Increased Current From the Fan Header

joesgarage11 · Post by **joesgarage11** » Sun Aug 15, 2004 4:10 pm

I'd like to run two case fans with a total current draw of 1.5A at 12 volts from a fan header on an Abit NF-7 motherboard. The header can supply only about 0.3A.

I'm using Speedfan and would like to use it to control the case fans, but need a circuit to take the PWM signal from the fan header and use it to drive the fans from a separate +12V supply.

The signal from the fan header looks like a square wave with ringing on the leading and trailing edges when viewed with an oscilloscope and changes, consistent with PWM, when the fan speed is varied with Speedfan. I tried an NPN TIP31A transistor as an emitter-follower driven from the fan header with a 200 ohm base resistor to limit current, but couldn't get it to do anything but run a fan at full speed.

silvervarg · Post by **silvervarg** » Mon Aug 16, 2004 4:53 am

It should work fine to control a transistor from the PWM signal and take the feed from a molex plug.
I would suggest to use a NPN Darlington transistor with reduced voltage to control it and put the transistor on the ground side. I would prefere to pick a cheap transistor with hfe at least 500.

Code: Select all

+12 MB header
  |
  |     +12 Molex
  |       |
  |       |
  R1      FAN
  |       |
  --------T
  |       |
  R2      |
  |       |
 GND     GND

I suggest to use R1=3.3k, R2=2.2k, T=BD677A.

Or you could just skip it all and buy a commercial fan controoler, but then you can't use speedfan to control it.

joesgarage11 · Post by **joesgarage11** » Mon Aug 16, 2004 9:28 am

Thanks, silvervarg.

It looks like I need to have R2 to pull the signal low. That would explain why I only got full speed from the fan. I'll try your circuit tonight.

With R1 at 3.3k, I need an Hfe of at least 400. I don't have any darlington transistors right now, but will try a discrete darlington made with the TIP31A and a 2N3904 transistor.

cpemma · Post by **cpemma** » Mon Aug 16, 2004 1:58 pm

Just out of curiousity, what's the PWM frequency?

silvervarg · Post by **silvervarg** » Tue Aug 17, 2004 4:26 am

I can't tell the PWM frequency for sure on a particular motherboard, but it is usually in the range 25-40Hz if I recall correctly.

efcoins · Post by **efcoins** » Tue Aug 17, 2004 6:35 am

I'd like to run two case fans with a total current draw of 1.5A at 12 volts

Are you sure, some mighty powerful and noisy fans that you have.
1.5W is more SPCR like

joesgarage11 · Post by **joesgarage11** » Tue Aug 17, 2004 10:00 am

1.5W is more SPCR like

Lol, that's for sure. When using Speedfan, the fans run at full speed before the program starts and on exit from the program, so whatever is connected to the fan header will have to be able to handle the current then. In normal use the fans would be turned way down.

The fans in question are two 120mm NMB 'Smartfans', 750 mA apiece.

cpemma · Post by **cpemma** » Wed Aug 18, 2004 6:56 am

silvervarg wrote:I can't tell the PWM frequency for sure on a particular motherboard, but it is usually in the range 25-40Hz if I recall correctly.

I was hoping joesgarage11 could be more specific for the NF-7

joesgarage11 wrote:The signal from the fan header looks like a square wave with ringing on the leading and trailing edges when viewed with an oscilloscope ...

joesgarage11 · Post by **joesgarage11** » Wed Aug 18, 2004 9:28 am

I'll try to get an idea of the frequency. My 'scope is an old EICO that isn't calibrated, but I may be able to get a reading with my frequency counter.

Silvervarg, I tried your circuit last night with a discrete NPN darlington running an 80mm NMB fan and had no sucess. Same as before, full speed or no speed. Using just a PN2222 transistor gave the same results. Tried connecting the fan directly to the fan header and it worked fine that way.

I'll see if I can get a photo posted of the header's waveform tonight.

lenny · Post by **lenny** » Wed Aug 18, 2004 11:37 am

joesgarage11 wrote:The fans in question are two 120mm NMB 'Smartfans', 750 mA apiece.

I saw these on eBay - aren't they thermally controlled?

joesgarage11 · Post by **joesgarage11** » Wed Aug 18, 2004 12:38 pm

Yup, that sounds like them. I got mine from Jameco. Something like US $5.00 each on sale a couple of months ago.

They used to be temperature controlled, but the tiny thermistor on the fan could not stand up to my 'modifications'.

After ripping off one wire from one of the thermistors and losing the other one completely, I gave up and replaced them with fixed resistors.

Interestingly, the value of the resistors had to be different on each fan to make the fans run at the same speed with the same voltage. 10K for one and 8.2K for the other.

joesgarage11 · Post by **joesgarage11** » Wed Aug 18, 2004 4:34 pm

I'm frustrated now...

I hooked up the oscilloscope to the fan header to look at the waveform. It was there, but very low voltage, <0.25V. Connected a fan, and it won't spin at all.

Did I blow the fan header? I was quite careful last time I tested and never let the red connection from the header touch ground directly. It worked last night.

I need to think about that for a while.

cpemma · Post by **cpemma** » Sun Aug 29, 2004 7:30 am

Thinking again about this, I thought the usual motherboard speed control system was PWM with a final bit of light smoothing to give a rough DC that would keep the speed signal working. That would explain the full-speed symptom, whatever you tried - load current through the resistor & transistor would be much lighter than with a fan, so the smoothing cap would stay fully-charged, so in both emitter-follower and switch configs you'd get full output.

What it doesn't explain is you seeing a square wave on the scope...

Increased Current From the Fan Header

Increased Current From the Fan Header

Re: Increased Current From the Fan Header