Identification of SMD SOT-23 component

Discussion in 'General Electronics Chat' started by Tera-Scale, Mar 26, 2016.

  1. Tera-Scale

    Thread Starter Active Member

    Jan 1, 2011
    Attached is a photo of an RS-422/RS-485 communication module. An human error caused the SMD component (circled in RED) shown in the photo to blow up. I have another exactly identical board where I could check the exact code number on the part that requires replacement.

    The code is: 712

    When I searched online I got the following list:

    I am assuming that this component is acting as a buffer for the signal as it is connecting the IC (MAX3491E) output to the output terminal. Therefore I would say it is an N-Channel MOSFET.

    I would really appreciate your verification..

  2. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    I get a Nds7002 nfet
  3. AnalogKid

    Distinguished Member

    Aug 1, 2013
    I vote for the Bourns transient suppressor.

  4. InspectorGadget

    Active Member

    Nov 5, 2010
    What kind of connector is J7? Can you take a picture of the connector end, the whole board? What kind of board is it? When in doubt, context would help an awful lot.
  5. Brevor

    Active Member

    Apr 9, 2011
    For once I think Dodgydave is correct, I have a drawer full of 2N7002's marked 712.
    Dodgydave likes this.
  6. SLK001

    Well-Known Member

    Nov 29, 2011
    I vote for testing the identical part right next to the blown device.

    Since the signal that goes into pin 1 of the device continues to somewhere else, I would say that it is a Bourns TVS. A quick test would be to see if pin 3 (the solo side pin) is connected to ground. If so, then it is NOT an N-CHAN MOSFET.
  7. Tera-Scale

    Thread Starter Active Member

    Jan 1, 2011
    I am attaching 3 photos: One of the whole board (not blown), RS-485 connector and an other explaining Pin 3 route. A voltage suppressor makes sense since the communication media of RS-485 is usually passed close to high current cables. This comm module will attach to BEMS (building energy management systems) controller. And will be used to communicate with electrical meters via RS-485 Modbus.

    The human error was that one of the wires (TA) was 'scraped' against a live (phase) wire -_- .. luckily the other 2 slaves (AEG Electrical meters) did not have any damage and they still communicate well. Only this component seems to be blown up.

    The pin marked with 'red short' in photo is in fact shorted with pin 14 of the IC which is normally ground. I think the capacitor with pin 14 is a decoupling. So Pin 3 on the unknown component is going to ground. Good observation SLK001.

    So, would all of you agree that it is a TVS?

    I also had to replace the MAX3491E on this board.. Let's say the 2 TVS are bad.. can I just test the board (in a controlled environment) without replacing the TVS ? Or at least should I remove them.. ?
  8. SLK001

    Well-Known Member

    Nov 29, 2011
    You should be able to test the board without the TVSs on the board. They are there to stop damaging transients. As long as your testing environment is controlled, your board should function normally. Remove the TVS if it is shorted - leave if it reads like a normal zener diode.