Speed Transducer

Discussion in 'The Projects Forum' started by odm4286, Apr 6, 2014.

  1. odm4286

    Thread Starter Active Member

    Sep 20, 2009
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    Hello everyone, was just looking for some constructive criticisms for this circuit. Its pretty simple, just a speed transducer. Is there a better way to do this or am I on the right track. Thanks ahead of time.

    [​IMG]
     
  2. #12

    Expert

    Nov 30, 2010
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    There seems to be a basic fault centered around D1. There is no current path for the base of Q1 because the diode opposes that flow. Perhaps you can tell us the intent and how the transducer works.
     
  3. odm4286

    Thread Starter Active Member

    Sep 20, 2009
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    D1 it's there to prevent back feeding t1, my hall effect sensor. Did i not set this up correctly? It works great in the simulator.
     
  4. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Your transistor is on all the time, because the base voltage goes to 5V,and the emitter is at 24V, you need a NPN type
     
  5. #12

    Expert

    Nov 30, 2010
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    May I quote you in the future? :D

    Dave and I agree that something is wrong, but it is so wrong that we focus on different parts. At least you told us it's a Hall Effect sensor.
     
  6. MaxHeadRoom

    Expert

    Jul 18, 2013
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    What is the nature of the transducer, most 3 wire devices have a open collector output or similar, if so, why not input directly into the PLC?
    Also if using a PLC, why not input the defeat switch (1 N.O. contact) into the PLC and do the logic internally?
    And if the output of the Transducer is O.C. NPN, you could be loading it?
    Use an Opto if isolation or voltage transition is needed.
    Max.
     
  7. crutschow

    Expert

    Mar 14, 2008
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    Don't know what simulator you are using but it's lying to you. As #12 noted, the diode blocks the base current through the transistor needed to turn it on (current flow from emitter to base) so it will always be off. And if you reverse the diode, then the transistor cannot be turned off unless the voltage from the Hall sensor output goes to +24V.
     
    #12 likes this.
  8. MaxHeadRoom

    Expert

    Jul 18, 2013
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    I think you should have sourced a 24v transducer!
    Max.
     
  9. #12

    Expert

    Nov 30, 2010
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    Here's one that has a limit at 24 volts, but it works at 5 volts, too. Open collector. No problem if the magnetic field is right.
     
  10. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    How about this?

    Edit:
    You could reduce R1 if Q1 doesn't turn on fully.
     
  11. #12

    Expert

    Nov 30, 2010
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    If you visualize that with R1 and Q1 being inside the sensor, then it's a good plan.
     
  12. MaxHeadRoom

    Expert

    Jul 18, 2013
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    If going with the small ones that #12 showed, Honeywell make a 4000 series that go to +24v.
    If your sensors are NPN, then if the PLC has the option, set it for source input (sink device).

    Max.
     
    Last edited: Apr 7, 2014
  13. odm4286

    Thread Starter Active Member

    Sep 20, 2009
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    hey guys thanks for the replies and sorry for the lack of information. The hall effect sensor im using is a OH090U

    Datasheet:
    http://pdf.datasheetcatalog.com/datasheets2/29/297921_1.pdf

    This sensor has an output voltage equal to input voltage when no magnetic field is detected. Anything above 90 gauss triggers the sensor and the output is then 0v.

    So my logic here was to have it high all the time unless a magnetic field was detected. Then it would go low, "closing" the pnp transistor and allowing current to flow from the emitter to the collector. So in the end you'd end up with pulsed DC which could be used to determine speed.

    The 6.8k resistor represents the input resistance of my PLC input card. Thanks again for the help, I hope this clears some things up. Let me know if I'm missing some basic transistor theory here, I'm still learning.
     
  14. #12

    Expert

    Nov 30, 2010
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    Pretty much the same thing as what I posted. Open collector, 50 ma current limitation.

    One, you don't add 20 pf just to make it look like the speed test circuit.
    Two, I don't think the 6.8 k resistor that Alec drew was intended to resemble the inside of your PLC.
    Three, If you're going to do the PNP circuit, both of the power supplies have to be the same voltage. Still, you're better off with Alec's circuit.
     
  15. MaxHeadRoom

    Expert

    Jul 18, 2013
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    The component you posted takes 24vdc? So why the 5v?
    Also if you can configure the PLC for a sink input device, just input direct to the PLC.
    With a PLC it does not matter what the logic level is, it can be reversed in the PLC.
    Also, if trying to capture a high speed pulse, the freq has to be lower than the PLC refresh rate, otherwise you miss pulses, the upper end PLC's have a Encoder or pulse module for capturing such events.
    Max.
     
    Last edited: Apr 7, 2014
  16. #12

    Expert

    Nov 30, 2010
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    The component he posted works on 5 volts, so why the 24 volts?

    Probably just showing my ignorance of PLC's.:rolleyes:
     
  17. MaxHeadRoom

    Expert

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    OH090U shows 24v VCC, unless I am missing something?
    If so, It could be fed from the PLC input supply and the 5v supply not needed?
    The make/model of PLC would also help.
    Max.
     
  18. odm4286

    Thread Starter Active Member

    Sep 20, 2009
    155
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    its a modicon m340. The 5V signal is to switch the 24V and yes I am trying to count pulses and yes Ive done the math and the PLC can handle the pulse rate. By the way this is just a learning project so it does not have to be perfect and I appreciate all the help, thanks guys
     
  19. odm4286

    Thread Starter Active Member

    Sep 20, 2009
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    Hey thanks for the idea but I'm not sure this would work as my input card in the PLC is grounded.
     
  20. #12

    Expert

    Nov 30, 2010
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    I just haven't figured out why this needs both 5V and 24V when the sensor will work with either of the voltages.
     
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