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Thread: Is a ULN2803 same as a SPST relay

  1. #1

    Default Is a ULN2803 same as a SPST relay

    Hi folks,

    I am a little new to electronics..

    Controlling a car thru PC parallel port is my first circuit..

    Then I was trying to get a stepper motor running..using ULN2803..

    Now I feel that this 2803 is similar to a SPST relay..

    2803 is capable of driving a normal motor if its input pin is high(5V as input) - am I right ?

    similarly a SPST relay switches the current flow if its coil is energised(for a 6V relay around 5V is enough to switch)..

    So Is a 2803 similar in operation to relay..

    I understand that in a relay the energising coil & switching circuit are actually seperate.. So a relay can be used to actually switch very higher volts & current..
    But in a 5V less current scenario(driving a dc motor).. both behave the same
    other than that is there any difference between the two ?

    Kindly explain me if I was wrong in any basic concept ?


  2. #2


    you go through my e-book

    MY EXPERIENCE IN AUTONOMOUS ROBOTICS. In that book i had written about relays clearly. You can get it from yahoo gp-booksbybibin. Read the whole book, then u will get good concept in electronics essential for robotics.

    The main difference b/w relays and uln's are

    Switching speed for relay abt 10hz (around) while that of uln is >10Khz
    relay is used to control high power and high voltage device, like controlling home equipments while uln's for small motors. See the ratings of the device

    Go through my book, you will get thing cleared

    Bibin John

  3. #3
    Super Moderator T-1000 docel's Avatar
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    Sep 2005
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    Default uln2803

    The ULN2803 is a 8 x SPST, low power ( relays consume 60 to 180ma power), non -isolated solid state switch.
    ^Anything is possible..
    after YOU prove it!!

  4. #4
    Member Advanced User
    Join Date
    Mar 2006


    adding to the above fundaes, it is unidirection in nature too, i.e. current can flow only in one direction

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