replace relay with npn transistor

Discussion in 'General Electronics Chat' started by tlllastar, Jun 1, 2010.

  1. tlllastar

    Thread Starter New Member

    May 31, 2010

    attached is relay circuit, designed to be driven by an npn transistor. My question is, is it possible to produce the same result if i replace the relay with another transistor? can anyone teach me how it should be designed? and the drawbacks of using a transistor instead of a relay?

    Thanks a lot for all your time and attention :)
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    What does the relay switch? Is there a load out there that draws current? What is the polarity of the voltage and the power needed?
  3. tlllastar

    Thread Starter New Member

    May 31, 2010
    thanks for the reply, beenthere.

    the circuit shows output of a push button. R4 is connected to an I/O port of a PIC, which sends pulse when press/push is detected. Vin is 12V.

    Relay is used in this case, due to considerations that the product/controller that the push button will connect to will be either NC till low pulse is received to execute action or it may be NO till high pulse is received.

    so my question is if a transistor can provide this function? Thanks alot =)

    Vin's 12V.
  4. Bernard

    AAC Fanatic!

    Aug 7, 2008
    Probably yes, but still need to know voltage levels on relay contacts & which contacts are loads.
  5. tlllastar

    Thread Starter New Member

    May 31, 2010
    if this push button is used on access control system, then voltage level on relay contact is 12V.

    may i know how i can connect a transistor to replace this relay?

    and how big a current can it withstand with a transistor instead of relay? Because we may be dealing with higher current if it's a system other than access control.

    Thanks alot
  6. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    The problem is that a transistor does not provide isolation.

    Without knowing EXACTLY how the load circuit operates - eg. by showing that circuit, at least around where the relay contact would normally be connected - it's not possible to predict what would be needed to drive it.
  7. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    Another factor to consider in the design is whether the NC part needs to conduct when the transistor switch(s) are not energized. That is, the NC contact of a relay conducts when there is no power at all to a relay.