transistor question?

Discussion in 'General Electronics Chat' started by jonisonvespa, Apr 1, 2012.

  1. jonisonvespa

    Thread Starter Member

    Mar 25, 2012
    49
    0
    hi
    im currently working on a project, it consists of monitoring the current of a single phase if the current falls below the pre set value then i will get an input to my arduino.

    im trying to use use a transistor once the motor is running at its slowest speed (min current), adjust the pot to turn on the transistor then input to arduino then is the motor falls below current i will loose the input. im using a current transducer the voltage out will be 2.5min to 4.5v max.

    what im stuck on is trying to work out the min current my transistor needs to turn on so i can work out the pot value here is the data sheet just need to know the symbol is this the simplest way to solve this maby other ways?

    thank you

    http://www.rapidonline.com/pdf/47-3372.pdf
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,347
    Hello,

    Can you post a schematic of the intended circuit?
    That way we can see if all is OK or if changes are needed.

    Bertus
     
  3. jonisonvespa

    Thread Starter Member

    Mar 25, 2012
    49
    0
    thanks bertus i will
     
  4. jonisonvespa

    Thread Starter Member

    Mar 25, 2012
    49
    0
  5. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,347
    Hello,

    What for input do you want to have for the Arduino?
    An analog (varying with the sensor output) or a digital (when a certain current is reached a high output)?

    Bertus
     
  6. panic mode

    Senior Member

    Oct 10, 2011
    1,321
    304
    read the datasheet, it says that load of transducer load MUST be greater than 2k. in other words you must prevent situation where transducer output i connected directly to base of transistor. (if you don't believe me, see what happens when you connect 2-3V directly to a diode or base-emitter section of BJT). so to prevent smoke, there should be (additional) fixed resistor between transducer output and potentiometer or between base of transistor and potentiometer.

    also as shown on diagram, potentiometer is practically useless because it does not give you needed adjustment - it does not vary voltage, it only changes current flowing into base of transistor. you can get much better response if the third terminal of potentiometer is tied to gnd.

    but why don't you bring analog output from transducer to an analog input of your arduino? then you can do comparison in code instead of external comparator (very sharp cut-off).
     
  7. jonisonvespa

    Thread Starter Member

    Mar 25, 2012
    49
    0
    i was thinking i could do it with an analog input so when the motor fails, the current falls then switches off the transistor then looses the input. could i still do it using this method?

    would like to do it this way as ive just started to learn to program the arduino so learning comparison may be a long way off but sounds the best way thanks panic
     
    Last edited: Apr 1, 2012
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