Npn NO proximity sensors


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    Default Npn NO proximity sensors

    I am new to the cnc world and was wondering instead of using a voltage divider circuit to drop the voltage from 12v to 5v would it be possible to use a buck converter to bring the voltage down to the necesary 5v for my BOB?

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    Default Re: Npn NO proximity sensors

    If you mean the supply voltage + 5Volt of the BOB, then the answer is YES. A buck converter will do it even better then a voltage divider.



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    Default Re: Npn NO proximity sensors

    Yes. My proximity sensors run on 12v and need to get the voltage down to 5v correct? I am using a DB25 1205 BOB.



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    Default Re: Npn NO proximity sensors

    Let me rephrase this...sorry. I am trying to step the voltage down from the 12v my proximity sensors output to 5v using a buck converter. My BOB is powered with 5v already.



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    Default Re: Npn NO proximity sensors

    Personally, I would use a 12v capable optocoupler to switch your existing 5V power to your BOB input. I think a Buck power supply is overkill and it may add some delay to the signal.

    Rod Webster
    www.vmn.com.au


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    Default Re: Npn NO proximity sensors

    So it's not the supply. Don't use the buck converter. Have a look into the manual and see if the inputs are optocoupler. If so, they might even accept the 12 V or just put a serial resistor ( about 10 Kohm will do )
    If it's a direct input on a IC then you can use a voltage divider 8K2 and 5K6 will do the job.



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    Default Re: Npn NO proximity sensors

    The inputs and outputs are all optoisolated



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    Default Re: Npn NO proximity sensors

    Npn NO proximity sensors-screenshot_20180508-205328-jpg

    - - - Updated - - -

    This is the board I have



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    Default Re: Npn NO proximity sensors

    The output of an NPN NO sensor is just a switch to ground. It doesn't have any output voltage of its own, so it doesn't need to be stepped down.



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    Default Re: Npn NO proximity sensors

    If your proximity sensors have true open collector outputs with no pull up resistors or LED indicators
    connected between the positive supply and output

    the sensor will act like a N/O switch between the output and ground terminals like this -

    Npn NO proximity sensors-npn-proximity-switch-jpg


    your break out board will be some thing like this -

    Npn NO proximity sensors-wantai-bob-circuit-jpg

    ( NOTE - the photos found on line did not make it clear if the 12 IC inputs from the printer port have the 10K resistors connected as pull up or pull down resistors - I have shown them as pull down resistors )

    the 5 inputs from your E-stop switch and proximity sensors etc
    are connected to a 330 ohm resistor and LED that is part of an opto-isolator
    when a switch or proximity switch grounds the input

    approximately 10mA will flow from the BOB's +5V supply .though the resistor & LED and the switch / sensor to ground and the light from the LED switches on the NPN photo transistor connected to the printer port input

    the board has two 5V power inputs
    one for the PC side of the BOB and one for the isolated
    input / output connections

    some versions of the board has two jumpers that enables the use of one 5V supply but this will by pass the galvanic isolation that is needed if you have problems with ground loops for instance

    John



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    Default Re: Npn NO proximity sensors

    John....would you mind if I private message you?



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    Default Re: Npn NO proximity sensors

    OK with me if you want to PM me
    the only thing is as I don't use any 3rd party to store pictures
    I have not found a way to add photos to private messages

    John



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    Default Re: Npn NO proximity sensors

    I have downloaded the data for your NBN8-12GM50-E2-V1 sensors
    NBN8-12GM50-E2-V1.pdf

    this confirms they have N/O NPN outputs
    Npn NO proximity sensors-nbn8-12gm50-e0-v1-2-jpg

    which can be wired in parallel and connected to a BOB input like this
    Npn NO proximity sensors-two-sensors-bob-jpg
    (provided the outputs are open collector with no pull resistor up to the +12V supply)

    John



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    Default Re: Npn NO proximity sensors

    Ok....so i can just run the sensor Black wire directly to my BOB with no resistor?



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    Default Re: Npn NO proximity sensors

    I was going to say connect the sensor output directly to the BOB input
    until I read the NBN8-12GM50-E2-V1 data sheet again

    the maximum reverse voltage for the LED inside the PC817 opto-isolator used on the breakout board is 6V

    I am not 100% sure but I assume the 0.5mA leakage current referred to on the sensor data sheet
    is from the positive supply to the output
    due to a component / circuit to protect the NPN output transistor from the back emf when it switches off an inductive load like a relay
    output leakage.pdf

    if I am correct, the leakage could damage the opto-isolator
    (as the cathode will be taken to +12V while the anode is at +5V reverse biasing the LED by 7V )

    what voltage do you measure between the negative supply and the output when the sensor is switched off ? (I am assuming you have a multimeter )

    if its a volt or two then you can connect the output directly to the BOB
    but if its around +10 to +12V
    I would add a diode with the cathode connected to the sensor output and anode to the BOB input

    any silicon diode like a 1N4148 or 1N4001 to 1N4007 will do
    (the 1N4001 is a 1A diode and has thicker wires that may be easier to connect to)

    Npn NO proximity sensors-two-sensors-bob-added-diode-jpg

    the problem with adding a resistor instead of the diode is
    you need a low value resistor to ensure the NPN photo transistor inside the opto-isolator is turned hard on
    but you will need a high value resistor to limit the reverse current through the LED when it breaks down

    John

    Last edited by john-100; 05-12-2018 at 04:57 PM. Reason: add ref to conflicting requirements for resistor value


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    Default Re: Npn NO proximity sensors

    Ok. I put a multimeter on the sensors output and ground. When no metal near the sensor it reads 11.70v. When i move metal to the sensor the meter reads .26v. I had some 1n4148 diodes and connected one up to the output as your drawing shows. I have .04v when no metal is near and .02 when the sensor activates.



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    Default Re: Npn NO proximity sensors

    "When no metal near the sensor it reads 11.70v. When i move metal to the sensor the meter reads 0.26v."
    makes sense if there is a high impedance between the sensor + supply and output

    "0 04v when no metal is near and .02 when the sensor activates"
    . looks like you have either a faulty component or a short circuit

    what do you have between the BOB's ground and an input with no sensors connected ?
    I would expect it to be about 3.2V and 5V


    try using two diodes like this

    Npn NO proximity sensors-2-sensors-2-diodes-connected-bob-jpg

    john



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    Default Re: Npn NO proximity sensors

    Ok John. When I took the readings on my multimeter the sensor was not hooked up to the BOB. I hooked one sensor to the input of the BOB following your diagram. When sensor is off I am getting 4.14v at the input. When the sensor is on I am getting 1.13v.



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    Default Re: Npn NO proximity sensors

    that's more like I expected

    this should sum up the results

    Npn NO proximity sensors-sensors-parallel-bob-input-jpg

    John



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    Default Re: Npn NO proximity sensors

    Should I put an external pull up resistor like your diagram suggests?



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