My circuit for using 3 pin fans on 4 pin PWM mobo headers

Control: management of fans, temp/rpm monitoring via soft/hardware

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justblair
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My circuit for using 3 pin fans on 4 pin PWM mobo headers

Post by justblair » Sat May 10, 2008 7:19 am

Been an age since I last posted here.

Apologies if I am teaching my Granny to suck eggs here, but I searched not just here but all over and couldn't find a circuit published anywhere that does this. The only similar circuit I could find anywhere was just blatantly wrong.

Image

Its a simple circuit that seems to work very well.

I checked out the standards for 4 pin mobo connections. The important things to note are.

The mobo header works in open drain on pin 4 to provide the PWM signal to the fan. The mobo's are capable of draining 5mv of current minimum.. That is why I used a 1k pullup resistor. I reckon i could have used higher no probs.

The fan itself requires a 3-5v pullup, ie the mobo pwm controller does not supply voltages. Hence the reason for the LM78L05 voltage regulator.

The circuit above uses a BFY51 medium power transistor. The 12v fan I have tried it on is rated at 0.3A. With an Hfe of 120 on my multimeter, and 5 mv of current at the base, this circuit should be good for supplying two fans at a push. I have several BFY51's that have higher HFe and so could comfortably supply two 120mm fans.

Why that particular transistor? Well its NPN. Can handle 800mv of current no problem, and most importantly I had some in my parts bin that I had no other use for.

The rest of the circuit is easy. As a motor (fan) is an inductive load, it does not require a current limiting resistor. The only other part of importance is the 4n4001 diode between the +/- pins of the 3 pin fan. The motor is an inductive load, so when swithing off produces voltage spikes. The diode is there to protect against them.

I made up this circuit this afternoon and it works as intended. Comparing it to the standard 3 pin PWM function on my Mobo there are some advantages to using this circuit.

The PWM from the 4 pin header seems smoother and quieter.

The 4 pin PWM does allow the fan to be controlled down to a much slower speed. On speedfan I managed to get the 120mm thermaltake fan I had (Origionally supplied with a Typhoon heatsink) controlled to 5% in free air and running fairly reliably. This was giving around 60rpm (the fan cannot report to the computer its rpm at these speeds. 6% was totally reliable and still a lot slower than the mobo's 3 pin header could control.

Disadvantages...

Speedfan cannot measure the RPM through the sense wire of the fan very well at all with this circuit. Spurious readings occur frequently when PWM is applied.

Its obviously not as easy as just plugging in to a 3 pin header. Though parts wise the costs are minimal.

I have to admit that if I had realised in the first place that speedfan would control my 3 pin header I probably would not have bothered designing this... But hey it is a bit of a novelty seing a 120mm fan spin so slowly you can count the rpm.

***Schematic edited due to carelessness of the author***
JB
Last edited by justblair on Sat May 10, 2008 10:46 am, edited 1 time in total.

Trekmeister
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Post by Trekmeister » Sat May 10, 2008 10:26 am

I can't make heads or tails of your construction, seems to me like you forgot something?

Should not the output of the voltage regulator be connected to the input connector of the fan? Or is the fan 12V input supposed to be connected to 12V from the computer? But then you would dump all the current through the transistor to ground and that might overheat it!

Or should the transistor be the other way around and connected to +12V or +5V?

Also should the diode not be connected in the opposite direction?


argh! I must be misunderstanding something, or you have made a series of mistakes in your schematic? Please enlighten me!

justblair
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Post by justblair » Sat May 10, 2008 10:35 am

Argh...

You are absolutley right there is a severe mistake in the schematic.

I will fix this before someone gets led astray...

I have just come back from getting toothache fixed. I must have been really muddled with the pain...

***That should be the schematic fixed no.. Sorry for anyconfusion caused...***

Trekmeister
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Post by Trekmeister » Sat May 10, 2008 3:01 pm

Now that makes a whole lot more sense to me! Looks like a good way to power a 3 pin fan from a 4 pin header.

I wonder if the voltage regulator is a bit overkill for generating a 3-5V pull-up. How about a using a zener diode instead perhaps?

I understand you only put this together from things you had handy, I am not critisizing your build. Also the only information I have about how the PWM signal works is from your post so I doubt I know as much as you do about it. :wink:

justblair
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Post by justblair » Sat May 10, 2008 3:14 pm

Trekmeister wrote:I wonder if the voltage regulator is a bit overkill for generating a 3-5V pull-up. How about a using a zener diode instead perhaps?

I understand you only put this together from things you had handy, I am not critisizing your build. Also the only information I have about how the PWM signal works is from your post so I doubt I know as much as you do about it. :wink:
Doh... I have zeners, I had not thought of using one here. I kinda used the first thing my eyes fell on. I could have made the circuit more compact. Good suggestion

Image

Here is a pretty rubbish picture taken on my camera phone of the circuit. This was my V2, which is about as compact as I could make it.

I'm down to 4% pwm reliable working on this fan/circuit combo. That is 77rpm (I am sad enough to time it) from 1360rpm fan. Even my camera phone can capture the fan blades.

JB

thejamppa
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Post by thejamppa » Sun May 11, 2008 2:00 am

77 rpm? Wow that low but it barely moves air that speed. But I am still impressed.

justblair
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Post by justblair » Sun May 11, 2008 2:25 am

thejamppa wrote:77 rpm? Wow that low but it barely moves air that speed. But I am still impressed.
Yes it is very low. I am playing around with speedfan this morning. With a single fan in my Antec fusion case/mini Ninja combo, 77 rpm is reliable, and will keep the system cool enough at idle. It ramps up though pretty much within seconds of opening an application. i suspect that I wont be using this setting on a permanent basis.

The slow speed is impressive, more for the novelty factor. I think that it does highlight the superiority of the 4 pin pwm header over the 3 pin. I am also pretty sure that I am getting a smoother response and less motor noise from the fan.

I have a bit of work to do yet on this.

I am also considering a fan replacement.

Of course now the world is my oyster as fan as fans go. Any two pin or three pin fan can now be controlled by the 4 pin pwm.

I did do something pretty stupid this morning when fitting the controller to the case. I had switched it off, but somehow restarted it, accidentally I touched my pliars accross the power and ground pins on the 4 pin header and released some magic smoke.

However I just took 12v power from elsewhere and the pwm still works perfectly.
JB

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