Cooling fan witchcraft

[Childs]

This is more of a curiosity. I'm replacing the cooling fans in a 24 port switch to make it quiet for the office. I've done it before on FC and 48 port Gb switches. So when I replace the two 40x20mm fans in this switch, I noticed only one would spin unless I move the fan, then both would spin. Turn it off, then back on, and only one fan would spin again.

So I looked at the original fans, and they were Delta EUB0405MD fans.
5V
6300RPM
.15A
7.2CFM

The fans I replaced them with were Scythe SY124020L
12V
3500RPM
.07A
4.86CFM

I am assuming the voltage differential between the two fans is causing the problem. So doing some research I see I need to add a resistor to the scythe to get the voltage to match the original fan. I don't care if it cools less, the environments will be cool enough and the load on the switches won't be that much. But the curiosity is the numbers, and the behavior of the fans in the switch. Why would a new fan have higher voltage but use less actual current? And does this differential in voltage explain the behavior I see in the switch? Or is it literally witchcraft, or some kind of voodoo?

 

[BonzaiDuck]

This is more of a curiosity. I'm replacing the cooling fans in a 24 port switch to make it quiet for the office. I've done it before on FC and 48 port Gb switches. So when I replace the two 40x20mm fans in this switch, I noticed only one would spin unless I move the fan, then both would spin. Turn it off, then back on, and only one fan would spin again.

So I looked at the original fans, and they were Delta EUB0405MD fans.
5V
6300RPM
.15A
7.2CFM

The fans I replaced them with were Scythe SY124020L
12V
3500RPM
.07A
4.86CFM

I am assuming the voltage differential between the two fans is causing the problem. So doing some research I see I need to add a resistor to the scythe to get the voltage to match the original fan. I don't care if it cools less, the environments will be cool enough and the load on the switches won't be that much. But the curiosity is the numbers, and the behavior of the fans in the switch. Why would a new fan have higher voltage but use less actual current? And does this differential in voltage explain the behavior I see in the switch? Or is it literally witchcraft, or some kind of voodoo?

Just for the last question: ". . . does the differential in voltage explain" it -- I would think so. All fans have a minimum "start" voltage as I understand it. So I guess the question would be "where ya gonna find 5V fans?"

You can make a 12V power connection stepped down to a 5V with a regulator, but I'm not sure how you do the reverse or if possible. Somebody would know. I don't think adding the resistor into the equation would change anything about the start voltage. Maybe I'm wrong about that . . .

I just never saw any 5V fans before seeing your link to the Delta, but then -- I never looked for any. . . . So . . . why can't you buy the Deltas? Those 40mm jobs aren't very expensive. Oh. you wanted to make it more quiet. I see . . .

 

[Childs]

It might make more sense to apply the resistors to the original fan then to simply reduce the RPM which should lower the noise. I did see some 12v fans with 5v start, but they are like $15 each. I should probably just get them since its not like you can buy 24 port managed switches for $30.

EDIT: I decided to buy some Sunon KDE0504PKV3.

5V
5200RPM
.12A
6.3CFM

Pretty close to the spec for the original fans, but at only 18dba, which is actually quieter than the Scythe fans I bought. Uses some sort of maglev system. Bought 6 at $5 each.

 

[BonzaiDuck]

It might make more sense to apply the resistors to the original fan then to simply reduce the RPM which should lower the noise. I did see some 12v fans with 5v start, but they are like $15 each. I should probably just get them since its not like you can buy 24 port managed switches for $30.

EDIT: I decided to buy some Sunon KDE0504PKV3.

5V
5200RPM
.12A
6.3CFM

Pretty close to the spec for the original fans, but at only 18dba, which is actually quieter than the Scythe fans I bought. Uses some sort of maglev system. Bought 6 at $5 each.

Good! Very good! If I thought there was a 5V model, I would have recommended something like a Sunon Mag-Lev fan.

I have a couple of the 12V 40x25mm model. Those are good little fans.

 

[Childs]

FYI, this ended up being somewhat of a hassle. The fans arrived, but with no fan connectors. I think no big deal I'll just go to Frys and get some. They apparently don't sell them! It looks like you can only get them for a reasonable price from eBay (China), but it'll take 14-30 days to arrive. So I solder the new fans to the old connectors and hook them up. 18db my ass, it was at least 28db, but probably more. Less than the original fans, but not by much. So now the plan is using resistors on the new fans to lower the voltage.

I figured I needed about 40 Ohms to reduce the voltage to 3v, which should drop the rpms to about what the Scythe was. Of course I don't have that resistor or anything close to it, and I figured I might need to play around and possible use smaller increments, so I buy a kit with a bunch from 1 ohm all the way to 1M. The kit and it arrives, I do some tests and find a good balance between noise and cfm, and go to put everything back together.

The damn screw holes on the new fan are super small! Difference of about size 4 (metric) vs size 6. And the fan housing is thin enough on the inner side that a bigger screw would probably crack the housing. So I go to a hardware store to get some smaller screws, but these screws are essentially sheet metal screws with no tip. The store doesn't have them in size 4, smallest is the one I already have. So I got some rubber washers (reduce some vibration noise), machine screws, washers and nuts.

So I ended up spending $30 for fans (6 fans, had to buy 5 min), $15 resistor kit, and $5 for fasteners. Plus $10 on the original Scythe fans, although I can reuse those on modern switches. It would have cost less than $1 if I just used a resistor and the existing fans. lol Live and learn I guess. A better mod would have been a couple of potentiometers mounted on the back, but by the time I thought of it I didn't want to bother with it anymore. All this because of 5v fans...

 

[fire400]

i'm at this point, too, debating about the case fans. thanks for sharing this.
i'm just going to slam a few dollars on sound dampeners.

i modified the molex connector splitting to 3 fans, using the yellow/red wire swap x2 trick, so three 120mm fans are now running at less than 50% with modest air flow to push most of any heat out from the chassis. i just ripped the yellow and black out and pushed them into the PSU wire molex and electrical taped it and zip tied, to save time/hassle.

it's not whisper quite, but it helps to keep the chassis noise a little less audible.