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Driving 50 fans from one pwm signal

post #1 of 58
Thread Starter 
I know that each fan is not supposed to source more than 5 (or possibly 8) ma, with 50 fans (rounding up for safety margin) nearly a half an amp would need to sunk by the pwm circuit. Even if the pwm circuit doesn't flame out, the capacitance on each fan would be additive reducing the rise and fall time significantly because of limited current flow.

I'm pretty sure that I need a buffer circuit to "amplify" the pwm signal. Note that the power will be taken directly from the PSU and only one RPM signal will be returned. The issue is driving a large number of pwm fans from a single pwm signal. The source of the signal will be the motherboard in one case and an arduino in another. The latter is more restrictive. The motherboard might be ok with 5 or so fans, but that is not nearly enough. I don't need a fan controller, all fans will spin at the same speed.

Does anyone know of an appropriate open drain / collector circuit that can be driven by a single pwm signal and be used to drive large numbers of fans?

I found one suggestion to use a hex non-inverting buffer, but it does not appear to be open drain / collector.
post #2 of 58
A few of these, and a few splitters to achieve the 50. I dont know of any other way tbh

http://www.akasa.com.tw/update.php?tpl=product/product.detail.tpl&no=181&type=Cables&type_sub=Fan%20Cable%20Adapters&model=AK-CBFA03-45
     
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post #3 of 58
Thread Starter 
That is just a cable. It takes the power from the PSU and routes the RPM signal correctly, but it shares the PWM signal. Five way might work, depending on the fans and the pwn signal source. However, it will not work for 50 fans. I don't need a cable, I will be building my own.

The pwm signal circuit on each fan both has a capacitance and requires a certain amount of current. It is small, but NOT negligible. These are additive when using multiple fans. So you need 50 times the current and need to be able to drive 50 times the capacitance to prevent the pwm wave from being smoothed too much. A little is ok, it will reduce the clicking noise if there is any by softening the edges of the wave. But 50 times is probably much too large a value.
post #4 of 58
5 will always work, 10 may or may not work. biggrin.gif

I have ran 7-8 fans with no problems. 9 became iffy. No overload of motherboard, just not enough signal strength for PWM circuit in fan to read so fan would run full speed. thumb.gif
post #5 of 58
Thread Starter 
My point exactly. And with the Ardiuno, it might not even be able to handle 2. It is much more limited than a motherboard. So, the booster circuit will be needed. I have some knowledge of electronics, but my design and analysis skills are very weak. Given a few hints, I can probably build it, but I don't know if I only need something as simple as a MOSFET driven by the psm signal or if something more elaborate is required.
post #6 of 58
http://pcbheaven.com/circuitpages/High_Frequency_PWM_Fan_Controller/ i have tried this circuit and it works well. not sure if it will solve your problem because of the current draw, but, it may give you some ideas.
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post #7 of 58

There was a thread on this at overclockers, here. Started in 2010, comments through March of this year. Alternate designs discussed. Also, knowledgeable people identified.

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post #8 of 58
Thread Starter 
Thanks. This is the part of the circuit that I already have. This is done by the motherboard and by the Arduino. This generates the pwm signal. It is NOT amplified. Also, there is an error in the circuit when used for fans. The circuit contains a pull-up resistor on the pwm signal. That function is required to be performed by the fan(s) and, more importantly, pulls the signal to 12v. The pwm fan specification (version 1.3) says that the maximum pull up is to 5.25v and the fan is allowed to source up to 5ma. Specified frequency is 25Khz, with 21 to 28 being acceptable. Using 25Khz makes any generated noise inaudible.
post #9 of 58
All you need is a 555 timer (tweak it of course) and any TO-220 BJT OR take the PWM signal and put it through a "amplifier" TO220 BJT biased for 10 gain
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post #10 of 58
Thread Starter 
Quote:
Originally Posted by DaveLT View Post

All you need is a 555 timer (tweak it of course) and any TO-220 BJT OR take the PWM signal and put it through a "amplifier" TO220 BJT biased for 10 gain

I don't know enough to "tweak" the 555. However, Bing on the thread referenced by ehume shows a 555 pwm amplifier in post 1606. Reading the 555 specs however, it appears that it can only sink 5ma at 5v (up to 200ma at 12v, but that is not relevant). Since the basic pwm circuit has to sink (if I remember correctly) 8ma, this doesn't do any good.

Because of that I don't see a 555 being useful. Wouldn't a MOSFET be better than a BJT since (while it uses current to charge the internal capacitance) it is voltage controlled rather than current controlled.

I am thinking that something like a pwm signal, pulled up to 5v could be used to drive a MOSFET, which in turn could sink as much current as needed. If that approach would work, then I am on the right track. I don't know enough to specify the pull up resistor or the MOSFET part number. I am assuming the need to drive 500ma (the 1.3 fan spec would require 250ma for 50 fans).
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