# Magnetic Pickup sensor MPU



## magmash (Jul 20, 2012)

Hi

We got a generator in to our shop for some repairs lately and while inspecting the wiring on the sensors i noticed this MPU (Magnetic Pickup Unit) that has 3 wires going out from it. I am used to them only having 2 wires, this got 3. The output signal is frequency as usual, So the question is what is the purpose of the third one, I am not able to imagine how the third one can be used 


Thanks in Advance


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## xpertpc (Oct 11, 2012)

Look up Hall effect sensor, basically a solid state magnetic pickup


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## xpertpc (Oct 11, 2012)

wire example would be something like this:
Brown, +24v
Blue, 24v Com
Black, +Output pulse


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## magmash (Jul 20, 2012)

xpertpc said:


> wire example would be something like this:
> Brown, +24v
> Blue, 24v Com
> Black, +Output pulse


That is exactly how the wiring looks like. But i don't get the idea, what is the difference between a hall effect sensor and a magnetic pickup ? aren't they the same thing ??


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## Big John (May 23, 2010)

A magnetic pickup is dead simple: All it's doing is waiting to a ferrous material to pass near the sensor head. The magnetic field changes the impedance of the sensor coil, and affects alternating current passing through the coil of wire, which changes the shape of the waveform. That's why they only need two lead because there is no separate excitation signal.

A Hall Effect sensor is much more versatile. It will produce a voltage for a changing or steady magnetic field, for example In DC amp clamps, hall effect sensors are what is used to gauge current strength. The have a separate excitation circuit which is why you have three conductors, two for power supply one for signal feedback. In my experience, the industrial speed pickups have always been electrically switched square-wave outputs.


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## dronai (Apr 11, 2011)

Big John said:


> A magnetic pickup is dead simple: All it's doing is waiting to a ferrous material to pass near the sensor head. The magnetic field changes the impedance of the sensor coil, and affects alternating current passing through the coil of wire, which changes the shape of the waveform. That's why they only need two lead because there is no separate excitation signal.
> 
> A Hall Effect sensor is much more versatile. It will produce a voltage for a changing or steady magnetic field, for example In DC amp clamps, hall effect sensors are what is used to gauge current strength. The have a separate excitation circuit which is why you have three conductors, two for power supply one for signal feedback. In my experience, the industrial speed pickups have always been electrically switched square-wave outputs.


 So giving feedback with a varying voltage for different switching depending on the output ?


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## Big John (May 23, 2010)

dronai said:


> So giving feedback with a varying voltage for different switching depending on the output ?


 With magnetic pickups, yes, varying voltage and frequency. 

With hall-effect sensors it depends on how the transducer circuit is built. The speed pickups I've worked with were constant-voltage but changing-frequency square wave. For something like a current sensor, I imagine the output is pretty linear and has no frequency component, so it's just a changing voltage.


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## dronai (Apr 11, 2011)

Seems like a Op Amp


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## NC Plc (Mar 24, 2014)

Oh, we have hall-effect mpu's here too. Didn't know the name of them though.


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## dronai (Apr 11, 2011)

NC EET said:


> Oh, we have hall-effect mpu's here too. Didn't know the name of them though.


Your move dude


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