should* be OK on one buck module at normal room temperature. Two would be significantly safer, and would give you margin for the fan current to increase e.g. due to a failing fan.However, as they are so cheap in five or ten packs, I'd probably use one per fan as its much easier to wire that way. Thermal losses are roughly proportional to I2so if you keep the current low enough the module wont heat up significantly, allowing you to simply insulate it with heatshrink sleeving over it about 2cm longer than the module,If you expect the local temperature round the module to be much higher than ambient, or its got to function during peak summer temperatures in a non-airconditioned location, *PLEASE* do some testing to check the temperature rise at your fuil load current then work out how close to its limits you'd be pushing it. The silicon is rated for max. 125 deg C, but I wouldn't trust the manufacturer to have fitted 105 deg C rated capacitors, and as they are in close proximity to the chip and the inductor both of which run hot at high load currents, I'd want to keep measured surface temperatures well under 85 deg C at the maximum local temperature it can be expected to have to run in. Considerable advantage may be gained by mounting the module edge on to and in the fan airflow.

author: Godzilla
author_rank: Contributor
timestamp: March 15, 2021, 12:39:13 pm
content: Im gonna use only 2 fans, for the ambient temperature i think that at max it is gonna be 30 degrees, and anyway i think that i will only use it in short periods of time (30min max).Since fans are pretty small i dont know if put the modules on the airflow will limit it too much... Anyway i will check the temperatures when ill try it. Is the temperature only proportional to the current? For example if im gonna reduce the voltage more to change the speed, will the module produce more heat cuz of more regulation?

author: Ian.M
author_rank: Super Contributor
timestamp: March 15, 2021, 02:47:09 pm
content: Changing the input to output voltage ratio while maintaining a constant load current changes the ratio of the internal pass transistor ON time to the external catch diode's forward conduction time. As the output voltage is increased, the transistor ON time increases and the diode conduction time decreases. This shifts dissipation between the 'LM2596' and the diode and also alters the total dissipation. The exact behavior is extremely complex as neither the catch diode nor the internal pass transistor are linear resistive devices while on/conducting.As the 'LM2586-ADJ' chips on these cheap modules are almost invariably fake, even T.I's WEBENCH design tool or their PSPICE model wont reliably give you the dissipation for a particular set of input and output voltages and load current. The fake chip is believed to be similar to T.I's older LM2576-ADJ chip, but without die photos to compare, there's no reason to suppose the LM2576 models would be more accurate.Therefore you'll have to determine the temperature rise experimentally for the load current and output voltage ranges you are interested in.

author: Godzilla
author_rank: Contributor
timestamp: March 15, 2021, 07:57:36 pm
content: OK, im gonna test them when they arrive!

author: james_s
author_rank: Super Contributor
timestamp: March 16, 2021, 05:09:21 am
content: Quote from: Ian.M on March 15, 2021, 02:47:09 pmThey blow up if you try to run them continuously at the full rated 3A, but as long as you derate them a bit they work fine. Unless these are really big fans I don't think there will be a problem. They've been reliable for me, and while I'm annoyed that they're misrepresented parts, for the price they're a bargain. I don't know why they don't just sell them under a legitimate part number, people will still buy them if they work.

author: archil
author_rank: Contributor
timestamp: March 16, 2021, 08:27:32 am
content: you need 7812 voltage regulator. schematic is simple, up to 1A all nice.

author: Zero999
author_rank: Super Contributor
timestamp: March 16, 2021, 08:31:17 am
content: If you're going to buy a DC:DC converter, then why not simply just buy the correct voltage power supply for the fan? Or skip the power supply altogether and use mains fans, as long as they're in a situation where the mains cabling can be adequately secured and insulated.

author: archil
author_rank: Contributor
timestamp: March 16, 2021, 09:11:12 am
content: you can also use dc-dc (step down converter with voltage regulation) and adjust fan speed with potentiometer, but fans must be connected in parallel (because dc-dc converters voltage drop).

author: Godzilla
author_rank: Contributor
timestamp: March 16, 2021, 10:31:48 am
content: Quote from: Zero999 on March 16, 2021, 08:31:17 amI just had an old printer power supply and 2 80mm fans lying around, so i wanted to use them. Also these buck converters are really cheap, i can buy 10 of them for less than 10€.Quote from: archil on March 16, 2021, 09:11:12 amYes, this is what im going to do, i'll just use one converter for each fan.

author: Ian.M
author_rank: Super Contributor
timestamp: March 16, 2021, 12:14:34 pm
content: Quote from: archil on March 16, 2021, 08:27:32 amThat has the same disadvantage as a 12V series Zener. We now know the fans draw between 0.15A and 0.2A so dropping 12V, that's a dissipation of between 1.8W and 2.4W for a single fan, double that for two in parallel, which is more than a TO-220 package in free air can handle without excessive temperature rise. Therefore if you ue a linear regulator, it *WILL* need heatsinking, so the total cost is likely to be higher than using 1€ buck modules. Also, the input and output decoupling capacitors shown in the 7812 datasheet typical application circuit are *NOT* optional. With the wire lengths involved and a load that's probably fairly inductive, there is a significant risk of the regulator being unstable without them, and no-one wants even a small risk of one acting as a HF band power oscillator modulated by the varying current draw of the fan's BLDC motor anywhere near any sensitive circuits!OTOH 7812 regulators rarely go short-circuit, (in my experience their typical failure mode is going hi-Z), so the odds of the fans surviving a regulator failure are considerably higher than if buck modules are used. However if the 7812 regulators aren't adequately heatsinked, the odds of failure of one run continuously close to its thermal limit are IMHO higher than a lightly loaded 1€ buck module running relatively cool, in spite of the greater complexity of the buck module and the dubius source of its semiconductors.

author: CaptDon
author_rank: Super Contributor
timestamp: March 16, 2021, 01:44:09 pm
content: I have a somewhat unique situation of exactly two 12vdc fans in series running on 24vdc. The fans are physically bolted together with one feeding the other and used to push air through the radiator of an 'external anode' transmitting tube. The 12vdc fans are much cheaper than the 24vdc version. Each Fan has a zener across it for balance at startup. I think they are slightly over 14 volt zeners at 1 watt. Normally they are not conducting so no additional loss. The magnification of airflow and available pressure is incredible with the two fans bolted together for 'series airflow'. The 'easy to start' fan will move enough air to help get the sluggish fan up to equalization quickly!!

author: Godzilla
author_rank: Contributor
timestamp: March 16, 2021, 02:10:11 pm
content: Quote from: CaptDon on March 16, 2021, 01:44:09 pmInteresting, for series u mean one behind the other? I honestly didnt think about fans placement yet...

author: CaptDon
author_rank: Super Contributor
timestamp: March 16, 2021, 05:31:42 pm
content: Yes, one fan behind the other. The second fan is receiving a somewhat condensed charge of air and so it seems to work slightly more efficiently and adds additional flow and pressure boost. I don't think you get twice the CFM, probably not, but for cooling the transmitter tube the pressure boost does wonders as the tube's radiator has about a 1.5" water column of pressure drop at rated airflow. We have found it is more efficient to 'push' air across the tube since this makes the air more dense and able to remove more heat as opposed to 'pulling' the air through.

author: james_s
author_rank: Super Contributor
timestamp: March 16, 2021, 05:49:33 pm
content: Quote from: Ian.M on March 16, 2021, 12:14:34 pmIt may be rare but it can happen. I had a 7805 go short and killed a somewhat expensive bit of hardware with 9V when I was a teen.

author: gpr
author_rank: Contributor
timestamp: March 17, 2021, 12:21:50 pm
content: Quote from: Ian.M on March 16, 2021, 12:14:34 pmIf you use 12v regulator to generate stable middle point and power one fan from upper 24v-12v, another from 12v-0v, linear regulator will not have to dissipate a lot of power. Upper fan would behave like "pass fan". Of course, if one fan fails, it will be hard for such regulator. I'm not suggesting this, just some thoughts.But we can use switching regulator to generate middle point. Load on such regulator will be near zero if fans are equal, up to one fan's load in worst case. Does it sound like an acceptable solution in practice?UPDATE: it seems, if "bottom" fan fails, or just happen to require less power than "upper", regulator should be able to sink current.

author: Ian.M
author_rank: Super Contributor
timestamp: March 17, 2021, 12:49:47 pm
content: Quote from: gpr on March 17, 2021, 12:21:50 pm7812 regulators cant sink current so your idea wont work unless you can guarantee that the lower fan will *ALWAYS* draw more current than the upper fan.Most switching buck regulators cant sink current either. You'd need one capable of two quadrant operation. Some designed for synchronous rectification can sink current, but as synchronous rectification is primarily offered to improve efficiency of low voltage and/or very high current switching regulators, you will have some difficulty finding one. Finding an affordable pre-built module will be near impossible.

author: Zero999
author_rank: Super Contributor
timestamp: March 17, 2021, 01:25:02 pm
content: It can be done with a couple of emitter followers, but I prefer the zener idea, due to simplicity.EDIT:The circuit originally attached was wrong. Here's the corrected one.

author: bobbydazzler
author_rank: Regular Contributor
timestamp: March 17, 2021, 01:36:53 pm
content: Most modern pc computer fans don't use anywhere near their max rated current except maybe momentarily during spinup. I just tested a 4 pin silent wings 3 120mm fan which is rated for 0.2A according to what's written on the fan but it only pulled between 50-60mA while using 12v input power and when I reduced the voltage to 7.1v the current dropped to between 30-40mA. I saw 90mA briefly during power on with the 12v supplied voltage.

author: gpr
author_rank: Contributor
timestamp: March 17, 2021, 02:15:25 pm
content: Quote from: Zero999 on March 17, 2021, 01:25:02 pmI suppose you should swap pnp and npn transistors in your schema. Also, followers will work only when middle point between fans moves more than Vbe drop from +12v. Not a big deal for fans though.

author: Zero999
author_rank: Super Contributor
timestamp: March 17, 2021, 04:53:13 pm
content: Quote from: gpr on March 17, 2021, 02:15:25 pmThat would result in magic smoke.QuoteTrue, but that's the point. When both fans are running they should roughly have 12V across each of them and the transistors won't conduct.

author: Ian.M
author_rank: Super Contributor
timestamp: March 17, 2021, 06:46:40 pm
content: Quote from: Zero999 on March 17, 2021, 04:53:13 pmIt already results in magic smoke, or at least crowbarring the 24V supply if the reverse hFEof both transistors is high enough to avoid melting their base bond wires! Once you've fixed it by swapping the transistors so the NPN collector goes to +24V and the PNP collector to groundas is normal for emitter followers, you still have the problem that with a Thévenin equivalent resistance of 5K at their bases, *IF* the hFEis 100, the fans will 'see' a 50 ohm equivalent resistance at the emitters, which is an additional 5V drop for every 100mA of imbalance. Its crying out for a pair of power Darlingtons to keep the equivalent resistance at the emitters low enough to be useful. An improved base bias network with a Vbe multiplier could be used to reduce the +/- 2 Vbe deadband about Vcc/2, though it would be a tradeoff against increased quiescent current and careful design and construction would be needed to eliminate the risk of thermal runaway.As can be seen from the above discussion, a rail splitter that's simple in concept is rarely simple to implement if you want good performance from it, so if you need something more effective than the 15V power Zeners (originally suggested by David back in reply #15), it may be advisable to bite the bullet and use a power OPAMP., as at least that gets it down to three components: two resistors for the Vcc/2 divider, and the OPAMP itself strapped as a unit gain non-inverting buffer.

author: Zero999
author_rank: Super Contributor
timestamp: March 17, 2021, 09:20:57 pm
content: Quote from: Ian.M on March 17, 2021, 06:46:40 pmThanks for spotting that. I can't believe I made such as silly mistake.I've renamed the wrong schematic, so it no longer pops up, but it's still there for entertainment value and so everyone knows what you're talking about. Here's the corrected version.QuoteIt's not that bad, since the transistors will only conduct during start-up and the large capacitors will provide a much lower impedance to the bases, as the motors get up to speed. Yes, I did think about Darlington pairs, but wanted to keep it simple, although it would eliminate the need for the capacitors so, perhaps the TIP121 and TIP126 could be used.QuoteYes, an op-amp could be used, or even an audio amplifier IC, but it's a bit overkill.

author: Ian.M
author_rank: Super Contributor
timestamp: March 19, 2021, 03:24:54 pm
content: Quote from: Zero999 on March 17, 2021, 09:20:57 pmIt may be overkill, and not particularly cheap, but the advantage of a single package to heatsink, and a circuit so simple it can be wired direct to the OPAMP's pins without a PCB *MAY* make it worth considering if for any reason switching regulators or even a switched mode rail splitter have been ruled out.