How can I sense backwards-connected (DC) devices?

You first could apply a little power, voltage and current limited, of either or maybe both polarities, and observe the resulting voltage drop. The normal 12 volts and a resistive divider should be enough. Experiment with some fans and see if this provides sufficient info about reversal. This could be automated, or implemented with a couple of test switches/buttons/LEDs.

Maybe just a current+voltage limit would do, again maybe a resistive divider. Hook the ps to the fan with the limit engaged and see if it spins a little. Maybe a limited-power reversal won't zap things.

John

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Classic solution: 

Look at page 8 of the data sheet.

Yup. A smoke detector and a microphone to catch the *KABLAM* :-)

Oh, different thing ... let me try again furtehr below.

Conducting tests is what needs to be done. Run a fan normally but as John said, crank up the voltage slowly. However, in addition try to find out a signature that shows up in the load current. Some commutating chip starting it's clock, whatever. Maybe that kind of noise could be used and the absence of it would indicate a possible wrong polarity.

Then find out how much reverse it can take. Of course, there is no guarantee that the damage is zero, or that all of the fans of same type will survive.

I'd do the tests above. Magic smoke isn't the only concern. Sometimes reverse polarity can damage parts but they still run. Sort of, because the lifetime or performance might had been compromised. Just like an RF transistor that had a zener breakdown at the BE junction will often still work but not as good anymore.

[...]

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e

yeh, assuming the output fets have an anti parallel diode (don't they all?) I'd think reverse should look like a pair of diodes in series with the coils The fets are rated for 250mA so the diodes should be able to handle a bit of current

with power the right way, nothing much should happen until the voltage gets the minimum 3V and the part starts running

-Lasse

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A typical soft failure in a semiconductor is a leaky junction. In a lot of cases, the part works, but the shutdown power spec is off the chart.

The failure analysis group can certainly tell how you damaged a chip. I've been involved in a few of these detective jobs. But how a customer gets access to the FA group is beyond me since I've never been a semiconductor customer, well other than Digikey. ;-) I assume you need to be a volume player.

In most cases, you can put a diode in series with the load, in the device side, so that reverse voltage will never reach the sensitive part. If you connect it backwards, no damage, but it won't work, so reverse the wires. The voltage drop across the diode will be 0.6 V or so.

If you need it to work either way, you can connect a bridge rectifier such that the voltage applied to the device is of the correct polarity no matter how it is connected. This will drop about 1.2 Volts in the two conducting diodes of the bridge rectifier component.

This latter scheme may not work if the device is connected to other systems, and if the power supply is grounded to other components.

Jon

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The problem with that approach is that it if the DUT is connected to
the brige backwards there's no difference between that and connecting
it to the supply backwards, sans bridge.

Most bipolar ICs will have substrate diodes that will happily conduct moderate amounts of current in the wrong direction, without long-term effects. A few tens of mA, limited to a couple of volts, will probably turn on such diodes and make their presence measurable.

John

Frnak McKenney formulated on Wednesday :

Seems the real answer is employ more inteligent testers and pay them to do it right. There will always be mistakes and no circuit you can devise would cover every situation.

--
John G

"Frnak McKenney" kirjoitti viestissä:5LadnXXdKPqdwL7QnZ2dnUVZ snipped-for-privacy@earthlink.com...

I tested some fans from my box of old fans: Adda, Delta, Top and Tech(JL), Top Motor, Yate Loon, Superred, Xinruilian, Panaflo, Comair Rotron

About half had protection diodes and other half drew under half amp at 12V and did not die in 30 s in reverse polarity. They will most likely overheat if in reverse polarity for long time.

One Papst (made in W-Germany, so quite old one) and one Japan Servo tangential fan tried to draw much current. They would likely die fast if the power supply is not current limited.

-ek

John,

Thanks for the prompt reply.

I've got some (two-wire) 12V PSU fans I scavenged the other day. After a bit of testing I discovered that:

1) A CCR2032-powered ohmmeter reports 33megohms across the fan, which may tell m more about my fingers holding the leads to the wires than anything else. 2) A cheap and vaguely-regulated switchable-voltage wall-wart set to "6V" isn't enough to turn these, but "7.5V" will. While I don't trust the wall-wart setting to be exact, it does indicate that there _is_ a minimum voltage. (Too bad -- if these were DC PM motors, the fan would just turn backwards: A Clue! )

Okay. So I need to run some tests... er, I need to clean the junque from the last five projects off the workbench so I have some speace to work in and run some tests.

What I'll be looking for is current flow under normal and reversed conditions at (say) 3V.

Thanks...

Frank

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Lasse,

Thanks for the reply.

[...] [...] [...]

If antiparallel diodes are standard, then I should be able to see a clear difference in current flow between "normal" and "screwed up"... er, "reverse-connected".

Thanks. That's reassuring, since my supply of scavenged fans for testing is limited.

Frank

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Joerg,

Thanks for jo> Frnak McKenney wrote:

I'll bet you were one of the guys who wrapped ten-ohm resistors with electrical tape and jammed them into dorm outlets after throwing the circuit breaker on the victim's room.

[...] [...]

Not reassuring. The connector for the fans I'm initally concerned with is polarized, with the pins in [Minus,Plus,Sensor] order, but I'm not aware of any guarantee that every fan is wired that way.

_Is_ there a reverse voltage high enough to measure but low enough so as to be trusted not to damage this type of fan? If so, do you have any suggestions for determining it? I don't mind losing a few fans in the process.

Frank

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  jail or fines. In Washington, meanwhile, most regulators and
  Members of Congress remain on the job, often with enhanced power.
      -- "Bernanke's Second Chance" / Wall Street Journal 08/26/09

[...] [...]

JG,

You're absolutely right, but I'm building this for a friend who has several cabinets' worth of RAID-configured drives (she's a photographer) and each drive has one or two fans, plus there are case fans to worry about. Friends are a lot harder to replace than testers.

Frank

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ek,

Thanks for resp>

Wow!

Thank you very much for the time you invested (and the risk to your collection) in doing this. It was definitely "above and beyond the call of duty".

I can believe that. I'm impressed that any electronic circuitry could be reverse-connected, draw half an Ampere, and still not die. I've been thinking of tests lasting perhaps a second or two.

Based on your tests (and thank you again for the effort you put in) and some of the other comments posted here, I'm thinking that the test might be as simple as having a moderate-sized resistor in series with the motor and monitoring the voltage dropped across the resistor. I've seen 12V fans rated from 0.06A-0.15A, so an imagined 240 ohm resistor in series with a 0.15A fan would (were the fan purely resistive) drop the fan voltage to 3V and the current to (12/320) or about 40mA.

Once the circuit has established ("made a reasonable guess" ) that the fan is connected correctly, shorting across this resistor will deliver full voltage/current to the fan for further testing.

If I have some time free this evening, I'll experiment with my own (much smaller) colleciton of fans and see what might be appropriate for "the circuit".

Thank you agin.

Frank

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I always had good luck with red and black wires...and hiring repair people with at least one working eye.

At the risk of stating the obvious...How about the following strategy? Replace the fan that ain't turning or is making excessive noise.

If you're intent on testing something, measure the supply voltage. If the volts are there and the fan ain't turning, it's pretty obvious where the problem lies.

No, I always lived off campus :-)

Can't someone just look that the red and the black wires are connected correctly? Seems easier to me. If done wrong too often it's time to have a chat.

You'd have to talk to the manufacturer of the commutation chip but it is unlikely that they'll give you anything in writing.

--
Regards, Joerg

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The idea would be the device would be properly connected to the bridge's output and tested, and then that connection would not be changed again. I know of some systems where this was done after some preamps that ran off + and - 12 V were smoked by improper connection.

Jon