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Hall effect sensor failure mode

Hall effect sensor failure mode

Hall effect sensor failure mode

(OP)
My Fisher Paykel dishdrawer uses a Hall effect sensor to check motor rotation. The service manual says it should have a resistance of 3 to 4 Mohm measured +ve centre, -ve outer, for each of the two outers.

Instead my Chinese Fuke brand (I see what you did there) meter tells me it is about -3 Mohm. That is not enormously helpful, does a typical DVM get confused like that?

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Hall effect sensor failure mode

What do you get with a 3 meg-Ohm test resistor?

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Hall effect sensor failure mode

"does a typical DVM get confused like that"

Yes, when measuring semiconductor devices, the cheap ones can give weird results. Even the good ones will give weird results if you have voltages on the device other than from the meter. As IR says, make sure you isolate the leads and re-test.

RE: Hall effect sensor failure mode

(OP)
It is disconnected but still physically attached to the motor, so it would be easy to get stray voltages to it, especially given the humidity.

The whole thing is DC, off a switched mode power supply, so even when the mains plug is pulled out the DC side is still somewhat live.

OK, I'll pull the little bugger out and put it on the table. thanks.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Hall effect sensor failure mode

I've always suspected that at the heart of it, the DVM is using R = V/I, where the V is applied by the DVM and the I is measured. So if the DVM applies a tiny little voltage and the component has enough stray voltage on it to overcome and push a poofteenth of an ampere back through the DVM, then you'll get negative resistance.

I'm curious about why the absolute value happened to be spot on - coincidence?

RE: Hall effect sensor failure mode

Maybe the (-) is a reminder that ohmmeters typically use a battery polarity backwards from what you'd expect.

Mike Halloran
Pembroke Pines, FL, USA

RE: Hall effect sensor failure mode

Two things:

A (-) negative reading when the DVM is set to Ohms indicates that there is a current still in the circuit, spoofing the resistance measurements, as others have said. When taking an interest in semiconductors it's nice to get a DVM that can test them properly. Look for the ones that have a "-|>|-" setting.

A semiconductor like the Allegro HE sensor has even odds of failing to short or to open. Since the semiconductor is just bonded to a continuous shunt, there's low risk of the shunt failing open. However, the cross-section of the shunt is very small compared to the supposed current rating of the sensor assembly. Lots of opportunities for error, especially with the screw terminal variety. I know of some folks who have used them with small wind turbines, but I wouldn't dare to. Allowing the power line circuit of a wind turbine to fail open would invite disaster. Since you're using solar, an open circuit isn't inherently disastrous. Unless certain consequences of the open-circuit are.

STF

RE: Hall effect sensor failure mode

(OP)
This one is for the dishwasher, not solar.

Anyway I have pulled the thing out and in the process of a slightly aggressive inspection, ie prodding it with the probes, the rather corroded sensor fell off the rather corroded circuit board.

So, I've ordered the part, but it is on back order, and also ordered 10 of each hall effect sensor I could find on ebay, for about 10c each.

Frankly I'm not wildly impressed that a PCB in a dishwasher under the pump unit is installed without encapsulation. The rest of the machine is neat, that detail is gross

The chip appears to read W10 4878 but I can't find that using google.

The real mystery is what the Hell are the 4 pads between the chip and the socket? presumably some abandoned wiring from a previous system.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Hall effect sensor failure mode

Are there no components (surface mount capacitors/resistors) soldered on the backside connecting those?

Ah yes, corrosion - I've had the voltage/current generated by (ok, take your pick - thermocouple emf, galvanic (corrosion) cell, cosmic rays) spoof a sensitive circuit into triggering at random moments.

RE: Hall effect sensor failure mode

Here's my guess on the empty pads.

The circuit card was designed to also accommodate an older or alternate type of Hall effect sernsors that might have required external resistors, to be installed on the empty pads. But the newer or other alternate Hall effect sensor used here are fully integrated and the same circuit card is used simply to connect the sensor IC to the wiring connector.

Pure speculation.

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