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| PWM Fan - can it run directly off 3.3v, 5v or 12v inputs/GND? | |
| Posted by: Trash80toHP_Mini on 2026-04-11 15:24:18 I've got a case with 3 PWM fans and an incompatible (Beige G3) logic board. I'd like two fans to run off a power level a step below the third in any combination of the three voltages available on an ITX PSU. Does this amount to a simple hotwire kluge or do they need a controller to function? | |
Posted by: luRaichu on 2026-04-11 15:47:28Does this amount to a simple hotwire klugeYes. Just hook up the red wire to your voltage and the black wire to ground | |
| Posted by: Byrd on 2026-04-11 16:59:05 ... they will work fine but full RPM, so check the specs Trash80 that you're not buying a wind turbine | |
Posted by: Trash80toHP_Mini on 2026-04-11 18:02:58... they will work fine but full RPMAssuming they're 12v max fans per PWM spec, they won't be full RPM @5v or 3.3v, no? so check the specs Trash80 that you're not buying a wind turbineI imagine I could build a hovercraft with a IIsi using these things. They were set up blow straight into this 2U server case thru a full frontal dust filter. I have them blowing out the backside, still sucking air in thru a different full frontal dust filter.ð  Ductwork will direct air straight thru the heatsink at 5v driven RPM, the other two are 3.3v RPM gravy I think. | |
| Posted by: Byrd on 2026-04-11 18:38:05 A 12V fan at 5V might not spin up, need to check specs of unit | |
| Posted by: Trash80toHP_Mini on 2026-04-11 19:37:11 Might it be as simple as adding resistors to limit the voltage reaching the fans from a 12v feed? I'd imagine that a control board with rheostats might suffice for a controller? | |
| Posted by: ymk on 2026-04-11 19:49:49 If you need speed control, get a PWM controller. If you just want to slow them down, fans can be grounded to 3.3V or 5V. | |
Posted by: Trash80toHP_Mini on 2026-04-11 19:57:26If you need speed control, get a PWM controller.No need for that. If you just want to slow them down, fans can be grounded to 3.3V or 5V.That's exactly what I was thinking in my original post. ð I have to use an ATX 24pin to 20pin adapter cable for the BG3 and I've got some short 24M-24F cables. The unused pins are 3.3v, 5v,12v and GND. To figure this out, I'm just gonna lop those wire off the 24 pin source ahead or the 24pin to 20pin adapter cable and fiddle with it. ðĪŠ | |
Posted by: Trash80toHP_Mini on 2026-04-12 08:48:47If you need speed control, get a PWM controller.Thanks much for that advice: KISS principle applied . . . $7.50 shipped and arriving tomorrow.  No soldering or wire kluge involved, pair should tuck right in under neath the Optical/Floppy bay or over the video connector? | |
| Posted by: herd on 2026-04-12 09:17:40 I like to use the temperature feedback controllers. The fans only run as fast as needed. | |
| Posted by: Trash80toHP_Mini on 2026-04-12 13:23:20 Makes sense for modern systems, but is there a way to sense temperature of a Beige G3 CPU or need to do so? Even with a "hot" accelerator, ducting air directly thru the heatsink, that fan running at half speed ought to suffice? Noise should be less than my MDD, no? | |
Posted by: Phipli on 2026-04-12 13:29:57Is there a way to sense temperature of a Beige G3 CPU or need to do so?even with a "hot" accelerator? Noise should be less than my MDD, no?You can do it in software with a lot of G3s (built in sensor). But probably easier to just drill a hole in the CPU heatsink and embed a thermistor in epoxy in the hole. Then if you want a weekend project, use the thermistor to control the duty cycle of a bistable 555 timer circuit. Or just buy a premade temperature to PWM circuit. (The ones you've bought might work but are actually designed for driving normal two wire DC fans or motors with PWM, rather than specifically PWM fans). Perhaps this? No experience of it myself, it is the type of thing I'd make my own so that I could tune it exactly how I wanted. 
12V Dual Ways / PWM PC CPU Fan Digital Temperature Control Speed Controller | eBay UKTFL (No. 1): The lowest temperature channel PWM setting, when ON state FAN1 PWM minimum is 40%, when OFF the minimum PWM of FAN1 is 20%. Support all within 12V 5A fans, synchronous rectifier output, high efficiency!
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(The blue board) Best solution though would be to use a MacSD because ymk has put all sorts of cool and customisable fan control features in it. | |
| Posted by: Trash80toHP_Mini on 2026-04-12 14:46:39 Interesting about the fan controller, thanks! I guess we'll see how that works out, it explains how they hook up to 12v/GND inputs, was wondering about that. MacSD does indeed look cool, but mass storage is a 2.5" Savvio UltraSCSI Server Drive/Apple Card like the high end MiniTower setups. I've got a USB multicard reader going in there that does Compact Flash/SD, MicroSD, two other types I've yet to figure out and USB out to take care of bridge machine duty. So three cards in the removable three slot riser cage, UltraSCSI, USB and a killer VidCard. Won't need to open that up unless something goes wrong. Remove four machine screws and it'll pop up for removing internal cabling and the address sensing cards in slots two and three. If I get my USB capable Whisper card up and running, can I skip the PCI USB card? | |
Posted by: ymk on 2026-04-12 17:07:20(The ones you've bought might work but are actually designed for driving normal two wire DC fans or motors with PWM, rather than specifically PWM fans). Agreed. It's better to use a fan's built-in PWM switching since you usually get better speed control and less motor whine. | |
| Posted by: Trash80toHP_Mini on 2026-04-12 17:20:05 Noted, Plan B initiated:  Noctua NF-R8 redux-1800, High Performance Cooling Fan, 3-Pin, 1800 RPM$14 x 3 = KISS? What's the third pin do, all the fans here I can think of around here are only two wire?edit: 1200 RPM ultra quiet Noctuas are the same price, shouldn't really need more than that, no. PSU has its own fan. | |
| Posted by: ymk on 2026-04-12 18:06:22 Third pin is RPM feedback. Fourth pin is PWM control. That's a non-PWM fan and I believe it comes with some inline adapters to reduce speed. | |
| Posted by: Trash80toHP_Mini on 2026-04-12 19:46:18 Thanks! So I should be able to treat them like two pin fans using the non-PWM controllers above? Going with the ultra quiet 1200 RPM fans. "1200rpm 3-pin version with excellent balance of performance and quietness, speed can be controlled with optional NA-SRC10 Low-Noise Adaptors or by reducing voltage" Overnight Prime delivery is getting here a day late . . . again . . . so it goes . . . | |
| Posted by: ymk on 2026-04-12 19:52:49 Yes, you can ignore the third wire. | |
| Posted by: Trash80toHP_Mini on 2026-04-15 14:40:26 Thanks for the help, gang. Planning to try the PWM fans in there first just cuz. If they don't work out the Quiet Noctuas seem the ticket. Did a quick prototype of the Front Panel. It'll be mounted lower, and thinner with size appropriate standoffs for the components. Need a bit more clearance for the speaker/mic jacks. Probably aluminum flat bar stock, but the siren song of clear Plexi is hard to resist and costs nothing . . .  All three fan connectors and one of the four pin Molex power connectors will be pruned to the appropriate length for insertion in the fan speed controller's blocks. Bare copper is probably best, but tempted to terminate the twisted wire leads with solder. Should I leave 'em bare? edit Clear Plexi Hinges, clear polycarbonate flat head machine screws and lock nuts will be ordered when I get back in town. | |
Posted by: Phipli on 2026-04-15 15:00:29Bare copper is probably best, but tempted to terminate the twisted wire leads with solder. Should I leave 'em bare?Ferrules on the ends are probably technically best for screw terminals, or ring / fork crimps depending on the type of terminal, but second to that, don't solder them, just plain stranded wire is better than soldered. Is what I believe I've been told. | |
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