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how can i change an npn sensor to pnp

Discussion in 'General Electronics Chat' started by markthe spark, Oct 13, 2004.

  1. markthe spark

    markthe spark New Member

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    hi,
    we have a npn sensor with 3 wires (24vdc)
    but our machine is from japan and is pnp
    can we change this sensor to pnp somehow or by using
    relays?
     
  2. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Sensors aren't NPN or PNP, those are transistors - your question doesn't make any sense!.

    I'm presuming your sensor doesn't match your machine?, but we need to know more details about both to offer advice. For a start, what kind of sensor is it?, is it an analogue sensor? (with a variable voltage output), or a 'digital' sensor, with a simple ON or OFF output?.
     
  3. markthe spark

    markthe spark New Member

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    hi,
    this is a digital photo electric cell(allen bradley), 10 t0 30v dc, 3 wire
    npn with the switch wire just on /off, when connected to our machine
    the sensor just stays on all the time, as the manufacture's state there
    machine is pnp, can we make this into a pnp or get it to switch on and off
    or do we need to buy a pnp type, hope this helps
     
  4. dave

    Dave New Member

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  5. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    What type number is the sensor, we can then look on the net for details!.
     
  6. JohnBrown

    JohnBrown New Member

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    Nigel Goodwin wrote:
    Umm, they are referred to as NPN and PNP as it happens, as I remember from my forays into PLC's and automated handling systems. It refers to the open collector(drain?) output configuration. I guess that these sensors are still sometimes used in non-computer systems.
     
  7. markthe spark

    markthe spark New Member

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    hi ,
    allen bradley p.e.c. type 42BA S2LNAE-A2
     
  8. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Looking at the specs this should work fine, it's a very simple thing to do.

    The transistor isn't at all critical, any small signal PNP should be fine, depending on the load it's required to pass.
     

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  9. markthe spark

    markthe spark New Member

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    thanks for the help
     
  10. panic mode

    panic mode Well-Known Member

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    Why don't you simply pick sensor with NPN output?
    Just call your local reps and tell them what you need.
    (polarity, sensing distance, diffuse with background suppression etc.)
    Take alook at the http://www.ab.com/catalogs/sensors/catalognumberpdf/ch1/1-99_1-102.pdf

    You will notice that 42BA S2LNAE-A2 is NPN so you don't have to change anything. You can use PNP output to drive small relay (700HK slim line from Allen Bradley for example or their twin G2R-SND from Omron).
    Use relay contact to turn on PLC input. Even better - use compact relay in form of terminal block or polarity converter block (they are not as popular and hence cost more than relay).

    You can also add just simple resistor to PNP sensor (load for open collector - 2.2k 1/2watt is fine) and invert the bit in PLC.
    You might be tempted to simply switch "light on/dark on" option on your sensor instead of editing program. Be sure this is safe for the process/machine.

    NPN is very common for sensors and PLC comming from Japan and all their equipment I've worked on so far was always using NPN types.
    In Europe and America PNP is much more popular but
    it's not like you can't find the NPNs as well. I use them all the
    time for all Honda and Toyota projects. You are more likely to have
    better selection and more readily available sensors if you stick
    with Japanese parts (Omron for example). Allen Bradley is
    leading brand in America but if you face the long delivery time, try to convert the signal via relay or Nigel's circuit.

    This is not preffered method since requirements in industrial enviroment are a bit different and it wouldn't last long when handled by most electricians I know but it would work.
    If you go this route just make sure to pick 1/2watt resistors and try to
    put it on DIN rail mountable bases (Phoenix Contact or Weidmuller).
    Also add LED to show operation and one or two diodes to protect it from
    reversed polarity. I would also add current limiting resistor in the collector circuit so the shorts on the output wouldn't kill it.

    Have fun

    You might want to check industrial automation forums. They normally handle this type of questions:
    www.mrplc.com
    www.plcs.net
     
  11. markthe spark

    markthe spark New Member

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    thanks , thats great, i know what to do now,
    new this website but its really helpful cheers
     

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