Question Pwm Vs DC Vs Auto

[Jaydeesus]

I went into my bios looking for a fan curved setting,I encountered auto,pwm and dc,it was set to auto but I tried the other 2 and saw no difference,should I keep my cpu fan at auto or change it to something else.Also my 3 pin in take fans were set to dc,I tried pwm and they were very loud,is it ok if I have them set to DC?Pc fansC cpu fan:Auto,is that ok?

 

[Lucretia19]

I mean the minimum rpm I can get on my cpu heatsink fan is much lower using DC mode instead of PWM for some reason.

I'll assume you meant that in DC mode the slowest the fan will spin is 450 rpm, unless you slow it all the way down to 0 rpm.

 

[prophet51]

Yes 450rpm is the lowest rpm it will spin but I also have the fan turn off below 37c cpu temp.

 

[Lucretia19]

@prophet51: Yes, setting the cpu fan to 0 rpm when the cpu is relatively cool makes a lot of sense to me. (1) My understanding is that temperature changes are more harmful to a component than a steady temperature (due to expansion and contraction) so why make cpu temperature changes bigger than necessary by fan-cooling the cpu to a more extreme low (unnecessary contraction)? (2) Spinning the fan costs a bit of electricity and some wear-and-tear on the fan bearing, and generates a bit of heat inside the case.

On the other hand, there may be reasons that I don't know, in favor of keeping the fan spinning. For example, if the load on the cpu suddenly becomes heavy (like starting a video encoding), perhaps the time that it takes the fan to respond will be significantly longer if the fan has to climb from 0 rpm.

 

[prophet51]

I've tested aida64 stress test with the fan at 0rpm and it's OK the fan easily ramps up in speed fast enough. The main reason I set 0rpm fan off below 37c is to reduce the speed dust builds up in the fan/heatsink.

 

[Paperdoc]

prophet51, I am intrigued by your statement, "It also lets my cpu fan completely stop when the cpu's below 37c ". A chassis fan header generally uses the temperature sensor on the mobo, and has NO knowledge of the internal CPU chip temperature. So, how does it know that the CPU is below 37C? MAYBE you have a mobo that allows your CHA_FAN header to use the CPU temp instead?

 

[prophet51]

My case fan lets me use the cpu socket temp. I can't use the cpu tdie sensor even when I'm plugged into the cpu fan header.

 

[Lucretia19]

The main reason I set 0rpm fan off below 37c is to reduce the speed dust builds up in the fan/heatsink.

Do you have dust filter(s) in front of your intake fan(s), and is there positive air pressure inside the case that would deter dust from entering through unfiltered gaps in the case? (To maintain positive pressure, the intake fan(s) should be set to spin fast enough that air is pulled in faster than the exhaust fan(s) try to push air out.)

 

[prophet51]

No I only have a single exhaust fan.

 

[Lucretia19]

No I only have a single exhaust fan.

I assume you also mean you have no intake fans. Given a choice between an intake fan and an exhaust fan, I would choose to have an intake fan, because that would establish positive pressure inside the case and provides a single point to filter to keep out dust. The negative pressure that an exhaust fan establishes pulls dust into the case unless you filter the many openings of the case; it's much easier to filter intake fans (assuming positive pressure) than to filter the many openings (without positive pressure).

Does your case have a mounting point for an intake fan? If you don't want to buy another fan, would the fan you're using as an exhaust fan fit the intake fan mounting point?

If your case doesn't have a mounting point for an intake fan or if the fan being used as an exhaust fan wouldn't fit the mounting point, you could consider turning the exhaust fan around so it will function as an intake fan, and putting a filter outside it.

In my case, the ssd drive and hard drive are mounted right behind the two intake fans (at the lower front of the case) so another benefit of the intake fans is better cooling of the drives. The two intake fans are the fastest spinning fans in my system (to establish positive pressure and cool the drives); I set the exhaust fan to spin slowly when the cpu and vrmos are both relatively cool.

 

[prophet51]

Yeah but my 120mm exhaust fan is my psu. I'm pretty happy with the thermal performance of my case I have all the side panels and front panels removed with a towel placed over the top of it to stop dust falling in. The good thing about this setup is I can leave a small gap at the bottom for the gpu heat to escape so I don't need a case fan really, and my cpu runs really cool too.

 

[Lucretia19]

Paperdoc wrote:

A chassis fan header generally uses the temperature sensor on the mobo, and has NO knowledge of the internal CPU chip temperature.

Why do you believe that's generally true? The BIOS on my MSI X470 Gaming Plus allows me to choose from four temperature sources for each fan header, and one of the four is the cpu temperature. The FanControl software I mentioned gives even more choices for each fan header.

 

[Lucretia19]

prophet51 wrote:

I'm pretty happy with the thermal performance of my case I have all the side panels and front panels removed with a towel placed over the top of it to stop dust falling in.

Sounds unusual but I won't argue with success. It might be more accurate, though, to say you're happy with the thermal performance of the little that remains of your case.

 

[prophet51]

Yeah my case is pretty ghetto, here's a pic with the towel covering removed. I forgot that I do have a front case fan but it plugs into a molex connector and runs 1500rpm non stop so I had to unplug it. I have the old towel covering all the way down to just below the GPU.

https://imgur.com/a/IAXmvu1

View: https://imgur.com/a/IAXmvu1

 

[prophet51]

Also here's a printscreen of my bios, it lets me choose either cpu or motherboard for the chassis 2 fan header. The cpu is just the socket temp it's not the on die sensor and I think motherboard is the chipset temp.
https://ibb.co/0VL1bf9

 

[Lucretia19]

I do have a front case fan but it plugs into a molex connector and runs 1500rpm nonstop so I had to unplug it.

Reasonably quiet PWM fans have gotten pretty cheap, if you ever decide to replace the molex fan. Or perhaps you could construct a fanheader-to-molex adapter cable, and set the fanheader to DC mode at less than 100%. To reduce the negative pressure or establish some positive pressure.

Do you have any concern about EMI radiation causing interference with your neighbors' equipment? If so, perhaps it would make sense, instead of a towel, to drill holes in the sides of the case so the sides would allow air flow but block EMI. Or a sheet of aluminum foil on each side, with some holes cut for airflow.

For the sake of completeness, I should also have asked whether the room in which the pc is located is air conditioned, and if so, is the air conditioner loud enough that case fans would be quieter (and less expensive to run).