Drive a relay from an open-drain

Discussion in 'General Electronics Chat' started by code4fun, Sep 25, 2014.

  1. code4fun

    Thread Starter New Member

    Sep 25, 2014
    4
    0
    Hello all,

    I am new to electronics and i need some support.

    I have a digital integrated chip(shift register) with an DMOS open-drain output (from the datasheet -> 33 V and 100 mA continuous sink-current capability) and I want to drive a relay.

    The output of the drain is connected to the relay, and then the relay is connected to the power supply 5V.

    When the transistor from the digital integrate pulls the line to the ground, the output drain is 0 logic, but the relay is not switching.

    What am I doing wrong ? The schematic I use is recommended for driving relays using this shift register (from the application notes of the shift register).
     
  2. pwdixon

    Member

    Oct 11, 2012
    488
    56
    You need to give more details like specific part numbers for the transistor and the relay. I would guess your relay is just not being driven hard enough. If you short the relay coil to 0V does it switch?
     
  3. code4fun

    Thread Starter New Member

    Sep 25, 2014
    4
    0
    OK, so there is a TPIC6C595 shift register and I use to drive RM05-4AS that has 4 inputs and 4 outputs
     
  4. pwdixon

    Member

    Oct 11, 2012
    488
    56
    If you short the relay coil to 0V does it switch?
     
  5. code4fun

    Thread Starter New Member

    Sep 25, 2014
    4
    0
    I double checked and the wiring of the relay was not good :( thanks
     
  6. tom_s

    Member

    Jun 27, 2014
    285
    333
    we've all had those days

    this old fella uses alzheimers as an excuse :rolleyes:
     
  7. ScottWang

    Moderator

    Aug 23, 2012
    4,855
    767
    RM05-4AS-4/4 need the current 24.6~30mA, and each output of TPIC6C595 is 100mA, I think the output of TPIC6C595 can't handle the RM05-4AS-4/4, when the output of TPIC6C595 output 4 high, you should doing the right operation to operating the TPIC6C595, when TPIC6C595 output 4 high to drive RM05-4AS-4/4, using your finger to sense the temperature, if it's too hot then you should adding the PNP bjt or P type mosfet to drive RM05-4AS-4/4.
     
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