2N7000 PWM Cooling Fan Control

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inglewoodpete

Senior Member
I am in the process of designing a fan controller for my Topfield 4400 PVR.

As background, I upgraded the hard drive from 80GB to 320GB. The specs said there was very little change in power consumption for that particular HD model. However, there must be additional heat that is tipping the Toppy over its limits. When cooped up in the video cabinet, the picture colour starts to lose its blues. When I open the doors on the video cabinet and hang the PVR partly off the shelf, the colours all restore indicating an overheating problem. The wife is not too impressed with my current cooling solution :)

My proposed solution involves a 14M using PWM to control a computer cooling fan (the Toppy does not come with a fan). There will be a couple of DS18B20s and an audio integrator to detect when the PVR is not in standby mode. The idea being, the fan only revs up when it is really needed, to keep fan noise at a minimum.

My question is about PWM using a 2N7000. The fan is a 12v 1.2w (Ie 100mA continuous). Reading the data sheet, the 2N7000 is rated at 200mA continuous (500mA pulsed). Is this suitable for PWM control of a DC motor? Does the reactive load affect the power rating of the FET? I'm trying to avoid using a larger FET with all the driver issues that raises.

-Peter
 
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moxhamj

New Member
Sounds like it should work fine. I doubt there would be much reactive load at 100mA. I guess one problem could be noise from the fan, and generally a slower bigger fan should be less noisy. Quality would be a factor too - ball bearings etc.

I did some work once at the bionic ear institute in Melbourne and the techies there said that real computers always have the cases removed. But maybe don't suggest that to your better half - it might cause angst.
 
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Dippy

Moderator
I have a Toppy TF5800. Superb.

PWMing a computer case fan? Mmm... I think you'll have to experiment as I've heard some fans won't be happy because of the built-in drive electronicss to make them brushless fans.
I'm not even sure it would work properly. Quite honestly I wouldn't do it. But I can't stop you.

You can buy temp sensitive fans for a fiver anyway so why faff? (they will have the temp sensing as part of their drive electronics which is far superior to external control)


I have been fiddling around with some 40mm fans for a project and quite honestly they are very quiet. esp the 10mm deep ones.

May I just suggest somthing on the subject of small fans:-
Finger guards: Don't use those cheap plastic ones (like sold by Rapid) as the 'body' plastic is so fat that it cuts the flow down h-u-g-e-l-y and the filtered version even more. Use the wire guards. Improved flow and lower noise.
A space/plenum. If you must use cheap plastic guards or filters fit some kind of spacer so that there is a slight plenum between filter and fan blades.
EBMPapst fans are much better balanced than cheap Anonymous fans - I have bought five different Beijing makes and whilst they are great value for money and work fine, they ain't so smooth as EBMPapst. But as everyone on this Forum is so tight then...
 
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Mycroft2152

Senior Member
My motherboard has a built in power saver fan controller setting. It will vary the speed of the fans depending on the temp inside the case.

Myc
 
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Michael 2727

Senior Member
Peter what is the Stall current of your fan ?

DC Brushless computer fans are electronically driven,
and as Dippy pointed out they may not like a PWM
supply.
I used 3x 1N4004 diodes to slow down a 12V PC fan
in my chicken incubator, below around 8V the fan
spitts the dummy.

I would go for an IRF540 myself, cheap & easy to source.
(driving such a low load current, may work down to 4V gate ?)
 
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inglewoodpete

Senior Member
I saw Dippy's post just before heading home from work. I had not thought of the fan being a brushless one but had thought to emulate the setup used in many PCs (as Mycroft suggested).

I managed to squeeze about 45 mins of experimenting late yesterday evening. The fan is a low noise 60mm Sunon brand one I got from Altronics. When connected directly to 12v, it shifts quite a lot of air but is relatively loud, mainly blade/air noise.

I then tried it on 10kHz PWM at 50% and 100% using the 14M. The PWM produced 2 different speeds, with the 50% PWM caused an irritating 'hiss' sort of sound from the motor. I guess the motor's electronics didn't like the chopped supply.

I then tried the same test, but with 20kHz PWM. Much, much better: just fan blade noise. Importantly, the 2N7000 temperature doesn't appear to change ("digital" thermometer: ie my finger! ;) )

I then tried a stepping between PWM levels to simulate temperature changes, using code:
Code: 
Init: PulsOut 1, 250          'LED added to show any resets
      '
Main: For b0 = 5 to 1 Step -1 '5 Speeds
         b1 = b0 * 40         'PWM at 40, 80, 120, 160, 200
         PWMOut 2, 49, b1     '20KHz @ 20/40/60/80/100%
         Pause 6000           '6 seconds: give fan time to settle at new speed
      Next b0
      Goto Main
The interesting thing was that when the speed was stepped down, most times the PICAXE would reset. Stepping the speed upwards was fine, so possibly some back-emf when the motor overruns when the PWM is reduced? I first tried a reversed diode across the fan motor: no improvement. Then I tried a 3.3uF capacitor across the motor. This radically reduced the torque of the fan but the speed did not reduce proportionately.

A 100nF capacitor across the fan seemed to make everything work at desired: lowest noise, best speed regulation and best reliability. It's still early days but it looks promising.
 
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BCJKiwi

Senior Member
These fans seem to be variable voltage devices. Have an 80mm Sunon KD1208PBT 1-6A. It is rated at 2.6W.

It starts and runs happily anywhere from 6 to 12VDC. Below around 5.7V it starts to vibrate.

Adding the right capacitance to a FAST PWM will smooth the PWM better approximating DC so you are on the right track.
 
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Dippy

Moderator
Ah yes, Sunon. That famous brand (???).

Yes, putting a healthy cap almost turns your PWM into a S/mode power supply. I just hope the cap can stand it. Probably be OK at those low levels.
 
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inglewoodpete

Senior Member
Sunon are quite common in this part of the world. They seem to do the job. I can imagine they sound a bit like Hyundai did 20 years ago. I know what my reaction was to the anglicised 'Goldstar' and 'Diamond' brands that came from China in '60s and '70s.

Yes, putting a healthy cap almost turns your PWM into a S/mode power supply. I just hope the cap can stand it. Probably be OK at those low levels.
Click to expand...
As I mentioned previously, a 100n capacitor across the fan was much more successful than 3.3uF. Not exactly what I'd expect.
 
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boriz

Senior Member
A PICAXE is definitely overkill. But why not :)

Rather than PWM, you could use switchable parallel current sources. IE: Two MOSFETS driven by two PICAXE outputs, each with a resistor from the drain to the fan –VE. (Assuming the fan +VE is connected to +12v). When both are on, the fan gets twice the current and turns at twice the speed as when only one is on. (well, it’s probably not a linear relationship, but you get the idea). Or you could use 3 MOSFETs for more speed options.

I’m not suggesting this is the best method. I offer it only as an idea for lower noise levels.
 
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inglewoodpete

Senior Member
I decided on a PICAXE because I think the PVR power supply might not be up to giving another 100mA at 12v. The PICAXE will detect audio signal out of the PVR, indicating that it is not in standby mode. The PICAXE will switch in a 3 or 5VA transformer to supply the fan as well regulate the temperature via fan speed.

2 or 3 FETs and resistors are an option if the PWM is not successful. Mrs IP is off to her sewing group tonight, so I'll have another play on the breadboard.

Noise is not a problem with PWM set at 20kHz, apart from when the fan is flat out.
 
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BeanieBots

Moderator
Be careful with higher capacitance.
Your FET will charge up the cap as fast as it's Ron will allow. Both FET and cap must be able to handle that current. Also, the motor will discharge the cap much slower than it is charged. The result is a 'skewing' of the voltage 'seen' by the motor vs the duty rate.
 
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Dippy

Moderator
Exactly my point. I wasn't referring to the actual capacitance value....
As BB is a Swithed Mode Man he knew what I was on about.
(I'm glad someone does...)

Hey IP, don't forget some voltage sensing otherwise your fan will come on when the Toppy is off and you talk to the Mrs.
You could get her to embroider a filter for it.
 
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wapo54001

Senior Member
Maybe your fan/Picaxe project should be separated from your fan-in-the-living-room requirement.

The "silent PC" movement has generated lots of interest in inaudible fans and they are out there in numbers, mostly running on straight 12vdc but also reduced to 7v and 5v. I think that pwm is not recommended.

Check out the fan section of http://www.silentpcreview.com

A 100ma draw tells me that this fan is a rather high speed unit not intended to run quietly.
 
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inglewoodpete

Senior Member
Many thanks for your contributions.

The fan draws about 94mA of 12v with 100% PWM (20kHz) and 53mA at 20% modulation. This is with a 0.1uF cap across the fan.

BB:
Be careful with higher capacitance.
Your FET will charge up the cap as fast as it's Ron will allow.
Click to expand...
That makes sense. When I put a 3.3uF cap across the fan, its speed does not change much for a large variation in % modulation.

I am now happy that the fan speed can be controlled with the configuration above.

Some may consider that a PICAXE is overkill. I dont want to poke around too much with the PVR's motherboard, which uses a switchmode power supply. I can tap off 5v from the PVR to run the PICAXE and the opamp audio detector fairly safely. Then use the PICAXE to control switching of a small mains transformer, which will power the fan.

The PVR outputs audio from anything it is recording or playing. The audio detector plugs into the RCA connectors on the back panel and monitors any activity. (Ie PVR in Standby = no audio activity). Audio activity will cause the PICAXE to switch on the additional power and fan. The PICAXE software will turn off the additional power after a few minutes of inactivity on the audio outputs.
 
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Andrew Cowan

Senior Member
Alternativly, you could run a slow PWM - 10hz or so. That would ensure that it never gets up to full speed, thus does not get too loud.

Andrew
 
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boriz

Senior Member
“...and an audio integrator to detect when the PVR is not in standby mode. The idea being, the fan only revs up when it is really needed…”

Surely it’s only ‘really needed’ when the hardware is too hot. Better to have the system controlled exclusively by temperature and use a quiet fan.
 
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