Defeat an open collector sense line?

[PeterEFry]

Cheesy question:
I have a network appliance with a 1U form factor power supply with a couple bad fans -- they rotate, but with excessive noise and vibration (it seems to be their normal mode, but it's hard to tell without samples for comparison). I would like to replace them -- in fact I have new (better in every way) fans installed -- but I received a rude awakening: The PS uses fans with a locked-rotor sensor, not a tachometer.

Grrr. I'd like to defeat the sensor input.

According to the original fans' data sheet, they supply an open collector (NPN, emitter -> ground); judging by my tests, this seems to be accurate. I would expect a 5V (most likely) or 12V pull-up for the shutoff logic, with a 1k or 2k resistor. Now, I'm a straight-software-logic kind of guy, so my first impulse is to simply ground the pins, but the possibility of excessive current flow worries me. If I'm right, I get a few mA, but if I'm wrong... So here I am. What would y'all recommend? Remember, I want a constant "on" (pulled low) state, and I have to cram the solution into a tiny, already-too-full box. I also would rather not disassemble (i.e. destroy) the original fans (they're not readily available, and a replacement PS is $750), so reverse-engineering their internals is a bit problematic. Same with the PS itself -- naturally, it's all glued together.

I'm going to make a few more checks, but I'm not sure if they'll provide me any useful data: Resistance between my sensor pins and 5V & 12V, just to see if I have a clear path to the pull-up resistor; perhaps current flow using the old fans.

Thanks.

Peter E. Fry

 

[itsmoked]

Can you hook a voltmeter between to the sense wire and the chassis ground to get some idea of what the running installed signal is?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[MikeHalloran]

Ground the sense wire with a 100mA fuse.
Looks like a 1/8W resistor, solders in like a resistor. Costs a buck.



Mike Halloran
Pembroke Pines, FL, USA

 

[OperaHouse]

Any open collector should be less than a couple mA. Use a 100 ohm resistor to ground. Cheap and even connecting it wrong wouldn't damage anything. Post back with the result. I haven't run into any of these three wire fans yet. I love these defective power supplies for project and would like to know what this third wire actually detects.

 

[PeterEFry]

Good points about the fuse and the resistor -- thanks. I have some 250mA fuses lying around for another project, but when I think about it, the 100 Ohm resistor yields a maximum 120mA on an open 12V line, which is the most I can have. So I threw it in, and got .3mA. Perfect. My ground points aren't perfect, so I may use an 82 Ohm resistor instead -- I'll test it again once I've built the wiring. Funny -- it looks like an AC signal. Eh -- WTH, it works, and I'm back in business. Thanks again.

Most of your three-wire fans have a tachometer -- a square wave out driven off of a Hall-effect sensor with a transistor. Comair Rotron has some good docs. 2 pulses/rev standard. The original fans in this PS have a locked-rotor alarm wire instead -- a special-order option. I'd recommend grabbing the datasheets for your particular fans. In my case, I looked after the fact, and I had to rip all of my stuff apart and rebuild it -- a personal peeve. The info wasn't hard to find -- a Google search got me:

http://www.nidec.co.jp/news/english/product/fm/fm_pdflist.html

...which has a part number key and datasheets. Not all manufacturers have this data handy for OEM parts -- another peeve. Me, I'm a believer in COTS (but not C[insert acronym for excrement here]OTS -- parts have to perform).

In my experience, the tach implementations vary and aren't terribly reliable -- no standards. But hey.

 

[nbucska]

There are two possibilities:
1.) Error = 0 No error =1
2.) error = 1 No error =0

The first is the most likely. Try to disconnect the sense line.

If this doesn't work case 2.) applies - so GND the line.
No resistor or fuse is necessary.


Plesae read FAQ240-1032
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