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Defeat an open collector sense line?

Defeat an open collector sense line?

Defeat an open collector sense line?

(OP)
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

RE: Defeat an open collector sense line?

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

RE: Defeat an open collector sense line?

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

RE: Defeat an open collector sense line?

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.

RE: Defeat an open collector sense line?

(OP)
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.

RE: Defeat an open collector sense line?

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
My WEB: <http://geocities.com/nbucska/>

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