Router temperatures

[StR]

this is when I truly wish I had CAD skills.
I have been imagining designing a manifold/duct that attaches to the bottom of our AC86 routers where the stand "leg" hides the bottom air vents. place a fan blowing into it, and thus, bringing cool air into the bottom, and allowing it to escape through the back and top vents.
(perhaps my description is adequate to inspire someone with the skills I lack...)
I'd wager a standard 12v fan running on the 5v of the USB port would push more than enough air through to drop temps into the "acceptable" range.

When you are trying to come up with the engineering solution, you need to formulate clearly, what you are trying to achieve. While offering your solution here, you've never said what you are trying to improve or optimize.
Why am I saying "optimize"? Because there is at least one obvious simple solution that works and that several people have used: just having any fan lowers the temperatures in the router.

I think, I understand what you are trying to do from your description: you are trying to optimize for the minimum active air flow needed while reducing the temperatures (of all WiFi chips & CPU) to the "acceptable" range. Did I get it right?

My main question about that: what would that gain you - over a reasonable off-the-shelf USB fan - either with low-noise dual-ball bearings (like AC Infinity MULTIFAN series), or SSO bearing (such as Noctua)?
They are quiet enough for most people and already provide reasonable temperatures.


In my mind, there are two classes of engineering solutions for the cooling of the routers: (1) fanless and (2) with fans. ASUS engineers have [marginally] failed to create adequate fanless solution (at least for the updated firmware). One can either improve on the fanless solution (which was recently discussed in this thread), or use a fan.
With the ultrathin electronics like laptops, tablets, and phones, they are trying to optimize (minimize) the fan size. For the majority of the situations, this is not a critical constraint in case of the routers discussed here.

The factors that may need to be considered here are noise level and dust accumulation. The noise levels with the aforementioned fans appear to be satisfactory, so no improvement seems to be needed there.
I do not see an obvious indication that your design would improve the dust situation.

So, what are you trying to achieve with the proposed design?
(I am not criticizing the design, I am just missing the goal here.)

 

[mromero]

If you leave it upright and waft a little air at the back side, particularly the right side, you don't do much more for the front side of the board and a lot for the back side, but at least you don't tamper with the front side.

The rear end is where the cpu, radios and common heatsink are located. The analogy of a heatsink and fan mounted on a computer cpu applies.

 

[anotherengineer]

My AC68U now
2.4 GHz: 44°C - 5 GHz: disabled - CPU: 67°C

Got an 80mm Noctua NF-A8 5V in mail today with USB.................one of these days when I get around to putting it on I will report back.

ooo just plugged it into charged for a test, I think it's 2200rpm, blows good but a bit of noise, maybe I will put on one of the low noise adapters first.

edit 2 - well low noise adapters did not work - probably cause they were for 12V model fans, so put it on there full speed and noise, might put an inline resistor or pot later on in the future to adjust speed/noise and increase fan life.

Probably take 30 minutes for temps to stabilize, but 5 minutes later they are.......

2.4 GHz: 37°C - 5 GHz: disabled - CPU: 49°C

edit 3

2.4 GHz: 35°C - 5 GHz: disabled - CPU: 46°C

I guess that's about it

 

[StR]

When you should test what is more efficient: blowing fresh air inside or extracting hot air from the router.

I agree with people who said that "blowing" is more efficient than "sucking" -- using the fans that we are discussing.
As a proof, here is the plot of the temperature rise after I flipped the fan attached to the back of the RT-AC86U router - in the configuration I described above.
The temperatures went from 38C, 46C, 51C to 40C, 48C, 56C, respectively (those are sustained numbers).
It is noteworthy that both fan orientations produce sufficient cooling, and the temperature difference between the two is much smaller than the temperature drop due to addition of the fan. (For reference: the corresponding temperatures without the fan were 54C, 58C, 90C.)

However, I disagree with some of the arguments why blowing the air in is more efficient here than sucking it out.

I think this has been discussed before. Looking at the build construction the air is drawn into the back of the router and goes out on top. Therefore I'm blowing air in through the back, so it leaves the router at the top. I suspect you will only try to defy physics when sucking air out of the router at the back when it's constructed to release its warmth at the top.

Using this argument, one might conclude that sucking at the top would have been equally efficient as blowing at the back.
The most significant shortcoming of this argument is that the argument related to the "natural" air flow is not applicable here, - at least in case of rather powerful fans, such as 120 mm (and probably even 80mm) fans - Infinity S3, Noctua, etc., because you are changing the regime of the air flow from free (aka natural) convection to forced convection in a drastic way (as opposed to just adding a little bit of a flow).

Blow, not suck.

Air is the medium to move the heat away from router internals much like the water/coolant mixture works in internal combustion engines. air absorbs the heat. you want to add more cool air into the router to take the heat away. "exhausting heat" without sufficiently replenishing the cooling mass doesn't work as well as pushing cool air through the machine. In fact, the egress has to be restricted to an extent to allow the cooling medium to absorb enough heat to justify the energy spent to push it through. like someone above said, they could run their fan faster, but the noise would bother them and it might not help cooling. it doesn't take a hurricane - just enough of a pressure increase inside the case will do the trick to put more air in contact with hot surfaces for long enough for a transfer between them and to keep the heat moving away from where it hurts things by building up.

This argument has some reasonable thoughts, but they are mixed with some incorrect ones, yielding a mess.
I am not going to analyze it in detail, but just one fact: the amount of air coming into the router is equal to that coming out of the router; there is no "build-up" of air inside. There is not need for restricted egress; rather, in the regime being discussed here, any flow restriction decreases cooling efficiency. Removing the back cover and providing uniform air flow along the fins. So, for the purpose of cooling with a fan, removing the back cover and blowing along the heatsinks/chips would be the best.


The main three factors for air cooling of an object are (1) the temperature of the cool air (or more exactly the temperature difference between the cooled object and the air) and (2) the air flow velocity (measured in LFM - linear foot per minute or in m/s - meter per second) at the hot surface (spot), and (3) the area of the hot surface (surface of the hot object) in contact with the air.

The linear velocity is related to air flow rate - usually measured in CFM (cubic feet per meter minute[*]) [US] or cubic meters per hour (m³/h) [SI units in most of the rest of the world], but it depends on the geometry. (There are also other nuances, e.g. you want to have a turbulent flow near the hot spot - for more efficient heat transfer from the surface and laminar flow away from it - for more efficient heat removal.)

In my opinion, the main factor why blowing the air into the router yields better cooling compared to sucking it out is the [constricted] geometry of the router and its internal elements and the geometry of the typical fans discussed in the thread: by blowing you produce a higher air velocity directly over the heatsinks/chips, and you create a more turbulent air flow at the heatsinks as compared to suction mode.

It is noteworthy that under different conditions (geometry of the enclosure and hot components), you might be able to achieve similar or even better cooling using suction.
There are many discussions and examples on the internet. I can refer you to, e.g. this detailed answer:
https://electronics.stackexchange.com/a/305659 and yet another answer referenced therein - for enclosures like we have: https://electronics.stackexchange.com/a/6382 .

----
[*] Thanks to @heysoundude for noticing the typo!

 

[StR]

Haven't paid attention to this in a long time, but I feel the urge to dip in here and then run away. I disagree with this and think @heysoundude and @Hazel are more in the right.

There are two sides to the board. I'll refer to the "front side" of the board as the side facing the front of the router (thiggins refers to it as the "bottom"), and the "back side" being the side on which the LAN ports, etc. are mounted (aka the "top"). Teardown The front side has a metal plate. In its designed position, the vents along the bottom, below the LAN ports, feed both sides of the board, and the natural convection across both sides of the board exits the top, including, with respect to the front side of the board, especially the row of vents across the top in front of the antennas. There is more venting across the back of the router.

If you lay it down, then there is no convection across the front side of the board. However much you overpressure the back side, you aren't reaching the front side.

If you leave it upright and waft a little air at the back side, particularly the right side, you don't do much more for the front side of the board and a lot for the back side, but at least you don't tamper with the front side.

Let me ask you: what is on the "front" side of the board that needs cooling?
I don't know what's on the back, but according to that nice article by thiggins, all the heatsinks are on the "back"/"top" side of the board.

Additionally, - what is the purpose of the metal plate? I have some guesses, but it would help knowing what's on the back side of the board.
Maybe people who have opened their router(s) can shed some light on that "back side of the Moon"? (@thiggins, @Tech9, ...)


I cannot tell exactly without opening the router enclosure, but it looks that the board is mounted at the level at the front end of the grille openings at the top of the router when it is standing upright, between the antennas. I marked that level with the orange line on the image below. If this is correct, it means that no significant air flow is expected along the front side of the board. This would be consistent with no cooling needed there.

 

[mromero]

Got an 80mm Noctua NF-A8 5V in mail today with USB.................one of these days when I get around to putting it on I will report back.

Noctua NF-F12 PWM 5V 1500RPM 120mm Focused Flow Cooling Fan is what you may need.

 

[heysoundude]

The linear velocity is related to air flow rate - usually measured in CFM (cubic feet per meter) [US] or cubic meters per hour (m³/h) [SI units in most of the rest of the world], but it depends on the geometry. (There are also other nuances, e.g. you want to have a turbulent flow near the hot spot - for more efficient heat transfer from the surface and laminar flow away from it - for more efficient heat removal.)

That's Cubic Feet per Minute. cubic feet per meter is mixing units of measurement...speaking of messes.

 

[StR]

That's Cubic Feet per Minute. cubic feet per meter is mixing units of measurement...speaking of messes.

Thank you for noticing and helping to correct my typo!
Was it the only one I made?

 

[Tech9]

This would be consistent with no cooling needed there.

One often burning VRM is on the back side of AC86U PCB. The one that causes dead router with LAN 4 LED illuminated. There is no airflow there.

 

[StR]

One often burning VRM is on the back side of AC86U PCB. The one that causes dead router with LAN 4 LED illuminated. There is no airflow there.

Thank you!
I should've said "This would be consistent with no cooling *intended* there."

Just out of curiosity, is that the element exposed in the opening of the metal plate close to its center? (Thanks to thiggins for the photo)

What do you think is the purpose of the metal plate: structural? heat spread/conduction? electromagnetic screening?

 

[elorimer]

Maybe people who have opened their router(s) can shed some light on that "back side of the Moon"? (@thiggins, @Tech9, ...)

Pictures at 412 and 413 here: FCC Pix

So, the LEDs (about 2C if you turn off), 5 thermal pads transferring heat to that big plate from the other side of the board on which is mounted the two radios and the memory chip (but not the CPU, huh).

Also, from the picture at 410, it looks like that side of the board vents top and bottom. From the picture you posted I think the edge of the board is at the solid part of the housing above the red line: the horizontal top vents are exclusively the top of the board, and the vertical vents are exclusively the bottom of the board.

 

[RMerlin]

Just out of curiosity, is that the element exposed in the opening of the metal plate close to its center?

That`s the fourth antenna.

What do you think is the purpose of the metal plate: structural? heat spread/conduction?

Both EM shielding and head spreader. There are thermal pads underneath these recesses, with the chips actually on the other side of the pcboard (with a heatsink covering all three chips).

 

[elorimer]

That`s the fourth antenna.

The yellow boxed thing, yes, from the picture he pasted in. But I think he was referring to the gray thing with the two silver wings in the hole, not in the yellow box.

 

[heysoundude]

Thank you for noticing and helping to correct my typo!
Was it the only one I made?

What are the chances you wear the iron ring? with you talking about turbulent airflow over hot spots, can I assume a dissertation on your research into how increasing Reynolds number of the airflow through Asus upright router models prolongs their MTBF by dropping temperatures of internal componentry?

 

[StR]

The yellow boxed thing, yes, from the picture he pasted in. But I think he was referring to the gray thing with the two silver wings in the hole, not in the yellow box.

Yes that's what I meant.

 

[gattaca]

Since this thread seems to split frequently, please see this posting, item #7. I think there are 4-6+ "fan temp" thread floating around!

https://www.snbforums.com/threads/asus-rt-ac88u-to-hot.58405/page-3#post-665525

The 5V Noctura's are way too loud for my use. I've detailed several times the best Noctura options for 12V units which run well at 5V. Not ALL the 12V Noctura fans will start up at 5V!! I found only a few models that will and have been using them for several years now. Once connected by a USB to 2 pin fan connector (also listed), these are quiet, effective and fall into KIS. Stay safe, stay alive. Peace. Have a great weekend!

 

[Tech9]

Yes that's what I meant.

No, what burns is a synchronous step down converter MT3125. It's located under the shield, to the right of this cutout. The visible component is an inductor, connected to pins 1-2 of MT3125. I don't have an AC86U open at the moment, but the issue is very common and I can borrow a picture from other sources:

The aluminum plate covering the back side of the PCB is more heatspreader than RF shield. Radio modules are shielded already with grounded metal box on one side and grounded PCB layer on the other.

 

[taffeys]

Since this thread seems to split frequently, please see this posting, item #7. I think there are 4-6+ "fan temp" thread floating around!

https://www.snbforums.com/threads/asus-rt-ac88u-to-hot.58405/page-3#post-665525

The 5V Noctura's are way too loud for my use. I've detailed several times the best Noctura options for 12V units which run well at 5V. Not ALL the 12V Noctura fans will start up at 5V!! I found only a few models that will and have been using them for several years now. Once connected by a USB to 2 pin fan connector (also listed), these are quiet, effective and fall into KIS. Stay safe, stay alive. Peace. Have a great weekend!

Can you tell us the model #s of 12V Noctura fans that will start up at 5V? I have many as Noctura fans are my choice for all builds. Thanks!

 

[Deldarius]

While I applaud peoples desire to crack open a router to diddle with the internals, I find that a bit excessive. I find a $10-15 desk fan and a swath of the missus's pantyhose (swapped weekly) for a filter does the trick.

It may not be the most elegant nor the quietest solution, the results speak for themselves. The room is currently 80f/26c and the router is running at a nicely chill 44c-50c-54c.

The fan has enough spill over to keep my nas and modem cooled, me as well.

 

[slo-nic]